hello class so I'm gonna start this lesson talking about electrons so if you remember from unit 1 electrons occupy the electron cloud which is the area surrounding the nucleus and electrons have a negative charge and then they occupy things called orbitals which can be broken down into sub shells and shells and if you're not remembering these things then please take please take some time to review your unit 1 material before moving into unit 2 we're gonna build on a lot of the ideas that we started in unit 1 and continue to add onto them into unit 3 so it's really important that you take some time and in particular if you know there were things that you did not know how to answer on the exam take some time review them and let's finish getting them down because we're going to be using these ideas moving forward so in particular we talked about how electrons are in the electron cloud and they have this structure so in particular we know that every atom has some electrons that are very close to the nucleus and these electrons are what we call core electrons and since core electrons are very close to the nucleus they're at a very low energy and hopefully that makes sense we talked about how orbitals that the closer the orbital is to the nucleus the lower the energy of the orbital and in particular the lower the energy of the electrons in that orbital so elect core electrons are low energy electrons close to the nucleus now because these core electrons are so low in energy we find that they don't tend to actually participate in chemistry it's important that they be there but because they are already so stable it's hard to convince them to change very much in other words if you want to think about it this way if all of your needs were met and if you knew all of your needs would be met moving into the future would it be easy to convince you to change anything in your life or more importantly would it be easy to convince you to come to school I would imagine not if all of your needs are satisfied and you know they will be satisfied it's hard to convince you to make a change or to work so this is true of core electrons as well because they're at a very low energy they're not going to change or do work the other type of electron though are valence electrons and you can think of valence electrons as being where the magic happens so these are higher energy electrons which occupy the outer s and P sub shells yeah so high-energy electrons which occupy the outer s and P sub shells so let's just kind of start by looking at chlorine I'm gonna grab a periodic table over here you can't see where over here is so the idea is we find chlorine on the periodic table and zoom in just a little bit here if we find chlorine on the periodic table according to the definition of valence electrons that we just discussed it is the outer s and piece so in other words chlorine is on the third row of the periodic table which means that its outer shell is the third shell so what we want to do is we want to count all of the electrons and chlorines third shell so starting at chlorine one two three four five electrons in the 3p and then we want to keep coming across the periodic table to the third row of the s-block and we know that's one two electrons so chlorine has five three P electrons and two three s electrons so all in all we would say chlorine has seven valence electrons okay so make sure you go back over that exercise and you understand where everything came from so chlorine has five three P's and two 3 s's and like I said we recall that valence electrons are the high-energy electrons which occupy the outer s and P so take a moment think about how many valence electrons fluorine has perhaps how many valence electrons bromine has right so try to count them up on your periodic table and I'm gonna take a brief pause here in the video to give you a chance to take a look at your periodic table and try to tell me how many valence electrons fluorine and bromine have so if we find fluorine on the periodic table it's on the second row which means its outer shell is the second shell so looking at the 2p would be 1 2 3 4 5 sliding over to the 2s 1 2 so all in all we could say that fluorine has 7 valence electrons as well and if we take a look at bromine we end up with a very similar situation start at bromine so bromine is on the fourth row of the periodic table so its outer shell is the fourth shell so 1 2 3 4 5 4 PS 1 2 4 s s so bromine also has 7 valence electrons so maybe at this point you're starting to see the important pattern I'm gonna zoom out a little bit here again maybe you're starting to see the important pattern so in particular one of the things we talked about in unit 1 were the 8 main groups of the elements so in particular chlorine fluorine and bromine are in group 7 so what we can say about group 7 is that they always have 7 valence electrons and this actually becomes a very easy way for us to determine how many valence electrons any given element has without having to count it up in the orbitals every single time so in particular the group number equals the number of valence electrons so you know any member of Group one starting with hydrogen then lithium and potassium and sodium and rubidium and cesium and francium any member of Group one has exactly one valence electron any member of group two has exactly two valence electrons any member of group three s3 and so hopefully you're seeing that pattern again group number is the number of valence electrons so if you're ever unsure how many valence electrons a specific element has check its group number and that tells you how many valence electrons the element has so what are these valence electrons have to do with chemistry or the rest of chemistry in particular there's a rule that we use called the octet rule and given that you can see that it's called the octet rule hopefully you notice or hopefully you imagine that the number eight is a very important number in this rule so in particular before we found out that the valence electrons are the outermost s and P electrons for any given atom the octet rule states that all atoms desire a filled outer shell in other words a filled valence shell which means that it wants that outer s and those outer Peas to be completely full so something I'd like you to think about is how many electrons is that so the valence shell is the outer s and the outer P sub shells and we learned in unit 1 that an S sub shell contains one orbital and that each orbital can hold two electrons so an S sub shell can hold two electrons we also learned that ap sub shell contains three orbitals each of which can hold two electrons so they can hold six electrons in total meaning that the valence shell is a total of four orbitals one s and three P's which can hold up to a total of eight electrons so what this is saying is that every atom is trying to achieve a situation where it has eight valence electrons instead of the number of valence electrons that it normally has in other words all atoms want eight valence electrons now there are two important exceptions to this rule so take a look at your periodic table think about the definition of valence electrons and in particular think about the definition of the octet rule which atoms would be exceptions to that rule kinda hoping that you're thinking hydrogen and helium because what you'll notice is is that their outer shell only contains an s-orbital in other words they're on the first row of the periodic table and the first row of the periodic table does not go into the p-block and so a P sub show is not included in their valence show so hydrogen and helium want two electrons we'd call that a duet so now I want to move into talking about exactly how atoms go about trying to satisfy the octet rule and so in particular they use chemical bonding and we'll talk about two types a lot in this class but there are really three types I want to talk about right now the first is an ionic bond and this is a bond that arises from the attraction of ions of opposite charge so in unit 1 we learned that columbic force describes the force of attraction or repulsion between ions and in particular we learned that if two ions have opposite charge they're extremely attracted to each other therefore what an ionic bond is is a cation that meets an N ion essentially they stay stuck together and in a little bit this will focus on the rest of our discussion or rather we will focus on this for the rest of our discussion and we'll talk about how ionic bonds go about satisfying the octet rule then we have covalent bonds which arise from the sharing of electrons between two atoms and finally metal bonds which pretty much as much as we're gonna talk about metal bonds is gonna be this right here so in other words please know what a metal bond is and that's gonna be about as much as we're gonna talk about metal bonds and that is a network of electrons between metal atoms so now I'd like to backtrack some and just talk about ionic bonding for the rest of the time and in particular what I want to start by talking about what I want to start talking about is how ionic bonding fulfills the octet rule so we know how to determine how many valence electrons any given atom has for instance consider sodium na find it on your periodic table what group is it in it's in Group one so since it's in Group one we know it has one valence electron now consider something like chlorine how many valence electrons does chlorine have it's in group seven so that means it has seven valence electrons next lesson I'm gonna discuss more about what I'm drawing here right now but for the time being hopefully you get what the image is conveying you have a sodium atom which has one valence electron and a chlorine atom which has seven so now we want to think about the octet rule the octet rule generally requires that something have eight valence electrons but the more accurate definition of the octet rule was that every atom wants a filled outer shell and so what we could realize is is we could accomplish that one of two ways so either an atom can gain electrons until it has eight valence electrons thus satisfying the octet rule or an atom can give up all of the valence electrons it does have leaving it with nothing but core electrons therefore satisfying the octet rule so here we're looking at sodium and chlorine sodium has one valence electron in order to satisfy the octet rule by gaining electrons it would need to gain seven electrons on the other hand in order to satisfy the octet rule by losing electrons it would only have to give up one on chlorine we've had kind of an opposite situation here chlorine is going to could either give up all seven or it could just take one so as a rule atoms are gonna do whatever it requires moving the fewer electrons in other words doing less work so what requires less work is that sodium give up its one valence electron to chlorine so that sodium now has no valence electrons and since it gave up one electron sodium is now a one plus cation chlorine gains one electron so that now it has eight valence electrons and so it is a chlorine one minus and then what we learned in a previous lesson was that ions of opposite charge attract each other so since sodium is a cation and chlorines an anion they'll stay stuck together and make the compound sodium chloride now which for the record is table salt so what happened was a sodium gave up its one valence electron to take on a one plus charge chlorine took on one electron to have a minus one charge and so these charges that our two atoms took in order to satisfy their octet rulz is what we call the common oxidation state so let's do a little practice together let's think about calcium and phosphorus and maybe let's go ahead and take a couple minutes and think about potassium and phosphorus okay so we want to figure out is what the chemical formula of the compound that would form if potassium and phosphorus interact and calcium and phosphorus interact so I'm going to pause for a couple seconds on the video to give you a chance to pause the video so that you can work this out if you're unsure then just let it keep playing and we'll work this out together and you'll get a little bit more practice on your homework so calcium is a group two element which means that it has a +2 oxidation state phosphorus is a group 3 element which means it has a minus 3 oxidation state so it's very similar to our situation with boron and oxygen in that calcium each calcium needs to give up two electrons and each phosphorous needs to take three so we know we can't have a one-to-one but every electron that we give up must be taken by something else okay so I'm gonna come down the paper and draw a little diagram for working this out a little bit more so we could think about calcium one two calcium one two third calcium one two and then we could look at the two phosphorus so each calcium is going to give up two electrons so in other words this one will come here let someone go there someone go here so I go there here and here so now our calcium's have all given up all of their electrons and each one of our phosphorus is has taken electron so that it now has eight all electrons that have moved have had somewhere to go so we know the chemical formula must be ca 3 P 2 so let's come back up here and look at potassium potassium is a group one elements so it's a one plus oxidation state phosphorus is a group 5 element again so it's a 3 minus so I know I'm gonna need 3 potassium so that I can cover one phosphorous so k 3p now what I'm hoping you may be starting to notice at this point it is a very simple fact which is that the oxidation state on one element here is the subscript on the other so because phosphorus is a three minus charge I know I need three potassium to balance it and then since potassium is a one plus charge I know I need one phosphorous to balance it and this trick will actually work each and every time the only thing we occasionally have to watch out for our situations like calcium and oxygen calcium is a 2 plus oxygen is a 2 minus so if we do the crisscross and we say okay since oxygen is a 2 minus charge I know I need to calcium's and since calcium is a two plus charge I know I need two oxygens in this situation we can actually reduce it a little bit further this is not truly an incorrect answer it's just there's a better answer and so since we could reduce it down it's more accurate to say it's ca 101 in other words if you end up with subs when you're doing ionic bonding specifically if you end up with subscripts that share a divisor then reduce them and hopefully that makes pretty good sense you'll have a little more practice in your homework and there are some good examples in your written notes so take a look at those if you're unsure one final thing that I'd like to talk about before ending this particular lecture is polyatomic ions so the ions we've been talking about so far are all mono atomic ions in other words a single atom that has a charge but then there are also polyatomic ions which can make ionic bonds and what this means is is they are a group of atoms that Nets a charge and so what can be difficult to think about with these is we must treat these groups of atoms as though they are one thing that is inseparable in other words your car is made of many many parts and yet you don't separate your car into the parts whenever you talk about it or if you were gonna work on your car you generally or for instance if you needed to have your car worked on biomechanic you would not take the engine out of the car and then take the engine to the mechanic you would take the car to the mechanic okay so in other words think of it as one thing like a car and it's true that it's made up as many parts but if you take it apart it's no longer a car so in other words if you have a car without wheels I might argue that you almost have a car but you do not have a car because a car is a device that can transport you from A to B on four wheels in much the same way these polyatomic ions can't be separated from each other if you separate the atoms that make up the ion it is no longer the iron much like if you take away parts of a car it no longer functions as a car thus it's not a car so a couple of common polyatomic ions I'll mention here you have a longer chart of them in your notes you do not need to memorize these they will be given to you as references on your exam so they each have a name you don't need to know their names and you don't need to memorize the charges you just need to be able to recognize polyatomic ions when you see so in particular a very common polyatomic cation is ammonium and so one thing I want to point out is this one plus charge does not belong to the hydrogen's it does not belong to the nitrogen it belongs to ammonium in other words you can think about it as being a group of 1 nitrogen and 4 hydrogen's all of which has a 1 plus charge on another common one that's an anion would be phosphate and phosphate is four oxygens and one phosphorus that all together have a three minus charge on them what's nice about polyatomic ions is beyond getting past the idea that these are groups of atoms that have a charge they don't work any differently from mono mono atomic ions in other words let's think about the chemical formula if potassium and phosphate were mixed together so potassium we know has a 1 plus oxidation state so since phosphate has a 3 minus that I know that means I'm going to need 3 potassium it's ok 3 and since potassium is 1 plus I'll need one phosphate which is the entire group of peor now let's look at another example that's a little bit different this time let's look at calcium and phosphate calcium is group 2 so it has a 2 plus oxidation state now again since I have a 3 minus charge on phosphate I know that means I'm gonna need 3 calcium's and then since I have a two plus charge on calcium I know that means I need two phosphates so phosphate should be put in parentheses this time with a 2 on the outside and so this tells us that I need two groups of pio for now we didn't have to put it in parenthesis over here because there's just an understood one you can put it in parenthesis if it makes you feel better and I'll count it right on your exam but blackboard will not accept this as an answer during your homework so in other words only use parentheses if it's necessary and always use parentheses if it's necessary but don't use parentheses if you don't need to because there's an understood one there so we'll do one last example and then all in the video let's look at ammonium and sulfate because sulfate has a two minus charge I know that means I need to ammonium so since I have more than one of them I know I'm in I know I'm going to need to put ammonium in parenthesis and there are two of those followed by sulfate so4 and since there's only one sulfate I don't need to put it in parenthesis this time so hopefully you've followed everything in the video there are more examples and more and more explanation in your notes if you need a little bit more help and then as always send me an email if you have some questions and I'll do my best to help you I always invite you to come to my office and I very much appreciate when you guys do make the time to come see me to me off in my office I'd love to meet you and I'd love to have you meet me and in particular I'll be how I'll be done grading your exams soon and I'd love for you to come and look at your exam so we could talk about the entire exam together but just send me an email and we'll make an appointment