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Current time:0:00Total duration:11:04

Video transcript

let's think a little bit about the molecule sodium sodium chloride if we look at the periodic table right over here we see that sodium is a group one element it's an alkali metal it has one valence electron it's also not too electronegative it's sitting here on the left hand side of the periodic table we know the general trend for electronegativity is that it increases as we go to the top right so these elements over here generally like to hog electrons these elements over here generally like to give away electrons these are electro positive these are electronegative so sodium right over here with its one with its one valence electron it's a very good candidate for giving away an electron on the other side of the periodic table we have chlorine we have chlorine right over here it is a group seven element it's as a halogen it would love nothing more than to gain an electron so that it can get to the magic number of eight valence electrons it is very electronegative so what you can imagine would happen is if these two things were to interact the chlorine would NAB sodium's extra electron over here and it would become the chloride anion and then sodium would become the sodium cation cation just as a positive ion anion is just a negative ion and so this one right over here is now positive sodium is now positive chloride is a negative ion and so they're going to be attracted to each other and they're going to form this ionic bond so this right over here is an ionic ionic bond now let's think about something that's not ionic that's more than that where where the electrons are not being fully nabbed from one mop from one atom to another but they're being shared and one of the most famous examples of that is water so we know water is h2o you have one oxygen bonded each water molecule is one oxygen bonded to to two hydrogen's and these two bonds are covalent bonds in each of these bonds you have a pair of electrons that are being shared by both the hydrogen and the oxygen being shared by both the hydrogen and the oxygen but we also know that this isn't a completely equal sharing of the electrons we look in the periodic on the periodic table here oxygen is far more electronegative than hydrogen is and so because of that the electrons are going to in these two bonds are going to spend more time around the oxygen than they are going to spend around the hydrogen's and we've seen this before this would give the oxygen end of the water molecule a partially negative charge that's the lowercase Greek letter Delta we use that for the notation of partially negative and on the hydrogen ends of the molecule we're going to have a partially partially positive charge now this is the reality but as we'll see later on and in future videos it's sometimes inconvenient to have this kind of partial messiness and so what I'm going to do right now is introduce you to what is fundamentally just an intellectual tool it's just a convention that chemists have invented that allow us to get our heads around a lot of reactions and allowed us to think about how is a reaction likely to occur and that intellectual tool is the idea the idea of oxidation states oxidation the idea of an oxidation state and what the hype what the oxidation state is is even if you're in a situation where you have covalent bonds you say well let I understand the partial those are partial charges these are covalent bonds electrons are being shared but I don't like this this partial stuff I want to just assume hypothetically what if this were what if these were ionic bonds and you say well okay if these had to be ionic bonds then the oxygen would NAB the electrons from these pairs and so the oxygen would have a fully the oxygen would have a fully negative charge a negative two charge and the hydrogen's would have a fully would have a fully positive charge would have a fully positive charge each and so if we were to write down the oxidation state or the oxidation states for the atoms in the water molecule let's write that down so h2 h2o we would say that oxygen has an oxidation state of negative two and water each or each hydrogen atom has an oxidation state of plus of plus one and notice the whole molecule is neutral and these things cancel out with each other positive 1 positive 1 that gets you to positive 2 then you have negative 2 they cancel out now the one thing I keep saying this is negative 2 but I wrote the negative after it if I wanted to write positive one as an oxidation state I would actually write it as one positive although you can assume that if someone just writes the positive and this is just a convention to write the sign after the number when we are writing actually ionic charges or oxidation states because an oxidation state is nothing but and a hypothetical ionic charge if you really had to if you were forced to assume these aren't covalent bonds but these are ionic bonds as once again I want to stress this is the reality these are covalent bonds these are partial charges the oxidation state intellectual tool that's forcing us to pretend like these are ionic these are ionic bonds and you might say okay well you know this this is kind of this kind of makes sense right over here this is this involved oxygen in some way that's why it's called oxidation states and that's how I initially conceptualized it when I when I first learned about this you said well look hydrogen lost each of these hydrogen's lost an electron top to oxygen so it makes sense that we say that the hydrogen each hydrogen got oxidized so hydrogen oxidized oxidized by oxygen by oxygen it makes sense that oxygen would oxidize something else this got done to it the some some operations that the charge was taken away by oxygen so it got oxidized now the other term on the other side of oxidized is reduced and the word reduced really comes from the idea that oxygens charge has been reduced so we could say Oh or we could say oxygen has been reduced reduced by the hydrogen's by by the hydrogen's and so there's a temptation here to say okay well okay this must always involve oxygen of some kind because it has it seems to have begin with the same words well that is not the case let's take for example let's take for example hydro if this is an aqua if this is an aqueous solution hydrofluoric acid hydrofluoric acid right over here you have a hydrogen covalently bonded to a fluorine now just like we saw in water fluorine is one of the most electronegative elements it's going to hog the electrons in this covalent bond so this is going to have a partial negative charge here this is going to have a partial negative charge here and this is going to have a partially positive charge but if we were going to think about it in terms of oxidation states we would say okay lets you know and push comes to shove if this had to be anionic if this had to be an ionic bond and not a covalent bond what would be the charges on each of these atoms we say well in that case hydrogen would lose an electron and it would have a full positive charge and fluorine would gain an electron would gain an electron and have a full negative charge this is the hypothetical once again the realities are partial it's a covalent bond but the hypothetical one is a full positive charge here and a full negative charge here and so we would say that the oxidation state in this molecule for hydrogen is plus one and the oxidation state for fluorine in this molecule is negative one and we would say we would say that hydrogen the hydrogen has been oxidized has been oxidized and we could even say it's been oxidized by the fluorine and we would say that the fluorine because it's charged is its hypothetical ionic charge has been reduced we would say fluorine has been fluorine might be reduced and you would say hey wait look oh wow we're using the word oxidized even though there is no oxygen to be seen here and one way to think about it if someone had told you that you had been Bernie Madoff that doesn't necessarily mean that you interacted with Bernie Madoff it means that someone did to you what Bernie Madoff would have done someone else someone took your money told you they were going to invest it and then put it into a Ponzi scheme even if that person is not Bernie Madoff you could say that you have been Bernie Madoff so here Florine did two hydrogen what oxygen tends to do it took an electron away it oxidized the hydrogen now if you that's how I tend to remember it the ox oxidized is you have done to you if something has been oxidized it's losing an electron and what I think about it is well that that's what oxygen would have done to you oxygen is very electronegative it tends to take electrons away from other atoms now there are other mnemonics that you might see for remembering what oxidation and reduction actually actually represent and I'll introduce those to you just because they might be helpful and they are introduced into in a bunch of and a bunch of chemistry classes one of the mnemonics is is Leo Leo the Lion says ger Leo the Lion and I'll write says in lowercase here because it's really not relevant but Leo the Lion says ger ger and this is to remember that losing an electron means you are being oxidized or losing electrons is oxidation and gaining electrons is reduction so that's just a mnemonic another one that's often used another one that's often used is oil oil rig oil rig and this essentially oxidation is losing electrons reduction is gaining electrons