hello everybody my name is Iman welcome back to my YouTube channel today we're starting our new and improved MCAT General chemistry playlist this first lecture we're going to call it chapter zero is going to provide some background information and a quick refresher of some important terms and Concepts in case it's been a while since you've taken chemistry this chapter will set us up for success as we cover the next 12 chapters and here specifically I'm going to go over units of measurement temperature classification of matter we're also going to go over subatomic particles here we'll discuss protons neutrons and electrons and then finally we will also cover atomic mass versus atomic weight and here we'll do a little bit of stochiometry as well so with that introduction let's go ahead and get started with this first objective titled units of measurement chemistry obviously involves experimentation and measurement and in order to communic at those measurements it's important to pay close attention to the units that are used to describe them every measurement must be expressed in appropriate units now the agreed upon unit system among scientists is the SI system which is based on the metric system and there are seven base units in the SI system I have them listed here now all other units that you might be thinking about or are familiar with those are just d iived units which are obtained by combining the SI base units so let's go over the SI base units for Mass the SI base unit is the kilogram abbreviated kg for length the SI base unit is the meter abbreviated lowercase M for time the SI base unit is the second and its abbreviated lowercase s the SI base unit for temperature is the Kelvin and it's abbreviated capital K for electric current the SI base unit is the Ampere and it's abbreviated capital A for amount of substance the SI base unit is the mole it's abbreviated m o and then last but certainly not least we have luminous intensity with its SI base unit being the Candela abbreviated CD now we can use SI units to obtain derived quantities some good examples being volume density and electric charge let's focus on volume here volume is the amount of space that's occupied by an object it's equal to the length time width time height and you would Express length in units of meters and width in units of meters and height in units of meters so what you have here is meters time meters time meters and here you get the derived units for volume to be C cubic M Now units they're frequently modified through the use of metric prefixes you can add these prefixes to make it more reasonable to refer to different values of length mass or time depending on the system you're referring to so looking at this um metric prefix figure we can go over a couple of important ones kilo is 10 ^ 3 Milla this is 10 ^ minus 3 micro is 10-6 nano is 10- 9 now again we just said that we can add these prefixes to make it more reasonable to refer to different values of length mass or time depending on the system you're referring to so for example if you are referring to a human cell you don't want to say hey a human cell is anywhere between 0.000000001 and 0.0000001 M when instead you can say that a human cell is anywhere between 1 to 10 micr m all right and so when we're saying that this value 0.000000001 is equal to 1 micromet that just means that 1 * 10 to Theus 6 gives us if we move the decimal over six places 1 2 3 4 5 6 it gives us back 0.000000 1 M all right so this is what we're talking about when we use metric prefixes to make it easier to refer to different values of length mass or time with that we can move into our second objective on temperature when we get to our chapter on thermodynamics and thermochemistry understanding temperature becomes really important but what exactly is temperature in everyday language we often use the term temperature to convey the sensation of hotness or coldness however in the realm of thermodynamics it has a more precise significance at the molecular level temperature correlates with the average kinetic energy of the constituent particles of a substance on a macroscopic scale the temperature differential between two objects it directs the flow of heat heat will spontaneously move from regions of higher temperature to those of lower temperature now how does this knowledge translate to the Practical function of thermometers well thermometers they've been used to quantify temperature of substances since the 18th century and some well-known substances include Fahrenheit Celsius and Kelvin both the fahrenheit and the Celsius scales are based on the phase changes of water which makes them super super convenient for everyday use now the Celsius scale it defines the freezing point of water to be 0° Celsius and the boiling point of water to be 100° C the Fahrenheit scale instead it defines the freezing point of water to be 32° fah and the boiling point of water to be 212° F now the Kelvin scale this is most commonly used for scientific measurements and it is one of the seven SI base units it defines as the zero reference point absolute zero this is the theoretical temperature at which there is no thermal energy and it sets the freezing point of water to be 373 Kelvin now here we have to note something important how do we convert between these units so if you want to calculate Celsius but what you have is Fahrenheit you can plug it into this equation right here 5 over9 multiplied by Fahrenheit - 32 now let's say that you have Celsius and you want to figure out Fahrenheit we can plug it into this equation right here 9 over5 * Celsius + 32 now if you're trying to get Kelvin after you know Celsius very easy Kelvin is just Celsius + 273 now something else I just want to note when we use a thermometer to measure temperature it's really essent essential to recognize that the thermometer it doesn't just directly gauge temperature no no it it rather gauges the volume of the substance it contains so common thermometers will often utilize the expansion and contraction of liquids like Mercury due to temperature changes to measure temperature and so that rise or fall of the liquid column of say Mercury it will correspond to shifts in temperature and so you work backwards from there so just a little fun fact now the next thing we want to talk about is the classification of matter matter can be defined as anything that occupies space and has mass and this is the fundamental material of the universe matter can exist in three primary States solid liquid and gas and we can argue plasma but we don't need to talk about that right now these States they're distinguished by the arrangement and the energy of their constituent particles so solids solids have tightly packed particles in a fixed structure liquids have closely packed particles that can flow and change shape and then gases they have a widely spaced particles that move freely now matter is very much complex and organized into several levels matter it can be divided into mixtures and pure substances mixtures are physical combinations of two or more substances where each retains its individual particles this can be categorized as heterogeneous mixtures and homogeneous mixtures heterogeneous mixtures have visibly different components or phases so think about a mixture of sand and water right you you can see the distinct Parts the sand is there the water is there you can see it and it can be physically separated homogeneous mixtures on the other hand also known as solutions they have a consistent composition throughout so think about like salt dissolved in water in homogeneous mixtures the individual components they're not visible you can't when you dissolve salt and water you can't see the salt and the water you really just see the water and so they're not visible and the thing is we can take uh homogeneous mixtures we can separate it into their pure substances but this requires physical methods like for example distillation through these separation methods you can get back your pure substances pure substances they have a uniform and definite composition and they can be further classified into elements and compounds elements are substances that consist of only one type of atom and they cannot be decomposed into simpler substances by chemical means so think about like hydrogen carbon oxygen gold these are elements elements are the building blocks of all matter and they're listed in the periodic table based on their atomic number now compounds compounds are pure substances that are formed from two or more elements chemically bonded in fixed proportions they have distinct properties that are going to be different from their constituent elements so for example water is a compound and it contains hydrogen and oxygen in a 2:1 ratio H2 compounds they can actually be broken down into simpler substances back into their elements but using chemical methods chemical methods are needed now going back to elements all right at the atomic level matter consists of atoms this is the smallest units retaining the properties of an element an atom it comprises of a nucleus and electrons the nucleus contains protons and neutrons and then we have electrons that orbit the nucleus protons are positively charged particles while neutrons are neutral and electrons are negatively charged particles that occupy the region of space around the nucleus called orbitals now electrons are fundamental particles protons and neutrons are not fundamental particles because they can be broken down further protons and the neutrons within the nucleus are made out of even smaller particles known as quarks now you don't need to know this for the MCAT I just want you to visualize this like hierarchy of matter quarks are fundamental particles that will combine in specific ways to form protons and neutrons we're not going to get into the details of that but this just helps you visualize looking at this looking at this you can begin to understand the hierarchical structure of matter from mixtures to pure substances elements and compounds down to atoms and even their subatomic particles and that provides us a comprehensive framework for exploring the composition the properties and the interactions of all material substances in the universe and actually in this next objective we're going to really be focusing in on understanding atoms and the different component on of atoms in terms of protons neutrons and electrons and to motivate this next objective we want to ask again what is an atom made of and how do the atoms of various elements differ we know that an atom is the smallest unit of ordinary matter that forms a chemical element and we know that an atom is composed of a nucleus made out of protons and neutrons and electrons orbit the nucleus we already said also that protons are positively charged neutrons have no charge and electrons are negatively charged now the protons and neutrons they Define the mass of the atom while the arrangement and movements of electrons determine the atom's chemical properties and reactivity now the first question we should ask is well how did we even reach this this conclusion and it was the overall work of many scientists that actually led us to our understanding of the atom we start off with John Dalton who proposed his atomic theory in 1803 he viewed atoms sort of like billiard balls atoms of different elements simply differed from each other by mass and nothing else they could not be subdivided any further it wasn't until JJ Thompson's work with cathod Ray tubes in 1898 that negatively charged particles called electrons were found to be present in atoms then in 1911 Ernest Rutherford further refined our view of the atom by showing that although atoms consist mostly of empty space at the center of every atom is an extremely small and dense nucleus which is positively charged and he was able to make these conclusions based off of his gold foil experiment then bore came along and developed his model which suggests that the negative charges orbit the nucleus of an atom which is made out of protons and neutrons there is more to that truly um because we actually can't pinpoint where the electrons are we can only talk about electron density probability and this is where the quantum mechanical model comes into place we can't say that we know where electrons are we can talk about where is the electron most likely to be found and this is actually a question that we talk about a lot in chapter one of this playlist now with that understanding a little bit of History let's really focus on the subatomic particles in a little bit more detail again an atom is the smallest identifiable unit of an element it is a neutral particle made made out of negatively charged electrons moving around a positively charged nucleus we talked about protons electrons and neutrons let's focus on protons really quickly it's found in the nucleus each proton has an amount of charge equal to the fundamental unit of charge which is by the way 1.6 * 10^ the -19 Kum and we denote this as a positive plus one charge that's associated for protons protons have a mass of approximately one atomic mass unit approximately all right and in kilograms that is written right here 1.6 72 * 10- 27 kilg now for neutrons neutrons are neutral they have no charge so their charge is zero a Neutron's mass is only slightly larger than that of the proton and together actually the proton and neutrons of the nucleus make up essentially almost the entire mass of an atom so here's the mass for a neutron written in amus and this is the mass of neutrons in kilogram now we want to talk about electrons electrons are really interesting they move through the space surrounding the nucleus and they're associated with varying levels of energy each electron has a charge equal in magnitude to that of a proton but with an opposite sign so we give it a minus1 charge in kums that charge is written right here now the mass of an electron is approximately 1 over 2,000 that of a proton they are very small and because of this because this subatomic particle's mass is so small the electrostatic force of attraction between unlike charges so between that proton and electron is far greater than than that of gravitational forces of attraction based on respective masses now something to know about electrons again is that they move around the nucleus at varying distances away from the nucleus the electrons that are closer to the nucleus are at lower energy levels and those that are further away are in higher shells that have higher energy we're going to revisit this in the next chapter now that we understand the atom though all right let's talk about a couple of important terms that are related to the atom atomic structure is defined by three critical numerical descriptors we have atomic number mass number and atomic weight now looking at the element on the periodic table for example looking at any element on the periodic table the atomic number also denoted as Z is the count of protons in the nucleus of an atom and this number is fundamental because it determines the chemical identity of the atom each element on the periodic table is characterized by a unique atomic number so if we take oxygen for example all oxygens are defined to have eight protons in their nucleus that is the defining characteristic of the oxygen element now the mass number sometimes denoted as a is the total number of protons and neutrons in an atom's nucleus neutrons and protons this sum gives us the mass number now the difference between the mass number and the atomic number is the number of neutrons in the nucleus and it's important to knowe that the mass number is not the same as the atomic weight now we're going to talk about atomic weight but really quickly I want to show you how else you might see this information communicated so you might also see where an element is written down an elemental symbol so for oxygen that would be o and here in the top left corner you're going to have the mass number which is about 16 for oxygen and in the bottom left corner you have the atomic number written so eight now let's talk about atomic weight this is a more nuanced concept it is the weighted average of all the masses of naturally occurring isotopes of an element since isotopes of an element have varying number of neutrons they also have different Mass numbers but the same atomic number same atomic number atomic weight it also takes into account the relative abundance of each isotope in nature so let me give you a proper definition of isotope Isotopes are variants of elements that are going to have the same number of protons but different number of neutrons leading to different Mass numbers so if we're talking about oxygen again all oxygen atoms they have eight protons in the nucleus but the number of neutrons that can change and that gives rise to Isotopes understanding atomic structure and Isotopes is really crucial for grasping the bigger picture of chemical behavior the variations in atomic number and mass numbers among Isotopes really influences the physical and chemical properties of the element which in turn affects reactions bonding Etc now let's go back to atomic weight let's spend some time here all right so now that we've defined Isotopes let's go back to our Definition of atomic weight we defined atomic weight as the weighted average of the masses of all naturally occurring isotopes of an element measured in atomic mass units so what we want to do here is break this down because the calculation is not so simple sometimes all right so to calculate atomic weight scientists are going to consider both the mass and the Natural Abundance of each isotope of the element so here's how it works first isotope masses first the exact mass of each isotope of the element is determined these masses are close to whole numbers but are slightly different due to binding energy within the nucleus that's besides the point we get the exact mass of each isotope of an element then two Natural Abundance each isotope Natural Abundance is then determined this is the percentage of each isotope that's going to occur naturally on Earth and then we can do the weighted average the atomic weight is just calculated by multiplying the mass of each isotope by its Natural Abundance and then adding these values together what the result is is the weighted average that represents the average mass of all the Isotopes as they occur naturally Let's do an example so for example if you have an element that has two different isotopes we're going to call isotope a and isotope B and you know that isotope a has a mass of 10 AMU and a Natural Abundance of 90% And you know that isotope B has a mass of 11 AMU and a Natural Abundance of 10% what is the atomic weight all right the atomic weight would be calculated as follows we're going to focus on isotope a we know that its weight it's its mass I'm sorry is 10 AMU and it has a Natural Abundance of 90% we're going to multiply that by 90% but in decimal form so 0.9 all right we're going to add that to our isotope B isotope B has a a mass of 11 AMU and a Natural Abundance of 10% or 0.1 in decimal form form all right we're going to add these two values together and what we get is the atomic weight if you plug this into a calculator this first one is 9 + 1.1 and that gives us 10.1 AMU so the atomic weight for this element that has two isotopes is 10.1 AMU all right so now we've talked about atomic number mass number and atomic weight there are a couple of more Concepts that we want to discuss now that is going to be super crucial before we move on and that is the following we want to really properly Define atomic mass unit moles and molar mass so we've said AMU and atomic mass unit quite a few times thus far in our lecture what does that mean the atomic mass unit is a standard unit of mass that quantifies the mass of atoms or molecules and it's defined as 12th of the mass of a carbon 12 atom all right carbon 12 isotope is stable and its abundance makes it a standard for measuring atomic masses one atomic mass unit is equal to 1.66 * 10- 27 kilog the atomic mass of an element it's usually found on the periodic table and it's expressed an AMU and it represents the average mass of all the Isotopes of that element taking into account their relative abundances all right so here we're tying in Concepts now then what is a mole this is going to be so important for chemistry all right a mole is a unit that measures the amount of a substance so just like the measure one dozen signals 12 insert object whether it's a dozen books which is 12 books or a dozen eggs which is 12 eggs eggs all right one mole is a unit that measures the amount of substance and one mole of any substance all right one mole of any substance equals 6.022 * 10 23 entities and that could be atoms molecules ions Etc this is known as avagadro's number and the mole allows chemist to count particles by weighing them the mass of one mole of a substance is equal to its molecular or atomic mass in grams and that leads us into talking about Mass molar mass molar mass is the mass of one mole of a substance element or compound and it's expressed in gram per mole now that we have defined these three terms how do we connect them all together and by that I mean how do we convert from one form into another so let's start by saying that you know the number of particles Les and you want to convert that to moles how do you do that in order to do that you divide by avagadro's number so you divide by avagadro's number if you know the number of moles and you want to go back to number of particles in order to do that you're going to multiply multiply by avagadro's number okay now let's say you know the number of moles and you are interested in converting to mass how do you go from moles to mass in order to do that you're going to multiply by molar mass now if you know the mass and you want to go back to moles you're going to have to divide by molar mass okay one more layer here what if you know the mass and you want to convert to number of particles in order to do that you're going to have to follow the arrows first you divide by molar mass and then you multiply by avagadro's number that's how you go from Mass to number of particles it involves two steps okay that's great what about the other way around what if you know the number of particles you're interested in the mass in order to do that again we follow the arrows first we divide by avagadro's number then we multiply by molar mass so that is the workflow in order to be able to move from one unit into another let's go ahead and Tackle an example problem together this one says suppose you have 3.11 * 10 23 particles or atoms of argon what is the mass of this sample all right so what we're given is we're given the number of particles and our goal is to get Mass let's go back to our figure and figure out what are the two steps that we need we have number of particles we have mass all right we're trying to go from number of particles to mass this involves two steps first we divide by avagadro's number then we multiply by molar mass we're going to write that down first we divide by avagadro's number then we multiply by molar mass and what we're actually going to set up here is what we're going to call stochiometry in order to convert all all the way from particles to mass in a setup that allows us to cancel and look at how we cancel units appropriately so the first thing that we're going to write is what we're given we know we have 3.11 * 10 23 particles now in order to convert from particles to mass there's an intermediate step where we go from particles to moles by dividing by avagadro's number and in order to do that well we know avag God's number is 6.022 * 10 23 particles per one mole right that's our relationship and what you notice here in this setup is that particles are going to cancel out we're left with units of mole but we're not done cuz the problem is not asking for moles it's asking for Mass so then we do our last step which is multiply by molar mass one mole of argon is equal to what is the molar mass of argon 39.95 G so one mole of argon is equal to 30 39.95 G and notice that moles cancel out here our remaining unit is grams of argon this is exactly what we want because it's asking us for the mass of this argon sample now what we do to solve this problem is this is just doing the numerator divided by the denominator so you will if to if you're wanting to plug this into a calculator you will first add in parentheses the numerators multiply them together so you're going to multiply this number by this number close parentheses then divide by the denominator which is this number right here avagadro's number and if you plug that into the calculator correctly what you're going to get is that the mass of this argon sample is 19975 G and there we have it we solved this problem let's go ahead and do another example here this problem is actually going to motivate our discussion on ions so let's read it it says determine the number of protons neutrons and electrons in a nickel 58 atom and in a nickel 60 atom so let's go ahead and discuss ions first and then we're going to come right back to this problem something to remember is that in order ordinary reactions the nucleus does not change but the number of electrons that an atom possesses can change and so it's safe to say that the movement of electrons constitute reactions and this is going to be a focus of most of our discussions in general chemistry and also in organic chemistry it's then important to pay close attention to whether a particle is a charged ion or a neutral atom because an ion possesses prop properties that are quite different from those of a neutral molecule an ion is a charged particle resulting from the fact that the number of protons is not equal to the number of electrons and there are two types we have cations and anions a positively charged atom is called a cation this happens when an atom loses an electron then we have a anion these are negatively charged atoms this happens when an atom gains an El electron and it's important to pay attention to whether a particle or a molecule is charged or not because an ion possesses properties that are quite different from those of a neutral atom with that being said let's move back to our problem here that says determine the number of protons neutrons and electrons in a nickel 58 atom and a nickel 60 so for our nickel 58 atom well let's remember that nickel the atom number for nickel is 28 and remember that the atomic number tells us the number of protons all right it tells us the number of protons now in a neutral atom of nickel which is what nickel 58 is here there's going to be 28 protons and 28 electrons so the number of protons is going to equal the number of electrons that means something really important if we have 28 protons but 58 here tells us the mass number we can figure out the number of neutrons by subt subtracting 58 from 28 and that gives us 30 this is our number of neutrons all right so for nickel 58 this is a neutral atom of nickel 58 is the mass number the atomic number for nickel is 28 atomic number tells us the number of prot protons in a neutral nickel atom the number of protons equals the number of electrons so we have 28 electrons and 28 protons 58 which is what we're given here this is the mass number this is protons plus neutrons if we know the number of protons we can figure out the number of neutrons by taking 58 the mass number subtracting it from the number of protons 28 and the number we get is the number of neutrons so that's how we worked through this problem for nickel 58 now let's do nickel 60 and it tells us here that this is a plus 2 cation what that means is our number of protons stays the same because we're still dealing with nickel and nickel is defined by having the same number of protons the same atomic number which is 28 it tells us this is a plus two cation that tells us how many electrons are lost remember cat ions form from loss of electrons when you lose an electron and this is a plus two cation which actually means you lose two electrons and so the number of electrons is no longer 28 it is going to be 26 that is the number of electrons all right fantastic and now we have the mass number here which is 60 we can figure out the number of neutrons by taking 60 subtracting it from the number of protons we determined and that gives us the number of neutrons 60 - 28 is 32 so so this has 32 neutrons and there we have it we've solved this problem and with that we've completed all of the topics that we wanted to in chapter zero with our background information that we need so that we can be successful in future chapters I hope this was helpful let me know if you have any questions comments concerns down below other than that good luck happy studying and have a beautiful beautiful day future doctors