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Let's have the molecule sodium chloride.

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Let's look at the periodic table.

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Sodium, it's in alkali metal.

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It has one valence electron.

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It's in Group 1.

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And so when it only has one, it really

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wants to give it away.

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And chlorine is a halogen, and it's only one away.

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It just needs one electron to have the full eight valence

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electrons in its outermost shell.

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So it really wants to take 1 electron.

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And we've talked about this story multiple times,

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especially it early in the chemistry playlist. So what we

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know what's going to happen.

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Sodium, since it wants to lose an electron, is going to lose

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its electron.

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And it's going to give it to chlorine.

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And chlorine is going to gain it.

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And so you're going to have a situation where sodium is

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going to have a plus 1 charge, because it lost an electron.

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And then chlorine will have a minus 1 charge because it

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gained an electron.

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And then they'll be attracted to each other because this one

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is now a positive atom and that one is

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now a negative atom.

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And just the Coulomb force will make them want to be with

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each other.

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And we call this, of course, an ionic bond.

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I haven't taught you anything you don't know yet.

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And in an ionic bond, you literally had a loss of

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electron from one compound to another.

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Now.

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that's a very cut-and-dry situation.

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But we saw other situations where wasn't quite as

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cut-and-dry.

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For example, H2O.

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Where you have a oxygen-- but you've seen this multiple

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times already, if you're watching

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this playlist in order.

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And it's bonded to 2 hydrogens.

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But we know that oxygen is much, much more-- in fact,

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nitrogen, oxygen, and fluorine are the most electronegative

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atoms, which means that they like to hog electrons the

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most. So when you have oxygen bonded with pretty much

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anything-- but in particular in this case, hydrogen-- it's

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going to to pull most of the electrons in its direction.

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So even though these are covalent bonds, the hydrogen

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is still sharing the electrons that it

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has bonded with oxygen.

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The electrons spend most of their time on the oxygen side

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of this whole affair.

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So you have a partial negative charge on the oxygen end of

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the molecule.

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And you have partial positive charges on the hydrogen.

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And we talked about this.

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And this is what leads to hydrogen

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bonding, and all of that.

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What we're going to do here is introduce-- it's almost an

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intellectual tool.

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We know that this isn't an ionic bond.

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We know that this is actually a covalent bond, that this

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atom is shared.

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But it spends most of its time at the more

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electronegative atom.

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That's why you have this partial negative charge.

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So we create this convention called oxidation states.

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Let me write that down.

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And I'll clarify what the word means in a second.

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And this is essentially assigning-- With an ionic

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compound, you naturally assign a charge, because each atom

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really does have a charge.

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But let's say we want to live in a world where we don't like

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this partial charge, partial negative.

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We want to say, look, if this were, hypothetically, an ionic

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bond, what would it look like?

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Who would gain the electron and who

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would lose the electron?

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So in the case of water, if you were forced to say OK, who

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gained the electron?

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You'd say oxygen gained 2 electrons from the hydrogens,

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and the hydrogens lost each 1 electron to the oxygen.

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So the hydrogen would have a plus 1 charge, each of them.

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And then the oxygen would have a minus 2 charge.

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Now, I want to be very clear with this.

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This isn't what really happened.

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This is just our little intellectual game that we're

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playing called an oxidation state.

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It's going to be really useful later to understand why some

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reactions occur.

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But I just want to be very clear.

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This is a hypothetical charge if these were ionic bonds.

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So you're just saying, whoever is the more electronegative--

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and remember, electronegativity, it goes

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from the bottom left to the top right.

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So these are the most electronegative atoms, which

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means they love to hog electrons the most. These are

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the least. Or you could also call them the most

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electropositive, which means they like to give away

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electrons the most.

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So what you would do is you say, OK.

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The more electronegative, let's just say that they

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actually get the electrons.

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And that the more electropositive atoms, they

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give the electrons.

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Even though we know that it's something in between.

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It's actually partial.

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Now these numbers.

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These hypothetical ionization of these hydrogen molecules.

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This is called their oxidation number.

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The oxidation number of hydrogen in H2O is plus 1.

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So we could write a plus 1 for each of the hydrogens there.

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And then oxidation number for the oxygen is minus 2.

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And the way we talk about it, we say that the hydrogen

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molecules here have been oxidized.

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And it's kind of like saying that you've been-- In this

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case, they were oxidized by oxygen.

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And this is actually a very confusing point, or at least

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it was to me initially.

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Because when I first learned about oxidation, I was like,

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oh, that's what oxygen does to things.

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Because the word has the word, oxygen, or at least the

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beginning part of oxygen.

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So I thought, oh, oxidation must mean what oxygen does to

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other atoms, which means it takes

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electrons away from them.

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Usually it takes 2 electrons for itself.

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Maybe it took 1 from each of the other atoms. So if you've

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been oxidized, you've had electrons taken away from you.

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And so you'll have a positive charge.

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Now, that interpretation is only partially true.

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The reality is it does not have to be oxygen that's doing

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the oxidation.

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So for example, let me do hydrogen fluoride.

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H F.

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I've shed my habit of writing fluorine as Fl.

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I've now remembered that its elemental symbol is just F.

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So hydrogen fluoride, if it was in an aqueous solution,

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it'd be hydrofluoric acid.

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That's hydrogen here bonding with fluorine, one of the most

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electronegative atoms. So what's going to happen here?

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Once again, the reality is that hydrogen is sharing--

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it's a covalent bond with fluorine.

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But the electron spends most of its time here, on the

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fluorine atom.

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So you're going to have a partial negative charge here,

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and a partial positive.

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But we don't like this partial, halfway game.

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We want to say, look.

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If this was, hypothetically, an ionic bond, if one of these

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people have to gain or lose an electron, how

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would it play out?

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In that situation, this guy likes to hog electrons more

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than hydrogen does.

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So the fluorine would gain an electron and have an oxidation

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number of minus 1.

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And the hydrogen would lose an electron, and have an

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oxidation number of plus 1.

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In this case, hydrogen has been oxidized.

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And notice, there's no oxygen to be seen.

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So the way I think about it, fluorine did to hydrogen what

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oxygen would have done.

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For example.

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If I say that you've been Bernie Madoff'ed.

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Bernie Madoff might not be the actual individual who's taking

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your money and putting it into a Ponzi scheme, but it would

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mean that someone else is doing the same thing that

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Bernie Madoff would have done to you.

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So in the same way, even though there's no oxygen here,

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fluorine has oxidized the hydrogen.

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Now, I'll introduce another word, and that's reduction.

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Reduction is the opposite of oxidation.

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I'll write it in blue.

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And this just means a reduction of charge.

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So in this hydrogen fluorine molecule, or hydrogen fluoride

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molecule, hydrogen has been oxidized.

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It's been oxidized because electrons have been

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taken away from it.

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That's what oxygen would have done to it.

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And so it has, now, a positive charge.

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And its oxidation number is plus 1.

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Because exactly one electron would have been taken away

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from this in a hypothetical ionic bond.

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Now, fluorine has been reduced.

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Its oxidation state was reduced by 1.

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If there was no hydrogen around, it

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would've been neutral.

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But now it has a minus 1 charge.

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Because in our hypothetical world of there's no partial

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charge, it's more electronegative.

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It took the atom from the hydrogen.

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So it has a minus 1 charge.

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Its charge has been reduced.

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Its oxidation state has been reduced.

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Your oxidation state is just that hypothetical charge.

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And that's how I think of it.

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Reduction means reduction in your hypothetical charge.

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Oxidation means, what would oxygen have done do you?

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Which means it would've taken away electrons, which would

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mean you have a positive charge.

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But if you look in some textbooks, or some teachers,

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they'll give you a mnemonic.

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Leo the lion says, GER.

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I'll say, says, in small, because it's really irrelevant

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to the mnemonic.

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And this is just a way of remembering that Leo means

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Losing Electrons is equal to Oxidation.

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And that Gaining Electrons is equal to Reduction.

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Now, to me, when you first said, oh wait, I'm gaining

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electrons, but I'm getting reduced.

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What's getting reduced?

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What's getting reduced is your charge, because

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electrons are negative.

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You're gaining something with a negative charge.

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So that's where the reduction comes from.

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Oxygen comes from the fact that oxygen will normally make

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you lose electrons.

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It will normally take electrons away from you, even

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though oxygen might not have anything

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to do with the reaction.

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Now, we can look at the periodic table, and we can

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guess, in most molecules, what an oxidation state of a given

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atom will be.

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All of these guys are alkali earth metals.

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And this is really a review.

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It's kind of taught as something new.

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But we know all of these guys.

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They love to give up electrons, because that makes

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them stable.

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Because they have this one electron in their outer shell.

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So they tend to have an oxidation state of plus 1,

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which means that they tend to give away an electron.

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For example, if I write sodium chloride.

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In this case, the oxidation state is truly reflective of

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their charge.

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Its charge is equal to its oxidation state.

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It gave away an electron.

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And that's true for all of the alkali metals right here.

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Remember, I don't include hydrogen there.

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Because hydrogen is a little bit of a special case.

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It could have been thrown here on the periodic table.

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Because it has one electron in its outermost shell, which is

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just its only shell.

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But it's also happy to get two, and have a configuration

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like helium.

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So you can almost view it as it's very close to

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completing its shell.

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So it sometimes has alkali metal-type properties.

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And sometimes it has halogen properties, where it wants to

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00:12:00,929 --> 00:12:02,399
gain electrons.

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So hydrogen-- Let's say hydrogen is bonding with one

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of these guys, right?

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So let's say you had lithium hydride.

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00:12:09,450 --> 00:12:16,040


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So in this case, lithium, is right here.

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It loves to lose its electrons.

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So it'll lose its electron.

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So it'll have an oxidation state of plus 1.

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And it will lose its electron to hydrogen.

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Because hydrogen is more electronegative than lithium.

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Because hydrogen, you give it 1 electron then it has

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electron configuration like helium.

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So hydrogen will be minus 1.

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In this situation, hydrogen bonds with people roughly on

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the left-hand side of the periodic table.

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So lithium-- just to review our terminology.

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Lithium was oxidized by the hydrogen.

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Hydrogen was reduced by the lithium.

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Its charge went down.

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Their oxidation number?

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Plus 1 for the lithium, minus 1 for the hydrogen.

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What is the total sum of an oxidation state

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00:13:07,860 --> 00:13:08,730
for a neutral molecule?

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In this case, you add up the charges and you get, oh, it's

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00:13:12,600 --> 00:13:14,210
equal to 0.

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And that's a big takeaway.

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Then, in general, if you have a neutral molecule-- let me

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write that down.

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00:13:21,929 --> 00:13:28,120


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I'll just say neutral compound.

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Then the oxidation states add to 0.

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And if you have a non-neutral compound, if you have a plus 1

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00:13:45,149 --> 00:13:48,829
charge, the oxidation states of all of the molecules in

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your compound are going to add up to your charge.

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Well, let me give another example.

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So in this case, I had hydrogen

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bonding with these guys.

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In that case, hydrogen is the one that took the electron.

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Let's say I had OH minus.

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304
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So hydrogen, in this case, when it's dealing with

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00:14:19,429 --> 00:14:21,229
something that's not on the left-hand side, when it's

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dealing with a super electronegative atom,

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hydrogen's oxidation state is plus 1.

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Right?

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Now, oxygen's typical oxidation state, when it bonds

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with almost everything else-- and I'll give a special case

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00:14:38,529 --> 00:14:40,769
in a few moments-- is minus 2.

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Because it normally takes two electrons from other things.

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We saw that with the water.

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Oxygen's oxidation state, in most cases, is minus 2.

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So even though it hasn't gained two electrons here, we

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would write its oxidation number as

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minus 2 for the oxygen.

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00:14:56,179 --> 00:14:59,419
And then you add up the two oxidation states,

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minus 2 plus 1.

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Well, actually I'll take that back.

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Oxygen has gained an extra electron because this is

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already an ion.

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So in this case, this OH, oxygen has stolen an electron

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from someone else.

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Let me draw that.

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And then oxygen normally only has 1, 2, 3, 4-- so oxygen has

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completed its valence shell.

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So essentially, it took an extra electron

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from some place else.

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So it definitely does have a minus 2 charge in this case.

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Oxygen has a minus 2 charge.

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00:15:35,480 --> 00:15:38,110
Hydrogen has a plus 1 charge, because this electron was

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taken from it by the oxygen.

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And so the total charge of this molecule is minus 2 plus

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1, which is minus 1.

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And that's why you have a minus 1 charge on OH.

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So you add up the oxidation states, and you're going to

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get the charge of the atom under question.

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Now, we already saw that most of these guys have an

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oxidation state of plus 1.

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These guys here, the alkaline earth metals, have an oxygen

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state of plus 2, because they like to give away two atoms.

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We already saw hydrogen.

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If they bond with people here, they take the electrons.

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So they get reduced.

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00:16:13,100 --> 00:16:15,159
So it has an oxidation state of minus 1.

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But if hydrogen bonds with these guys on the right-hand

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00:16:17,570 --> 00:16:19,310
side, it gives away the electrons.

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So it would have an oxidation state of plus 1.

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00:16:21,350 --> 00:16:23,279
We saw that with hydrogen fluoride.

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It would be the case with hydrogen

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00:16:24,110 --> 00:16:25,940
chloride, hydrogen bromide.

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00:16:25,940 --> 00:16:27,280
You saw the case with water.

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All of these cases, hydrogen is the one that's giving away

356
00:16:30,019 --> 00:16:30,740
the electrons.

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So it gets oxidized.

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00:16:32,500 --> 00:16:37,389
And these guys get reduced, or their the oxidizing agent.

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00:16:37,389 --> 00:16:40,990
Now, what about these guys over here?

360
00:16:40,990 --> 00:16:43,680
What about our halogens?

361
00:16:43,679 --> 00:16:48,319
Well, they all like to take electrons.

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00:16:48,320 --> 00:16:53,010
So their typical oxidation state is minus 1.

363
00:16:53,009 --> 00:16:55,169
Oxygen's typical oxidation state?

364
00:16:55,169 --> 00:16:57,409
We just saw it's minus 2.

365
00:16:57,409 --> 00:16:59,579
And I'll show you an example, which is really one of the few

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00:16:59,580 --> 00:17:01,100
cases where oxygen doesn't have a

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00:17:01,100 --> 00:17:03,300
minus 2 oxidation state.

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00:17:03,299 --> 00:17:05,430
And then there's a lot of these other elements, in

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00:17:05,430 --> 00:17:06,910
between, that can have multiple

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00:17:06,910 --> 00:17:08,269
different oxidation states.

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00:17:08,269 --> 00:17:09,598
And we'll see that in a lot of examples.

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00:17:09,598 --> 00:17:12,059
But if you know that hydrogen is plus or minus 1, if

373
00:17:12,059 --> 00:17:15,019
hydrogen is bonding with these guys, it's a plus 1. it gives

374
00:17:15,019 --> 00:17:16,180
away electrons.

375
00:17:16,180 --> 00:17:18,480
If it bonds with these guys, it's a minus 1.

376
00:17:18,480 --> 00:17:19,990
it takes the electrons.

377
00:17:19,990 --> 00:17:22,960
If you know oxygen tends to take two electrons, so it has

378
00:17:22,960 --> 00:17:24,779
a minus 2 oxidation state.

379
00:17:24,779 --> 00:17:26,779
And the halogens take one.

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00:17:26,779 --> 00:17:29,269
If you know that, you can normally figure out all of the

381
00:17:29,269 --> 00:17:30,920
other people in the compound.

382
00:17:30,920 --> 00:17:32,390
So what I'll do is I'll leave you there for now.

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00:17:32,390 --> 00:17:33,670
I realize I'm running over time.

384
00:17:33,670 --> 00:17:36,259
In the next video, we're going to do some maybe more

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00:17:36,259 --> 00:17:38,160
difficult examples.