This is AP Chemistry, Crumb Study Unit 1. In Unit 1, we're going to learn different forms of the matter, definition of an atom, atomic structures, different forms of an atom, which is called isotopes, and the energy levels of the shell. and subshells, electron configuration, and periodic trend. Let's get started.
We're going to start from the definition of an atom. We learn atom, we learn atomic structure and its property because atom is the building block of all the matter in the world. Okay, so any matter in the world, the glass, rubber, plastic, wood, metal, all of this, all of these consist of the small particles called atoms.
That's why we learn atom. So Dalton is the first scientist. ever in the history who suggests the idea of atom. He claimed that all the matter in the world consists of small particles called atom, well, which is correct.
And then he stated that atom is indivisible, it's the smallest unit. Well, that's not actually correct because later other scientists have proved that atom consists of smaller units, which is called nucleus and electron. And nucleus consists of protons and neutrons. Okay, so protons and neutrons, the proton carries the positive charge, neutron is neutral by the name suggests. okay they are tightly packed around each other and then they reside in the center of an atom and electrons move around these nucleus okay and then right in in between the electrons and the nucleus it's just empty space nothing is there So this is the very basic structure of an atom.
When we refer to something as an atom, we're always referring to something that's neutral. Atom can never carry a positive charge or negative charge. Atom is always neutral, which means...
for any of the atom, it has the same number of the protons and then the same number of the electrons. So the charges get cancelled out, the atom remains neutral. There are different types of atoms and then we call each of the type as element. Well, this is the periodic table of the elements.
You've seen this table so many times if you've learned chemistry in high school. Each of the cubicles represent different types of elements and let's break it down into different parts. So right in the center, we have the symbol. So let's say capital letter C represent the carbon. And then on the left top corner, or on top, right in the center, we have an integer with six.
So this number is called atomic number. And this is actually the most important thing when it comes to an element. So atomic number of an element is like their ID card, different elements are assigned different atomic number.
If you take a look at this periodic table, you will observe that no two elements have the same atomic number. of them have different atomic numbers. Atomic number, if any element has atomic number of 34, then it's selenium.
If it has the atomic number of 3, then it's the lithium. If it has atomic number of 5, it's the boron. Atomic number determines the identity.
That's the most important thing when it comes to an element. And this is the same as the number of the protons. Okay, for any of the atom, the proton number is always the same as the number of electron.
When it comes to proton and neutron, they have approximately the same number of electrons. So, if you have a neutron, approximately the same mass and approximately the same size. They're about the same.
On the other hand, electron is like very, very, very small. Electron has negligible mass and it doesn't even occupy that much of the space compared to the protons or the neutrons. When we calculate the mass of a single atom, we just simply add up the number of the protons and then the number of neutrons because their mass ratio is approximately one to one. So proton plus the number of the neutron is equal to atomic mass. This means I can calculate the number of the neutrons by subtracting proton from atomic mass.
So let's say for carbon, it has atomic number of 6, so it has 6 protons, and then it has atomic mass of 12.011. So 12 minus 6, that will be 6 nutrients. And since it's a carbon atom, it has the same number of electrons as the protons.
So it's 6 electrons. Then what about for fluorine? For fluorine, it has an atomic number of 9, so 9 protons.
It has the same number of electrons, so 9 electrons, and it has atomic mass of 19. So 19 minus 9, that will be 10 neutrons. So remember that for each of the elements, they might have the same number of the neutrons and protons. or they might have different number of nutrients and proteins. But these two numbers will be approximately the same.
And as the atomic number increases, the difference gets a little bit larger. Let's say, for example, Xanon has atomic number of 54. So it has 54 proteins. It's a pretty big number and it will have 54 electrons, of course, and the number of the neutrons will be 131 minus 54, which is equal to 77. Now you see a big difference between the number of the protons and number of the neutrons.
You will see the strands across the periodic table, but for most of the elements that we are familiar with at the top of the periodic table, they have approximately the same number of the protons and the neutrons. Well, for example, for xenon, its atomic mass looks a little bit weird to me. 131.294. So this is not a whole number. This is not an integer. Let's say for argon, that's 40. For neon, that's approximately 20. For fluorine, that's approximately 19. But xenon has 0.294, which cannot be really rounded to a whole number.
Okay. And there are some other elements like that, such as for chlorine. For chlorine, its atomic mass is 35.45 approximately. approximately 35.5 that's weird um so what does that mean does it mean that chlorine has um 17 protons 17 electrons and 35.5 minus 7 so 18.5 neutron is that what it means 18 and one half of the neutrons it doesn't really make sense because protons neutrons and electrons they're already like the smallest unit they they are not divisible you cannot break them down into smaller units like one half have the protons or one have the nutrients okay so why is that why is the reason that it has like 0.453 as an atomic mass it's because there are different multiple isotopes for chlorine element okay then what is an isotope so isotope of the same element means um there are different forms of um there are different different forms of atoms for the same element so they have the same identity they are like twins um so they have the same number of the protons, but different number of the neutrons.
For carbon, we have carbon 12 and carbon 13. They're twins. They have exactly the same identity. They look exactly the same, but one of them is a little bit skinnier. The other one is a little bit heavier. That's the only difference.
Okay. And for carbon, because it has atomic number of 6, it has 6 protons, and then 12 minus 6, which is equal to 6 neutrons and 6 electrons. For carbon-12, then what about for carbon-13?
It's exactly the same. They are twins. They have the same... identity and I told you that I said the number of the proton represent the ID. So it has six protons as well and six electrons.
But the number of neutrons is different. So 13 minus 6, that's 7. So carbon-13 has seven neutrons. So the number of the neutrons is the only difference when it comes to isotopes.
And how we identify isotopes of the same element is by using this machine called the mass spectrometer. So on the mass spectrometer, we have a detector and then different isotope will fall on a fall at different position of the detector so if you have like two spots then there are two isotopes if you have like three spots then there are three different isotopes and a mass spectrometry will generate a diagram like this so based on this diagram i can easily tell how many different isotopes there are okay the point is um well um i'm i'm interested in the average atomic mass i mean how did you get 35. four five three um as the average atomic mass of a chlorine okay so when we calculate the average atomic mass of any of the element i calculate the weighted average not a simple average weighted average of different isotopes of the same elements okay which means for each of the isotope um they actually um they occur in different percentage or different abundance in nature so i'll have to consider its percent percentage in nature and then calculate the average weight. Okay, so here is an example question.
The average atomic mass of naturally occurring neon is 20.18 amu. There are two common isotopes. One is neon 20 which has a mass of 19.99 amu the other one is neon 22 which has the mass of 21.99 amu calculate the percent abundance of each isotope okay so percent abundance is actually the percentage of each of the isotope in nature well there are only two isotopes in nature which means When you add up the percentage abundance of these two, you will get 100%.
Okay, so when you add up these two, you will get 100%. So if I set one of the abundance, the one of the percent abundance as x, then the other abundance will be 100% minus x, or which is simply 1 minus x. Okay, and as I said, how you calculate the average atomic mass is by calculating the weighted.
average. So what I'm going to do is I will multiply the mass of each of the isotope by its percent abundance. Okay, so I got these two products, and eventually I will add them up.
So I will add up these two products. And then that will be my average atomic mass. So 20.18 is equal to 19.99x. plus 21.99 multiplied by 1 minus x.
So 20.18 is equal to negative 2x plus 21.99. 2x is equal to 1.81, x is equal to 0.905. This means the percentage abundance of neon 20 is equal to 90.5% and the percent abundance of the second isotope neon 22 is 9.5%.
So that's it. So this is how you calculate the average atomic mass of an element based on their percent abundance. and the atomic mass of each of the isotopes. Okay, so here are like different types of the questions. This is actually a type of questions that you see often in AP chemistry exam.
You are given this diagram and then you're asked to calculate the average atomic mass or estimated or identify the element based on the estimated average atomic mass. Okay, so I'm gonna teach you like simple ways to do it. So the relative abundance, it's not... given in the form of the percentage right it's given in the form of the integer 9 6 1 it just represents the ratio the ratio of the isotope 70 72 74 is 9 2 6 2 1 so how i do this type of question is by multiplying on the mass of each of the isotope by on the the percent um the percent of the corresponding isotope so it will be 9 divided by 9 plus 6 plus 1 plus 72 multiply by 6 over 9 plus 6 plus 1. So this represents the percent of the 72 isotope in the whole sample plus 74 multiplied by 1 over 9 plus 6 plus 1. Okay, well, if you have a calculator, you can just easily come up with an answer.
The thing is, in MCQ questions, a lot of times you're not allowed to use a calculator. You're just allowed to estimate it. Okay, here is my method to estimate.
estimated so I'm going to take a look at the the highest picks so 70 is the highest pick 72 is the next highest pick and then the ratio is 3 to 2 the ratio of these two pick is 3 to 2 okay so when you take the average the weighted average of these two picks you will get a number that is even slightly less than 71 because 70 and 72 has average of 71 but you have more of the 70 so your average will be actually 7 less than 71 okay and then because you add them up together um now they're um now their weight will be 15 9 plus 6 that will be 15 so this pick you can get rid of these two picks you will have a new pick which is slightly less than 71 and then um it has the height of 15 and now you're going to take the average of these two again it's weighted average and because the taller pick the pink one is so much more taller it's 15 to 1 so um this pic is actually negligible so 71 well when you take the average of 74 it will increase a little bit but i will still um it will be still like somewhere like around 71 so i will say it's approximately 71 like slightly less Then 71. Okay, what about the next one? So it has only two peaks, which means it has only two isotopes. Okay, now this will be easier.
And then the peak with 10, atomic mass of 10, has the height of... of 25 and then the other one with a timing less 11 has the height of 100 so that will be 10 multiplied by 25 over 25 plus 100 plus 11 by 100 over 25 plus 100. So that will be 10 multiplied by 1 over 5 plus 11 multiplied by 4 over 5. So that will be 10.8 amu. So this will be the average atomic mass for this mass spectroscopy. Okay, next, here's another question.
Based on the mass spectrum of a pure element represented above, the average atomic mass of the element is closest to which of the following? Well, this is the type of question that you need to ask me rather than using a calculator. calculator to actually come up with the exact number of the atomic mass. So I have two picks, so two isotopes.
One is 185, the other one is 187, and then the relative abundance is 40% in 60. Okay, well, if you take the average of both, if both of them were like 50-50, then you will get 186. But now you have more of the 187 than 185, which means the weighted average will be greater than 186, slightly greater than 186. So it will be 186.3. Next, let's talk about different forms of the matter. We can classify all the matter in the world depending on their form.
their forms. Elements consist of the same type of the atoms only. Metals such as copper, zinc, silver, they just exist in the form of one single atom.
So just by copper, Cu, it represents the exact copper. And then the silver, it can be just represented in terms of Ag. There are some other substances that can be represented as the multiple atoms of the same element, such as the oxygen gas, O2. Oxygen gas consists of two oxygen atoms combined together, or hydrogen gas, H2, or nitrogen gas, N2, or ozone, O3.
Okay, so oxygen gas and ozone, these are two different substances, but both of them can be written in terms of the same element, of one single element, oxygen. So these are considered elements. They can be written in terms of the same type of the element, the same type of the atom.
Then what is a... compound. Compound is a substance that can be written in terms of two or more than two different types of atoms, such as when copper reacts with oxygen, it forms copper oxide.
Let's say water, it's when hydrogen and oxygen react, so H2O. So hydrogen, oxygen, these are two different types of compounds. Ethanol, the alcohol, it consists of carbon, hydrogen, oxygen, three different types of atoms.
So these are what we call better compounds. and both of them fall in the category of pure substance. When you mix multiple pure substances, then you get the mixture.
When you mix two or more different types of pure substances, then you get mixture. Well, so you will get a bunch of different types of mixture. You can just mix two different types of elements. So let's say I mix copper and silver to make...
jewelry. If I mix oxygen and then the ozone then it's still the mixture even though it consists of the same atom oxygen but it's still considered mixture because oxygen and ozone they actually can be written as two different types of the chemical formula. So they're two separate substance. When you mix the compounds together, let's say you are going to mix the water and then the alcohol.
So you have CH3, CH2OH, and then the water mixed together. This is still the mixture. So you can combine them like just in a way like whatever you want. You can combine elements with compounds. You can just mix the compounds with compounds.
You can mix elements. with elements and then you will get mixture and in the mixture you have two different categories as well one is homogeneous the other one is heterogeneous well based on the name homo we know that this is um the homo um means same right um so it means the mixture is These two different pure substances are mixed evenly so you can't tell the difference between them. So one of the best examples for homogenous mixture will be saline water. So in a beaker of saline water, you can't really tell which part is salt.
which part is water, they're thoroughly mixed with each other. Unless there is salt undissolved at the bottom of the solution. But let's assume that all the salt has been dissolved, you can't really tell the difference. And when you take...
taste this portion of the saline water and then this portion of the saline water, they taste exactly the same and then they look exactly the same. You can't tell the difference. This is what we call by homogenous.
When you mix the alcohol and water thoroughly, you will still get a homogenous mixture. You can't tell the difference which part is water, which part is alcohol. And the other one is heterogeneous. So heterogeneous is something that's not mixed evenly. Let's say salad.
So salad has the dressing on top of it, it has a cheese, it has olive oil, it has leaves, everything. Well, even though we mix it thoroughly, but you can still tell like the leaves apart from the cheese, apart from the olive oil. you can tell them apart. So the heterogeneous, so most of the mixtures in real world situation is actually heterogeneous, such as pizza.
So on pizza, you have dough at the bottom, you have sauce, and then you have cheese, and then you have like toppings. Okay, so these This is heterogeneous mixture. Or let's say when you have pasta and then the pasta mixed with let's say the cream and then the sauce and then the toppings, this is also heterogeneous mixture.
This is an example of the homogenous mixture. They are mixed evenly. In this heterogeneous mixture, you can see like a cluster of the compounds and a cluster of the elements. So they are not mixed thoroughly or evenly.
So this is what we call a heterogeneous mixture. Then how do we separate different mixtures from each other? If something is insoluble, then you can actually just separate it by using a funnel with a filter paper.
You are going to put a beaker at the bottom, and then by using a funnel and then the filter paper, You can just run the mixture and then eventually you will have whatever that's insoluble left in the filter paper and funnel. You will have the soluble mixture at the bottom. What if something is soluble, such as sodium chloride like table salt to dissolve in water?
You can just boil it. We call it by evaporation. by evaporation you're just removing the water and then you will have the solid table salt left or you can do distillation so distillation is used to separate two liquids such as alcohol and water so alcohol and water have different boiling point water boils at 100 celsius degree alcohol boils at 70 approximately 70 Celsius degree, you're gonna set the temperature around 80, slightly higher than the temperature of the alcohol.
Then eventually all the alcohol will be evaporated and then the leftover is the water and you're gonna collect the vapor evaporate evaporated vapor, you are going to collect it and then cool it down, and eventually you can collect the liquid form of alcohol. Barium in aqueous reacts with sulfate to produce barium sulfate in solid, which forms precipitate. A student obtains 10 gram sample of the white powder labeled barium chloride. After completely dissolving the powder in 50 milliliters of distilled water, the student adds excess sodium sulfate, which causes the precipitate of the barium sulfate to form. The student filters the barium sulfate, rinses it, and dries it until the mass is constant.
Which of the following scientific questions could best be answered based on the results of the experiment? First off, I know C and D are not the right answer. I can just immediately cross them out because it's asking about the solubility and this question has nothing to do with solubility. There are a couple of the keywords that you need to pay attention to.
It says excess sodium sulfate, So the limited agent in this question is the barium ions. You can only explore if the limited agent is pure or not pure, not the excess agent. So it's not going to be A, the answer will be B.
How we can determine if barium chloride is pure or not pure? Based on the mass of the barium chloride, you can calculate mass of the barium. This is theoretical. This is theoretically the amount of the barium that you expect to have in 10 grams of the barium chloride, and then this is what you actually get. This is experimental.
The experimental value will of course be less than or equal to the theoretical value. If it's the same, then it is pure. If mass of the experimental value is less than the mass of the theoretical value then it's that it means barium chloride used for the experiment is not pure how do we actually measure the matter so chemistry is a quantitative science which means when you do the chemical reaction you always want to know how much amount you are using or how much of the products you are going to get you can measure the matter by volume or by mass by volume you are going to measure using milliliters or liters by mass you are going to use grams most of the times in chemistry or you can actually count the number of the particles.
We just learned that matter consists of small particles called atoms, and each of the atoms carries exactly the same mass, and then they have the same size, they have the same properties. So we can actually count the number of the particles. The thing is, this is actually almost impossible because the atom size is so, so, so, so, so small.
It's even smaller than a grain of salt. It's barely possible for us to count the numbers exactly. Okay, so what we use...
instead is we measure the matter by mole. So mole is a unit that we use often in chemistry. That's just like a dozen.
So we know that a dozen always means 12. A dozen of x means 12x. So if I want to buy 24x, it's just two dozens of x. Then like three dozens of x is 36x.
We use mol as a packet to represent like a certain number of the particles. And this number is Avogadro's number. So NA, Avogadro's number, is equal to 6.022 times 10 to the power of 20. three so you have this many particles for one mole okay so number of moles is represented in terms of the small letter n so n is when you multiply this number by the avogadro's number you will always get the number of the particles. So one mole of any of the particle is equal to 6.022 times 10 to the power 23. And when I said particles, it can be atoms, it can be ions, it can be molecules.
When you are given the number of the mole, how you convert it to the number of the particles. is by multiplying it by the Avogadro's number. Then how do you convert it between the number of the particles and then the mass of this sample? We're going to multiply something called the molar mass. The molar mass is mass of any substance.
Again, this substance can be molecule, it can be ions, it can be atoms. Mass of any substance for one mole. we calculate, how we find the molar mass is when we take a look at the periodic table, the atomic mass, the atomic mass of any of the atoms represent its molar mass.
So let's say for boron, its molar mass will be 10.81 grams per mole. Let's say for nitrogen, its molar mass will be 14.01 grams per mole. This is the molar mass of all of the atoms. And if you have, let's say molecules such as as H2O, then you're simply going to add up their molar mass by the ratio.
So you have two of the hydrogen, 2.1008, and then you're going to add the molar mass of the oxygen, 16. So eventually, you will get 18.016 grams per mole for the H2O, for water. Okay, so when you multiply the number of the moles by the molar mass, the units... canceled out.
The units of the mole get canceled out, and then you will get the mass. Okay, so you will have to memorize this triangle, and you have to be very, very familiar with the conversion, the mole conversion. And if you're given the mass, then if you want to get number of the particles, then you'll have to do a two-step calculation. If you're given the number of the particles, and you want to know the mass, then you have to do the two-step calculation as well. So two formulas to remember.
Mass is... is equal to number of moles multiplied by molar mass. And then number of the particles is equal to number of the moles multiplied by the Avogadro's number. Okay. So here's an example question.
What is the number of the moles in 5 grams of calcium chloride? You are given the mass of the sample. It's 5.0 grams of calcium.
Mass is equal to number of moles multiplied by the molar mass. Then the number of the moles in will be equal to mass divided by molar mass. Or 5.0 grams of the calcium chloride divided by its molar mass.
Then what is the molar mass? On the periodic table, table, I will have to find the molar mass of the calcium and chloride. So I have calcium here and then the chloride here.
Then calcium chloride will have the molar mass of 40.08 plus 2 times 35.45, which is equal to 110.98 grams per mole. Okay, so I'm going to divide the mass of the calcium chloride by this. number the units get cancelled out then you will be left with the unit of the mole and the answer will be 0.0451 mole of the calcium chloride and next how many molecules are present in this sample.
Okay, so molecules, calcium chloride, it behaves as one single unit. This is the one molecule. The number of the molecules of the calcium chloride will be equal to the number of moles multiplied by Avogadro's number, which is equal to 2.71 multiplied by 10 to the power of 22 calcium chloride particles or molecules.
one calcium chloride particles, you have one calcium and then two chlorine atoms, which means in total you have three atoms in one calcium chloride molecules. Okay, so the number of the atoms will be equal to number of the molecules multiplied by three atoms per molecule. So this will be equal to 8.14 multiplied by 10 to the power of 22 atoms. The next question. A sample of calcium carbonate contains 4.86 moles, so it's given...
number of moles what is the mass and grams of this is simple so it give it the number of the mole is given it's asking me to find um the mass okay again i'm going to use the formula mass is equal to the number of moles multiplied by the molar mass then it will be 4.86 mole multiplied by the molar mass okay again um i will have to find calcium carbon and oxygen so c a c o 3 2 minus okay so carbon has the mass of 40.08 um carbon is 12.01 oxygen is 16 so it will be 40.08 plus 12.01 plus 3 times 16, 100.09 grams per mole. Okay, so I'm going to multiply the number of moles by that. This is equal to 486.44 grams. Okay, and what is the mass of the carbonate ions present?
So for one mole of... the calcium carbonate you have one mole of the carbonate ions right mm-hmm okay which means for for what and what is the molar mass of the calcium carbonate 100.09 so which means for 100.09 grams of the calcium carbonate because this these two actually represent the same amount of the calcium carbonate one mole or you can like measure in mass then it's 100.09 grams this is exactly the same amount then what about for the carbonate okay so the carbonate the molar mass of the carbonate will be on this part only which is 60. 60.01. So one mole of the carbonate is equal to 60.01 grams of the carbonate ions.
This ratio is the same as this ratio. This is how I converted the mole ratio into the mass ratio. 486.44 grams of the calcium carbonate.
multiplied by 60.01 grams of the carbonate in 100.9 grams of the calcium carbonate, 291.65 grams of the carbonate ion. So molecule and formula represents the smallest unit of a molecule. For example, C6H12O6 is glucose.
6 to 12. to 6. This is not the smallest whole number ratio. So if you convert it to the smallest whole number ratio, then it will be 1, 2, 2, 2, 1. So I just plug in all the atoms, and it will be carbon 1, hydrogen 2, and oxygen 1. C2H4, its empirical formula will be CH2 because the ratio of 2 to 4 can be simplified to 1 to 2. Let's say C2H6 ethane, its empirical formula will be CH3. Remember that only the molecular formula is what actually represents a molecule.
And then if a molecule wants to maintain the property, then it has to be in the form of the molecular formula. Then why do we need to know empirical formula? What is it used for? So empirical formula is especially useful when we're trying to figure out the chemical formula of an unknown substance. Let's say mass percentage of an unknown substance is 40%.
carbon, the 6.67% of hydrogen, 53.3% of oxygen. Well, sometimes the exact mass is given. Sometimes it's the mass percentage.
That's exactly the same. How we solve these type of problems is exactly the same. So what I'm going to do is I'm just going to take the number and then divide these numbers by the molar mass of each of the elements. 40 divided by the molar mass of carbon, which is 12.01.
Then it will be... 3.33. This is the number of the carbon, okay? And next, for the hydrogen, it will be 6.67 divided by the molar mass of the hydrogen, which is 1.008. Then you get 6.62 hydrogen.
And next, 53.33 divided by the molar mass of the oxygen, 16.33 oxygen. And now I'm going to calculate the whole number ratio of these number. 3.33 carbon to 6.62 hydrogen to 3.33 oxygen.
Okay, well, I'm going to divide each number by 3.33 because I have two of the 3.33. Then it will be one carbon to 1.99 hydrogen to one oxygen. Well, not 1.99 hydrogen is approximately just two. It can be run up to two.
So it will be 1 carbon to 2 hydrogen to 1 oxygen, which means the empirical formula for this substance will be carbon, hydrogen 2, and oxygen, CH2O. This is one of the most important method we use to figure out the identity of an unknown substance when you are given only the mass percent of different elements. The next question is, then how do you calculate the molecular formula?
First off, you need to have empirical formula. That's the very first step. And then you need to know the molar mass of this sample or this substance. And then you can figure out the molecular formula.
Okay, so let's say a compound with an empirical formula. CH2, the empirical formula is already given, we don't have to do any more calculation, has molar mass of 70 grams per mole. First off, what I'm going to do is calculate the molar mass of the empirical formula CH2, 12 plus 2 times 1, which is 14 grams per mole.
Okay, and then I'm going to divide the molar mass of the molecular formula by the molar mass of the empirical formula, 70 divided by 14. which is 5. So 5 will be the formula units. So I'm going to multiply this 5 to the empirical formula. So for the molecular formula, I will get C5H10.
So this is the molecular formula. Next, a 42 grams of sub-sample of a compound containing carbon and hydrogen only was analyzed. The results show that the sample contains 36 grams of the carbon and 6 grams of the hydrogen.
Which of the following questions about the compound can be answered using the result of the analysis? Okay, so this time you were given you are given the exact mass of the carbon and the exact mass of the hydrogen. Well, okay, so how you solve this type of question is exactly the same.
You are going to divide... the mass of the each of the compound by its molar mass so that will be three carbon and six grams of the hydrogen divided by one that will be six hydrogen So 3 carbon to 6 hydrogen, the ratio is 1 to 2. So I will get the empirical formula of CH2. Remember that this is empirical formula.
And then because we're not given the molar mass, so there is no way for me to figure out the molecular formula yet. Okay, so A, what was the volume of the sample? Well, there is no way for me to figure this out.
B, what is the molar mass? Again, this is not given. I can't figure it out based on the given information.
And C... what is the chemical stability you will have to do a series of chemical reaction to see if it reacts well or not so not c the only answer will be b what is empirical formula of the compound and then you already figured it out which is ch2 a student measures the mass of a sample of a metallic element m then the student heats the sample in air where it completely reacts to form the compound mo the student measures the mass of the compound that was formed which of the following question can be answered from the result of the experiment okay so this is a reaction between the metal m and then the oxygen o2 and then they form mo based on this okay first off you are given the mass of the sample um and then you are going to measure the mass of the compound again so you have the mass of the m mass of the mo okay um so based on this which information can be um can be answered um a what is is the density of m so density is equal to mass divided by volume you're not given the volume of the um the the element m and there's nowhere for me to figure this out so it's not a and b what is the molar mass of m okay um well for the molar mass molar mass is equal to mass divided by the number of the moles okay then i will have to figure out the number of the moles based on these two information. Okay, the thing is, based on this two data, I can figure out, I can just subtract the mass of the metal M in the compound, then I will get the mass of the oxygen.
And since it has the chemical formula of MO, which means the ratio of the number of the moles of the metal M and the number of moles of oxygen is equal to each other, number of moles of the oxygen. And this is the same as the the number of the moles of the metal M, which is mass divided by molar mass M. So you will be able to use this information to figure out the number of moles of the metal M, and then because you are given the mass of M, you can figure out the molar mass of M. So B is the right answer.
The next question. A jar labeled sodium chloride contains a powder. The table above contains information determined by analyzing a sample of the power. powder in the laboratory what information in a table is the most helpful in determining whether the powder is pure sodium chloride okay um okay so mass mass percent density and color okay so just by the mass there is no way for me to figure out if this is sodium chloride or something else because for all different types of the matter you can have exactly you can have 27.3 grams of all the matter in the world so it's not mass for sure that's is calculated by mass divided by volume. The thing is different substance might have the same density so it's not something specific or unique to a substance so that's not density and color of course not like sugar is also a white powder you can't really tell it from the color or the density so it should be only the mass percent of sodium.
A student obtains a mixture of the chlorides of two unknown metals X and Z the percent by mass of X and then the percent the percent mass of Z in the mixture is known. Okay, the percent might max of X, the percent mass of Z. Which of the following additional information is most helpful in calculating the mole percent of XCl and ZCl in the mixture?
And eight, the number of the isotopes of chlorine. Well, when we deal with the mole conversion, the conversion between the mass, molar mass, and NMS, we use the average atomic mass. So the number of the isotopes doesn't really matter. when it comes to the mole conversion. And B is the molar mass of X and Z.
And C is the density, mass divided by volume. Volume has nothing to do with this question. It's conversion between the mass, the number of moles, and then the molar mass. not C either.
Indeed, the percent by mass of C in the mixture. So this is the mixture of XCl and then the ZCl. So it actually means that in this mixture, you have three elements, X and Z and Cl.
This means when you know the mass percent of X and mass percent of Z, the mass percent of chlorine will be pretty obvious. It will be 100% the mass percent of X. and Z.
So this is very similar to how we calculate the empirical formula. You are given the mass percent, and then you're going to divide the mass percent by its molar mass, and then you're comparing these values of different elements. You're comparing this ratio of X and Z and CL, and then you can figure out their mole percent.
Okay, which means now what I need is molar mass of each of the compounds X and Z. So the answer is B. The next is...
atomic structure? Well, we already talked about the very basic atomic structure, which has the protons and neutrons in the center as nucleus, and then you have electrons surrounding it. So that was an oversimplified structure of an atom.
And now let's dive deep into the actual detailed structure. Okay, so again, it consists of the nucleus and electrons, and nucleus is right in the center, and then it consists of protons and neutrons. Proton carries the positive charge, neutron is neutral.
and the electron carries a negative charge. The thing is, a lot of times we think or we believe that electrons are just moving around the nucleus on one single, around one single orbital, which is actually not correct. So electrons move around the nucleus in certain shells or in certain orbitals only. Okay, so for example, electrons can move on this shell, it can move on the second shell, it can move on the third shell, but electrons cannot reside in between the cells.
It cannot be here or here or here. It has to be on this specific shell. Okay, and then for the specific shells, we call them energy levels.
So from the innermost shell, we call it N equals 1. And then the next innermost one, we call it N equals 2. The next innermost one, we call it N equals 3. And then the outermost one, we call it the valence shell for the outermost. We call it valence shell. So for the electrons that reside on the outermost shell, we call it valence electrons. So all of these atoms on n equals 4 are called valence shell electrons or just valence electrons.
Okay, the thing is, what's really interesting is within each of the shell, there are sub-shells. There are like sub-energy levels. Okay, and then the sub-energy levels are S, P, D, F. And then the energy increases from S to F.
So F is the highest sub-level and S is the lowest energy sub-level. And for N equals 1, we only have S shell. For N equals 2, we can have S and P sub-shells. For N equals 3, we have SPD, three sub-shells.
For N equals 4, we can have SPDF. And starting from N equals 4, N equals 5, N equals 6, all of those have SPDF, four different sub-levels. Okay, and based on this diagram, you can tell that as the number of the subshell increases, the number of the electrons in each of the shell also increases, and it's not proportional.
For S, you can fit in two electrons, and then it's the same for the second shell. For P, you can fit in six electrons. For D, you can fit in 10 electrons in total, and then for F, you can fit in 14 electrons in total. Okay so these can be actually very very confusing.
So here is a brief review of shell, subshell, and orbital. Again the shell, what we also call called by the principal quantum number, we represent it as n equals 1, 2, 3, it goes on. And then the innermost shell is n equals 1. The next one is n equals 2. The next one is n equals 3. And then for the subshell, it's S, P, D, F. So there are different subshells on each of... the shell.
As I said, on N equals 1, you have only S subshell. On N equals 2, we have S and P. On N equals 3, we have SPD. Starting from N equals 4, we have SPDF.
In each of the subshell, we can fit in multiple electrons. Okay, remember that SPD, they have different energy levels. So S is lower than P, P is lower than D.
But within each of the subshell, when you've, when when you plug in multiple electrons, they are on the same energy level. And then we call this orbitals. So for S, you can fit in two electrons, which means there is only one cubicle or one orbital. For P, you can plug in six electrons, which means there are three orbitals. And these three orbitals are just like the rooms in a hotel.
So there are like three rooms on one single level, and then you can fit in two people in each room. of the room okay so remember that um these three orbitals in p they are always on the same energy level the orbitals are always on the same energy level and the shell and subshell will be on different energy levels Next is electron configuration. So for any of the atom, you have to fill in the electrons in these energy levels and energy sub-levels.
Well, there are certain rules when you fill them in. You cannot just randomly plug in the electrons. any of the vacant orbitals.
No, you can't. There are like strict rules for that. The rule of thumb is you always have to fill in from the lowest energy level. So you will have to fill the 1s in full, and then you can fill in 2s, and then when 2s is full, and then you can next plug it. it in 2p and then when 2p is full and then you can plug in the electrons in 3s okay and then how do i know this how do i know the order of this okay then you're going to use this diagonal diagram so for this diagonal diagram um you are going to list and the energy subshells so you are going to put um the energy levels on the column n equals one n equals two n equals three n equals four and then equals on on the row you are going to put the sub-levels of S, P, D, F. Okay, and then you're going to use this diagonal to go from the right top corner to the left bottom corner, and you're going to follow the order of this diagonal.
So after 2S, 2P, and 3S, 3P, it's not 3D, even though 3D is on the same energy level as 3S and 3P, but because you're following the diagonal, you're actually going to 4S. Okay, so 3D is the highest out of on the energy level. energy level of 3 and 4s is the lowest out of the energy level 4 and there is like a slight overlap and 4s is actually at a slightly lower energy level compared to 3d so that's why you fill in the 4s first okay this is the first rule you need to follow um so here's an example let's try to plug in let's try to draw the electron configuration on this energy diagram for nitrogen so nitrogen has atomic number of seven so he has seven protons and seven electrons okay so again i will have to fit in on the lowest energy level first so i have to fit in the one s orbital um so s orbital s subshell has only one orbital which means two electrons and for two electrons in one orbital they have to be paired which means one electron will have a spin pointing up the other one has a spin pointing down so the speed number actually represents the exact um way and and an electron moves around the orbital. And then in one orbital, two electrons must have two different spins. Okay.
And then it will be 2s. And then again, I'm going to plug in two electrons in the orbital of 2s. And next is 2p.
There are three orbitals. Okay. So I cannot plug in two electrons in one orbital and then another electron in the second orbital.
No, this is not correct. so the second rule is you will always have to fill all the orbital on the same subshell And then you start to pair them, which means you will have to fill in all the three orbitals and then you can start plug in more electrons in the orbitals. Okay, for nitrogen, I have seven electrons only. So I have one, two, three, four, five, six, seven.
So that's it. Okay, can I plug in the electrons with a downward spin? Yes. So for this part, you can replace it with downward, downward, downward.
But remember that all of the three orbitals must have the same spin. So if you're going to draw some... something like downward, upward, downward, this is not correct. But all of them upward or all of them downward is fine.
Next, here's another example. Let's try to draw the electron configuration of magnesium. Okay, so magnesium has the atomic number of 12, which means it has 12 protons and 12 electrons.
Okay, again, starting from the the lowest energy level 1s it's up and down and then 2s up and down and 2p it will be up up up down down down okay and i have one up and one down so that's it for magnesium okay so this is how you fill in the energy um energy level diagram and then how i actually write the electron configuration will be For nitrogen, it will be 1s2, which means in 1s orbital, I'm going to plug in two electrons. And then next, 2s2. Again, it means in 2s orbital, I plug in two electrons.
And then 2p3, which means I'm plugging three electrons in 2p orbitals. Okay, then what about for magnesium? For magnesium, it will be 1s2, 2s2, 2p6, and then the last 3s2.
Okay, or sometimes... I can write this electron configuration with regard to the noble gas. Okay, so let's say for magnesium, it's in n equals 3. It's on the third energy level. Okay, then let's take a look at the...
take a look at a noble guess on n equals 2 which is neon okay then i can use use neon to write an abbreviated form of the electron configuration which is bracket neon okay so bracket neon represents the electron configuration of neon which is 1s2 2s2 and 2p6 so i just need to start from here so it's bracket neon and then 3s2 so this is a abbreviated form of the electron configuration and both of them are completely correct. Both are correct. Okay, next, why don't we try to draw the electron configuration of calcium 2+. Okay, remember that this is not atom-animer, this is ion.
Okay, so calcium has 20 protons, right? So calcium atom has 20 protons. But then now, okay, so it's supposed to have 20 electrons if it's an atom. But now it is an ion which carries 2 plus charge, which means it has lost 2 electrons. electrons.
So now it has only 18 electrons. So how you calculate the charge of an ion will be the charge of the proton plus the charge of the electron. Okay, so it's positive 2. So positive 2 means it has 18 electrons only.
Okay, so how I fit in electron 18 electrons will be again starting from 1s orbital up and down, 2s up and down, 2p up up up down down down, and then 3s up and down. 3p up up up down down down okay so how many electrons do we have for now I have two two six two six so that is eight plus twelve which is twenty electrons in total Sorry, that's, so how many electrons do we have for now? We have six electrons, six electrons, and six electrons. So we have 18 electrons for now, so that's it. Okay, we're going to stop here.
So how I write the electron configuration of calcium. Calcium 2 plus iron will be 1s2, 2s2, 2p6, 3s2, and 3p6. Or I can just use abbreviated form. So calcium is...
on n equals 4. So I'm going to find a noble gas in n equals 3, which is argon. Okay, so I can simply use bracket argon to represent the electron configuration of calcium 2 plus ion. Remember that this is an ion, that's why it has the same electron configuration as a noble gas.
Argon Okay, um, let's try it one more time. Then what about for on the let's say what about? Okay, what about what about boron boron? Okay, so boron has atomic mass of 5 so it has 5 protons and 5 electrons so in the 1s i'm going to plug it up and down two electrons 2s up down 2p it will be just up so electron configuration of boron will be 1 1s2, 2s2, and 2p1.
If I use the noble gas abbreviation, and boron is in n equals 2, so I need to find a noble gas in n equals 1, which is helium. So it will be bracket helium, and then 2s2, 2p1. How many unpaired electrons are in the atom represented by the electron configuration above? Remember that in each of the orbital, if there is only one single electron, then we call it a noble gas.
call this unpaired. And if there are up and down, then we call this paired electrons in each of the orbital. Okay, so for 1s2, 2s2, 2p6, 3s2, 3p6, all of the orbitals are full because in s orbital, because in s subshell, there is only one orbital, which means you can plug in a maximum of two electrons in each of the p shell. It has three orbitals. orbitals, which means we can plug in a maximum of 6 electrons.
So for this one, all of the S subshells and P subshells are full, which means there is no unpaired electrons, it will be 0. 1s2, 2s2, 2p6. 3s2 and then 3p6. So as shown in this electron configuration diagram, there is no unpaired electrons. The answer is zero.
Next, which of the following represent the electron configuration of an oxygen atom in the ground state? Okay, so the ground state. means it's not excited. So what I mean by the ground state or the excited state is, so what we draw up until now is the electron configuration of the ground state or the most basic state. But when an atom is excited, well usually it's excited.
excited by absorption of the energy such as a light or heat, then the electrons get very very excited, then it can jump to a higher energy level. So this is represented using the Bohr model on the hydrogen spectrum. So electrons can jump back and forth, but when the electrons jump up, then it will have to absorb energy.
When electrons jump down, it will have to release energy. So when you release or absorb energy it's usually in the form of the light or heat okay and for the oxygen um so first off oxygen has the atomic number of eight which means oxygen has eight protons and then eight electrons okay so um one two three four five six so i know immediately a is not correct because it has only eight electrons uh it because a it has only six electrons and for b it has only seven electrons So the answer is either C or D. So in C and D, both of them have eight electrons, eight valence electrons. In C and D, both have eight electrons. The thing is, in C, it has one, two, three, up, up, up, and then down.
on the other hand d has up down up down or up up and down down okay remember that we always have to fill all the orbitals first and then we can start to fill the for the electrons um and orbitals to form the paired electrons. So B is not correct, C is the right answer because the last orbital of 2p is vacant, then you cannot form this paired electrons. The next is photoelectron spectroscopy. So this is a very useful method that can be used to identify the identity of an element.
Usually it shows the electron distribution and energy on each of the subshells. So usually on the x-axis we will have binding energy which is the amount of energy energy required to remove an electron from a certain subshell. Okay, and then different subshells have different energies, as we just stated.
So higher the energy level, the more energy, and then the lower the energy level, the lower energy. Okay, so if you, if we're trying to remove the electron from a higher energy level, then more energy will Then if you're trying to remove the electron from a higher energy level, less amount of energy is required. This is because higher energy actually means it's at a higher potential energy or it's unstable. On the other hand, closer to the nucleus means that the energy level is low or it's very stable so if you want to get rid of electrons in this very stable um subshell it's going to take a lot of energy so um a greater binding energy means that it's the it's close it's closer to the nucleus. The left side is close to the nucleus and then the right side is close to the valence shell.
Remember the energy on each of the subshell is the same. Okay so if you want to remove the both of the electrons in let's say 2s then it will take the same amount of energy for both of the electrons. Okay then what does the peak represent?
So the peak represents the amount of energy required to remove the electrons. electrons in that subshell and then the height of the peak represents the relative number of the electrons so the more electrons you remove from the certain subshell the higher the peak so from the left side okay the left side the leftmost peak has the highest binding energy which means this is the innermost subshell of 1s and 1s and and then the next the next and the next peak means it's 2x okay to fill in the electrons in 2s you have to fill the 1s in full this means 1s pig has two electrons so this is 1s2 and then the next pig has the same height as 1s which means this is 2s2 and then the next peak right next to 2s okay so right above 2s we have the energy sub-level of 2p but it's the height of the pig is only one half of the 2s2 which means there's only one half of the electrons in 2p orbitals which means there is only one electron in the whole 2p subshell. This represents the photoelectron spectroscopy of a boron with 1s2, 2s2, 2p with three separate peaks because there are three sub energy levels. Then what about for sodium? Okay so sodium is in n equals three.
So if I try to write its electron configuration that is 1s2, 2s2, 2p6 and then 3s1. And let's see if this diagram matches or fits the electron configuration. Okay, so from the leftmost 104 kilojoules, 104 megajoules, so this corresponds to the innermost sub-level of 1s2 and then next 6.84 it corresponds to the next higher, next innermost energy subshuffle of subshell of 2s2 and then this highest peak which has the three times the height of these two peaks represent 2p6 because there are six electrons so it so the pig is three times higher than the peak that and that that's for two electrons and then the next will be 3s and because there is only one electron so the height of the pig is only one half of the 2s and 1s2 Okay, so here's the question. The photoelectron spectrum for the element boron is represented above. Which of the following best explains how the spectrum is consistent with the electron shell model of the atom?
Okay, A, the spectrum shell. shows an odd number of electrons. b the spectrum shows a single electron in two-piece subshell. c spectrum shows equal number of electrons in the first and second electron shells.
d spectrum shows three electrons with the same binding energy in the second electron shell. Well, let's try to analyze this photoelectron spectroscopy diagram first. So from the left to the right, the energy, binding energy gradually decreases, which means the left is close to the nucleus, it's the innermost shell, so it will be 1s2, next pic will be 2s2, next will be 2p, and because the height of the rightmost pic is only one half of the other two pics, it will be 2p1.
So it has the total number of electrons will be 2 plus 2 plus 1, which is 5. So A is actually correct, and B The spectrum shows a single electron in 2P. This is correct too. There is only one electron. See?
Spectrum shows equal number of electrons in the first and second electron shell. Remember the electron shell refers to n equals 1 and n equals 2. So 1S is in, 1S subshell is in first electron shell. And 2S and 2P are in second electron shells.
So there are only two electrons in the first shell, but three electrons in the second. second shell, so C is actually not correct. D, the spectrum shows three electrons with the same binding energy.
in the second electron shell. In the second electron shell, 2s and 2p represent two separate binding energies, so D is not correct. Then out of A and B, which one is the answer?
Actually, both are correct, but which one is the explanation for the spectrum that's consistent with an electron shell model? Okay, so the odd number of electrons doesn't really tell us anything about the shell model. So A is not the right answer.
It's B. So B tells us about the shell, like n equals 1, n equals 2, and then the subshells. And next, this is the last subtopic in Unit 1, periodic trend. So there are four categories that you are going to take a look at for a periodic trend.
One is atomic radius, or just the size of an atom. And then the second is ionization energy and then the third is electron affinity and then the last one is electronegativity so there are four categories so we're going to start from the first one atomic radius the Size of an atom is, of course, determined by where the valence shell electrons reside away from the nucleus, right? Okay, so we're going to use Coulomb's law to explain this. Again, the atom consists of the positively charged nucleus and then the negatively charged electrons. The thing is, there are multiple shells.
Okay, so in the innermost shell, there are two electrons. And then starting from the second shell, you can fit in a maximum of eight electrons. So let's say...
say this is a model of 1s2 and then 2s2. So you have two electrons on the valence shell. Okay, then how the atom size is determined is by the attraction force between the valence shell electron and the nucleus, okay, so which carries the positive charge.
We know that the light charges repel and all light charges attract. Okay, so the positively charged nucleus and then the negatively charged valence electrons attract each other. And then the amount of attractive force can be calculated using the Coulomb's law. The force is equal to the constant, the Coulomb's constant, K, multiply by charge 1, multiply by charge 2, divided by R squared.
So R represents the distance between the electrons and then the nucleus. And then the Q1 represents the charge of the nucleus, and then the Q2 represents the charge of... of the electron.
So let's say I'm going to compare the lithium and then the beryllium. So it carries the charge of positive three. Then it has two electrons on the first shell and then it has one electron on the second shell. So this is lithium.
Okay then what about beryllium? So beryllium actually has one more proton. So it has plus four as the charge of the nucleus and then it has two electrons on the innermost shell and then two electrons. on the second shell.
So this means that the Coulomb's force between the valence shell electrons and nucleus of lithium is smaller than the Coulomb's force experienced by the valence electrons and nucleus of the beryllium. Nucleus attract electrons more toward the center, so beryllium will result in a smaller size. So attractive force between the nucleus and valence electrons increases across the period.
When I say across the period, it always refers to left to the right. This means the radius decreases. So distance between valence electron and nucleus decreases across the period. What if I compare the lithium to...
the sodium. Okay, so lithium and sodium, both of them are in group one. One is in the shell of n equals two, one is in the shell of n equals three. Okay, so obviously, sodium has one more shell, which means the valence-trielectrons will be further from the nucleus.
So the reason why the So of course the distance between the nucleus and an electron will increase when you go down the periodic table. The number of the shells increases down the periodic table and thus Valence electrons experiences less attractive force from nucleus. This can also be explained by the repulsive force in between the core electrons.
So electrons on the outermost shells are called valence electrons, and all the other electrons inside are called core electrons. Okay, and then the more shell you have, the more core electrons you have, and then the repulsive force between core electrons increases, and this compensate for the attractive force between the valence electron and the nucleus. So it kind of diminish that attractive force, and we call this electron shield.
We call this increasing repulsive force between core electrons. We call this electron shield. This is a summary of the atomic radius.
So from left to the right across the periodic table, it gradually decreases and then from top to the bottom it gradually increases. The next will be the comparison of the atomic radius and ionic radius. There is no certain rule for this.
So for some of the atoms, their atomic radius is bigger. For some of the others, their ionic radius is bigger. Lidium or beryllium, so in general for For alkaline earth metal and then alkaline metal in group 1 and group 2, when they form an atom, they usually form cation because they're losing electron, which means the number of the shell is actually decreasing.
So let's say for lithium, its valence shell electron is on n equals 2. When it loses one electron, it actually decreases to n equals 1. So it lost one whole shell. So it's... size will actually decrease a lot.
And it's the same for all the other alkaline metal and alkaline earth metal. So let's say for magnesium, when it loses two electrons, it also lost one shell. So its size will decrease by a huge amount. On the other hand, for some other atoms such like halogen or the atoms in group 6, when they form an...
ion they usually gain electron and form anion. Let's say for chlorine it's gonna gain an electron and be converted to chloride ion. Okay then compared to its its size because of the extra electron its size will actually get bigger and it's the same for the oxygen. So oxygen gains two electron to form O2-so its size gets so much bigger. So you need to pay attention to if it's a cation or an anion, if it's losing an electron or gaining an electron.
The next, the second of the periodic table will be first ionization energy. Okay, so ionization energy is the amount of energy required to remove an electron. If I want to remove an electron from magnesium, then it will be magnesium converts into magnesium plus and an electron. Okay, so a lot of people think, oh, this is gaining an electron because you have plus electron, but that's not true.
It means magnesium is... going to split into two parts one is the electron and then one is the magnesium plus the magnesium cation okay and then you are gonna it's gonna take a certain amount of energy let's say you wanna break something then it always takes energy it's not releasing energy it doesn't have happen like just naturally you need to put in energy to make sure this like breaking process happens so that's the same for an atom if you want to remove an electron from an atom then you need to put in energy and it takes energy and that amount of energy is called the ionization energy and first ionization energy means when you remove the first electron from from an atom Okay, so you can keep removing electrons from magnesium. So for magnesium plus, you keep removing the electrons so it becomes magnesium 2 plus. Then this is the second ionization energy.
going to deal with the first ionization energy first okay so for the first ionization energy it increases from left to right obviously and then it decreases from top to the bottom Okay, this is a general trend. And this is because when you go from left to the right, remember that atomic radius decreases. So because of the increasing effective nuclear charge, it's going to be more and more difficult for me to remove the valence electron. From left to the right, the nuclear charge. increases from left to the right so it gets harder to remove an electron so it takes more energy but when you go from top to the bottom because the valence electron is is further away from the nucleus which means it doesn't experience that much attractive force so it's much easier to remove that electron from the atom.
There are couples of exceptions for first ionization energy. Okay, again, this is first ionization energy only. If you take a look at this diagram, you observe that that from left to the right, the general trend is increasing, but from group two to group three, there is a decrease, and then from group five to six, there is another decrease.
Across group five to six, you observe this trend, and across group two to group three, you also observe this trend of decrease, okay? And why is it? Okay, so this is the first ionization energy of the elements in N equals two shell, okay?
From leading... to beryllium increases but from group 2 to group 3 decrease and then increase increase decrease increase increase this trend holds true for all of the period And this is because how electrons are filled in the P subshells. Let's try to draw the electron configuration of beryllium and boron. Beryllium has electron configuration of 1s to 2s. If I try to draw the energy diagram, then it will be 1s up and down and then 2s up and down.
Okay, and for the boron, it has electron configuration of 1s2, 2s2, 2s2. 2p1. So again, it will be 1s up and down, 2s up and down, and then for 2p, you are going to fill in only one electron in three orbitals. Okay, so now if you are trying to calculate ionization energy, you need to remove these electrons. You need to remove this electron from boron and this electron from the beryllium.
Okay, so now the beryllium has fully filled 2s subshell, which means it's very, very stable. So if you want to break this stable state and remove that one electron to make it unstable, it's actually going to take a lot of energy. On the other hand, for the boron, because only one unpaired electron is in the whole 2p orbital, which makes it unstable. So if you want to knock off this electron and then make it into a stable state, then it's relatively easy.
So we call this that beryllium has fully filled. 2s subshell while boron has only one unpaired electron in 2p subshell so it is relatively easy to Remove the single electron in 2p of boron and make it stable. Then what about from nitrogen to oxygen? This is a trend that you observe from group 5 to group 6. Again, let's try to draw the electron configuration of nitrogen. Nitrogen is 1s2, 2s2, and then 2p3, while oxygen is 1s2, 2s2, 2p4.
Okay, I'm going to draw the energy level diagram. For nitrogen, 1s up and down, 2s up and down, and then... 2p, it is up, up, up.
For the oxygen, it's 1s up and down, 2s up and down, and then 2p up, up, up, down. Okay, so now when you're calculating again the first ionization energy, you are trying to remove the electron in the 2pz orbital of the nitrogen. And this...
one of the paired electron in 2ps orbital of the oxygen. Okay, so which one is easier? So even though like nitrogen doesn't have the fully filled orbital, it has half filled 2p orbital, which also makes it stable. Okay, so if you want to break this stable state and remove the electron, then it's going to be relatively difficult. the other hand for the oxygen it has this extra electron here even though it's paired and usually the paired electrons are stable but because the other two orbitals are not paired so this is actually more unstable.
If you want to knock off this electron and make it into a stable half-filled 2p subshell it's actually preferred so it's easier to remove this extra electron. So it is because nitrogen has half filled 2p subshells. So it is relatively difficult.
to remove an electron from 2p subshell of the nitrogen. Next is electron affinity. So electron affinity and ionization energy is a pair of opposite terms. So ionization energy is the amount of energy required to remove an electron. Then electron affinity is the energy change that occur when an atom gains electron.
So let's say fluorine gains an electron to form fluoride, then the amount of energy that is usually released is considered electron affinity. Then in this case, electron affinity will be a negative value. Okay, so from left there.
right electron affinity increases the last one will be electron negativity so electron negativity is very similar to electron affinity but it's actually different so this is the tendency this is the tendency of an atom to attract electrons especially in especially the shared electrons okay So electronegativity is used more often when it comes to the bondage between two atoms. Okay, so let's say two hydrogen atoms form a bond, okay? Then it means that two hydrogen atoms are actually sharing the electrons, okay? So this is the electron of the left hydrogen, this is the electron of the right hydrogen, and they are sharing two electrons with each other, okay? So for two hydrogen atoms, they have the same size, the same mass, and then they have the same electronegativity.
They attract the electrons with the same force, with the same attraction. So the electrons will be equally shared. The electrons will be right in the center.
But what if it's the hydrogen and fluorine? Okay, or hydrogen and chlorine. Okay, so chlorine has a much greater electronegativity, which means it's very, very strong.
It can just pull the electrons toward itself. So when hydrogen and chlorine share electron, the chlorine will just pull them. the electrons toward itself so hydrogen barely gets to share um that that shared electrons even though it contributes one electron and two to form the bond okay then this means electrons are shared unequally and this is because chlorine is more electron negative than hydrogen which means chlorine is stronger when it comes to attracting the atoms attracting the electrons um um in um when when they form a bond.
Okay, so if one atom is like too strong compared to the other atom, then eventually the electron will be just fully transferred to that atom and then the other atom will just lose an electron. Okay, so we're going to dive into electronegativity difference and the bonding in unit 2. We're going to learn different types of bonding. And then different types of chemical bonds actually is due to the difference in electronegativity.
Okay, so this is a chart for the electronegativity of the elements. And you can tell that when you go from left to the right, the general trend is electronegativity increases. This is because when you go from left to the right.
across the period, the effective nuclear charge increases, so a stronger attraction towards the electrons, okay, and from top to the bottom, it decreases, and of course, this is because the more shell it adds, the more shell it has, the weaker attractive force forms between the electrons and the nucleus, okay, so let's say, for example, hydrogen and fluorine, they have electronegativity difference of 4 minus 2. 1. So the electronegativity difference is 1.9. This is actually considered a very big difference in electronegativity, which means hydrogen is too weak and fluorine is too strong. So fluorine will actually just gain that electron away from the hydrogen, and hydrogen will be actually left with no electrons left. But when we compare the hydrogen and chlorine, even though the difference is still big, 3.0 minus...
2.1, but the but chlorine is not as strong as fluorine so they are still kind of sharing on the electron. But this is still unequal sharing of the electron. The electrons are dragged more toward the the chlorine.
In terms of the atomic structure, explain why the atomic radius of potassium is larger than that of sodium. Okay so on this periodic table, potassium is on the energy level of n equals four and then the sodium is on energy level of n equals three okay so potassium is larger because potassium has more energy shells or more energy levels or more shells um so um um so potassium experiences more shielding of the core electrons, which means the valence electrons of the potassium experiences less attractive force. towards the nucleus.
Okay, B, in terms of the atomic structure, explain why the first ionization energy of the potassium is less than that of calcium. Okay. So when we compare the potassium and calcium, they are on the same energy level, but potassium is alkaline metal and calcium is alkaline earth metal, which means calcium has one more presence compared to potassium.
So potassium has less effective nuclear charge. than the calcium. So it is easier to remove the valence electron from potassium compared to calcium.