in this module we're going to focus on the overall structure and composition of atoms atoms are the smallest unit of matter that have all the properties of a specific element now there are two main models when examining atoms the first is the model which is a simplified and overly simplified view of what an atom looks like and we call this the orbital model and this is usually what we default to it is really kind of a lie in reality this is not what uh the atom looks like but it's just much easier to understand and relate to and in reality the atom exists as a cloud now you'll notice that there are three main subatomic particles that we're going to examine and that's what we're going to do in the rest of the video so in the very dead center of the atom we call the dead center of the atom the nucleus now it's not a real structure like it is in a cell it's just the center of the atom and within that center of the atom you have two main subatomic particles the first particle is the proton and the proton has a positive charge and the number of protons dictates what element the atom is so this number of protons is actually referred to as the atomic number so for example if you have an atomic number of one it means you have one proton if you have one proton you are hydrogen okay so an atomic number of one tells you you have one proton and if you have one proton you are elemental hydrogen if for example your atomic number is six that means you have six protons and if you have six protons you are elemental carbon okay and a couple of the other ones eight is eight protons it means your oxygen seven protons or atomic number of seven you means you have seven protons and you are elemental nitrogen okay so if you change the number of protons you change the identity of that particular atom the second subatomic particle is the neutron so both the proton and the neutron are found within the nucleus the neutron has no charge and really the number of neutrons it's not something that we're going to focus a lot on in this class but neutrons really just contribute mass to the atom so when you look at a value on the periodic table oftentimes you'll have the letter the letter indicates what element we're talking about the number at the top left is usually the atomic number and there's oftentimes a number over here and this is a number that is referred to as the atomic mass now the atomic mass is not always going to be a whole number but the atomic mass is equal to the number of protons plus the number of neutrons so if for example we look at elemental hydrogen it has an atomic number of one meaning it has one proton and it has a mass of 1.01 well if the mass is 1.01 and i have one proton well if i solve for x for the number of neutrons that means i have 1 100th of a neutron well that's not really physically possible so why are atomic masses not whole numbers the reason for this is the atomic mass is an average of masses for all the elements for all the atoms within that element so what i mean by this is when you look at hydrogen the vast majority of hydrogens will have one proton they will have zero neutrons but there are some hydrogens which they still have one proton that dictates at their hydrogen but they might have one neutron or you might have a hydrogen that has one proton and it has two neutrons so all the neutrons have done is added mass so if you look at somebody who is big boned for example right it doesn't make them any less human they're just a human with a little bit more mass okay so we call these different variants isotopes and sometimes these isotopes are unstable in which case we call them radioactive isotopes because the neutrons might spontaneously degenerate and release massive amounts of energy but in terms of isotopes we're not going to focus on that aspect of chemistry in this class the takeaway is if you change the number of neutrons you're not changing the chemical properties you're not changing the identity all you're changing is mass so let's look at a couple of examples in this example here when you're changing the number of protons you change the element so hydrogen has one proton helium has two protons lithium has three protons okay you can't have a helium that has only one proton the number of protons dictates the element on the flip side if you change the number of neutrons you're not changing the identity so you can have hydrogen with zero neutrons you could have hydrogen with one neutron or two neutrons it does not change the identity now of interest here when you look at the example with hydrogen and helium notice how the number of protons matches the number of electrons and that's because on the periodic table there are no charges everything is neutral so our third subatomic particle is the electron and the electron is not in the nucleus the electron orbits the nucleus and moreover the electron has negligible mass so the number of electrons really doesn't add much in terms of mass that's why when we look at the atomic mass it was just the heavier subatomic particles the proton and the neutron electrons are negatively charged and their function focuses on chemical properties so while protons were identity neutrons were mass the number of electrons dictates the chemical properties associated with that atom so we're going to pay special attention to electrons so when we examine the periodic table there's a specific grouping of the periodic table into these various columns in our first column all of the elements have one electron well wait and wait a second if we change the number of protons we change identity how does lithium with an atomic number of three still have only one electron the reason for this is we always focus on the number of electrons in the outermost shell so that's an aspect of atomic chemistry that can get a little bit confusing so let's focus on that for a second let's say we have our nucleus okay so here's our nucleus and within our nucleus we're housing our protons and our neutrons now orbiting this is why we default to that orbital model because it's just easier to conceptualize let's say we have our first orbit just like the first planet that orbits our sun is mercury right you don't have all the planets in the same orbit you don't have all the same electrons in that first orbit so this first orbit is basically an energy shell so the distance the orbit is the distance the orbit is from the nucleus correlates to energy and so remember many of you probably when you think of einstein you think of the famous equation e equals m c squared well there's a relationship between mass and energy and since electrons don't have mass they have lots of energy okay so you can convert energy and mass and in electrons we've converted basically the mass into energy so the electrons have lots and lots of energy and the first point is the distance they are from the nucleus correlates to the level of energy the further away they are from the nucleus the more energy they have okay additionally within the orbit you have small little areas called orbitals orbitals are basically locations where the electron pairs up with another electron now why this is the case we're not going to go into that um if you are interested you could see me in office hours regarding that but in that first energy shell or first orbit you actually have one orbital and that one orbital can only house two electrons which means the first shell can only house two electrons now all the energy shells thereafter so the second energy shell the third the fourth so each shell or each orbit after the first has four orbitals meaning you can house excuse me a maximum of eight electrons and so some of you that have had chemistry in the past you've probably heard of the octet rule okay so each orbit after the first can house a total of eight electrons now whenever the electron shell or whenever the energy shell is full that valence shell is full it gives you a certain chemical property but our focus is always going to be on the outer most shell so when you look at lithium it has two electrons in that outer excuse me two electrons in that innermost shell and it has one electron in the outermost shell we are only focusing our attention on that outermost shell which is known as the valence shell okay that's why we group the periodic table the way we do is because it's the number of electrons in the valence shell not the total number of electrons but the number of electrons in the valence shell that dictate the chemical properties of that particular atom and so we can group them up and hydrogen lithium sodium they all have common chemical properties because they have one electron in the valence shell when you look at the right most elements helium neon argon notice how they all have a full shell so they will all have similar chemical properties you look at oxygen that's sort of a an important one notice the positioning of the electrons when you look at carbon nitrogen oxygen there is a specific placement of electrons so let's look at carbon for example carbon has an atomic number of six that number again dictates you have six protons it means you have six electrons because you're neutral right protons are positive electrons are negative okay now the atomic number is uh very important the atomic mass is 12.01 so that has six neutrons but our focus is not going to be on the neutrons so within the carbon nucleus let me just put the six protons there in that first energy shell in that first orbital i have two electrons well remember i have a total of six electrons i've just placed two of them so that means i need to place four more electrons so in that second shell notice how they're much further away from the nucleus so these electrons have a lot more energy to them however we have in this orbit for orbitals now we stabilize electrons when we pair them up but for carbon we don't pair the electrons up initially we have to place an electron in each orbital before we pair it up so you have four orbitals that are incomplete okay so in other words we can't put the two electrons here and two electrons here and then have two orbitals that are full and two orbitals that are completely empty this is how we need to place the electrons when we look at something like oxygen oxygen has an atomic number of eight therefore when we look at the outermost let's just look at the outermost in this case so i'm going to forget about the valence shell so if i assuming i'm going to focus on the valence shell not forget about it apologies here i have six electrons to place because remember i put two electrons in that first shell so i have six electrons to play with so here i place four electrons one in each orbital and then i can start to double up so these orbitals are complete and they're stable while the ones over here are incomplete and not stable and that is a common thread with all the atoms all the elements in a particular column so oxygen sulfur they all have two complete orbitals in that valence shell and two incomplete you look at nitrogen it has one complete and three incomplete phosphorus is the same okay so understanding this orbital model allows us to determine the chemical properties in reality you know as we've mentioned before the actual model is the cloud so even though we're divvying stuff up into orbitals we don't know it at any moment in time where the electrons are exactly so even though we draw this sort of like this neat little diagram there's really just a cloud of electrons and we don't know where they are exactly so in the next module we're going to look at molecular basics