Electronegativity | Atomic structure and properties | AP Chemistry | Khan Academy
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Electronegativity | Atomic structure and properties | AP Chemistry | Khan Academy

Voiceover: What I want to
talk about in this video are the notions of Electronegativity, electro, negati, negativity, and a closely, and a closely related idea of Electron Affinity,
electron affinity. And they’re so closely
related that in general, if something has a high electronegativity, they have a high electron affinity, but what does this mean? Well, electron affinity
is how much does that atom attract electrons, how much
does it like electrons? Does it want, does it
maybe want more electrons? Electronegativity is a
little bit more specific. It’s when that atom is
part of a covalent bond, when it is sharing
electrons with another atom, how likely is it or how badly does it want to hog the electrons
in that covalent bond? Now what do I mean by hogging electrons? So let me make, let me write this down. So how badly wants to hog, and this is an informal
definition clearly, hog electrons, keep the electrons, to spend more of their time closer to them then to the other party
in the covalent bond. And this is how, how
much they like electrons, or how much affinity they
have towards electrons. So how much they want electrons. And you can see that these are very, these are very related notions. This is within the context
of a covalent bond, how much electron affinity is there? Well this, you can think of it
as a slightly broader notion, but these two trends go absolutely
in line with each other. And to think about, to just think about electronegativity makes it
a little bit more tangible. Let’s think about one of the most famous sets of covalent bonds, and that’s what you see
in a water molecule. Water, as you probably know, is H two O, you have an oxygen atom, and you have two hydrogens. Each of the hydrogen’s
have one valence electron, and the oxygen has, we see
here, at it’s outermost shell, it has one, two, three, four,
five, six valence electrons. One, two, three, four,
five, six valence electrons. And so you can imagine,
hydrogen would be happy if it was able to somehow
pretend like it had another electron then it would have
an electron configuration a stable, first shell that
only requires two electrons, the rest of them require eight, hydrogen would feel, hey
I’m stable like helium if it could get another electron. And oxygen would feel,
hey I’m stable like neon if I could get two more electrons. And so what happens is they
share each other’s electrons. This, this electron can
be shared in conjunction with this electron for this hydrogen. So that hydrogen can kind
of feel like it’s using both and it gets more stable, it stabilizes the outer shell, or it stabilizes the hydrogen. And likewise, that electron could be, can be shared with the hydrogen, and the hydrogen can kind
of feel more like helium. And then this oxygen can feel like it’s a quid pro quo, it’s getting something in
exchange for something else. It’s getting the electron, an electron, it’s sharing an electron
from each of these hydrogens, and so it can feel like
it’s, that it stabilizes it, similar to a, similar to a neon. But when you have these covalent bonds, only in the case where they are equally electronegative would you have a case where maybe they’re sharing, and even there what happens in the rest of the molecule might matter, but when you have something like this, where you have oxygen and hydrogen, they don’t have the
same electronegativity. Oxygen likes to hog electrons
more than hydrogen does. And so these electrons are not gonna spend an even amount of time. Here I did it kind of just drawing these, you know, these valence
electrons as these dots. But as we know, the electrons are in this kind of blur around, around the, around the actual nuclei, around the atoms that make up the atoms. And so, in this type of a covalent bond, the electrons, the two electrons
that this bond represents, are going to spend more
time around the oxygen then they are going to
spend around the hydrogen. And these, these two
electrons are gonna spend more time around the oxygen, then are going to spend
around the hydrogen. And we know that because
oxygen is more electronegative, and we’ll talk about
the trends in a second. This is a really important
idea in chemistry, and especially later on as
you study organic chemistry. Because, because we know that oxygen is more electronegative, and the electrons spend more time around oxygen then around hydrogen, it creates a partial
negative charge on this side, and partial positive charges
on this side right over here, which is why water has many of
the properties that it does, and we go into much more in
depth in that in other videos. And also when you study organic chemistry, a lot of the likely reactions that are going to happen can be predicted, or a lot of the likely molecules that form can be predicted based
on elecronegativity. And especially when you start going into oxidation numbers
and things like that, electronegativity will tell you a lot. So now that we know what
electronegativity is, let’s think a little bit about what is, as we go through, as we start, as we go through, as
we go through a period, as say as we start in group one, and we go to group, and
as we go all the way all the way to, let’s say the halogens, all the way up to the yellow
column right over here, what do you think is going to be the trend for electronegativity? And once again, one way to think about it is to think about the extremes. Think about sodium, and
think about chlorine, and I encourage you to pause the video and think about that. Assuming you’ve had a go at it, and it’s in some ways the same idea, or it’s a similar idea
as ionization energy. Something like sodium
has only one electron in it’s outer most shell. It’d be hard for it to
complete that shell, and so to get to a stable
state it’s much easier for it to give away that
one electron that it has, so it can get to a stable
configuration like neon. So this one really wants
to give away an electron. And we saw in the video
on ionization energy, that’s why this has a
low ionization energy, it doesn’t take much
energy, in a gaseous state, to remove an electron from sodium. But chlorine is the opposite. It’s only one away from
completing it’s shell. The last thing it wants to
do is give away electron, it wants an electron really,
really, really, really badly so it can get to a configuration of argon, so it can complete it’s third shell. So the logic here is
that sodium wouldn’t mind giving away an electron, while chlorine really
would love an electron. So chlorine is more
likely to hog electrons, while sodium is very
unlikely to hog electrons. So this trend right here, when you go from the left to the right, your electronegativity, let me write this, your getting more electronegative. More electro, electronegative, as you, as you go to the right. Now what do you think
the trend is going to be as you go down, as you go down in a group? What do you think the trend
is going to be as you go down? Well I’ll give you a hint. Think about, think about
atomic radii, and given that, pause the video and think about what do you think the trend is? Are we gonna get more
or less electronegative as we move down? So once again I’m assuming
you’ve given a go at it, so as we know, from the
video on atomic radii, our atom is getting larger,
and larger, and larger, as we add more and more and more shells. And so cesium has one electron
in it’s outer most shell, in the sixth shell, while, say, lithium has one electron. Everything here, all
the group one elements, have one electron in
it’s outer most shell, but that fifty fifth electron, that one electron in the
outer most shell in cesium, is a lot further away then
the outer most electron in lithium or in hydrogen. And so because of that, it’s, well one, there’s more interference
between that electron and the nucleus from all the other
electrons in between them, and also it’s just further away, so it’s easier to kind of grab it off. So cesium is very likely to give up, it’s very likely to give up electrons. It’s much more likely to give
up electrons than hydrogen. So, as you go down a given group, you’re becoming less, less
electronegative, electronegative. So what, what are, based on this, what are going to be
the most electronegative of all the atoms? Well they’re going to be the ones that are in the top and the
right of the periodic table, they’re going to be these right over here. These are going to be
the most electronegative, Sometimes we don’t think as
much about the noble gases because they aren’t, they
aren’t really that reactive, they don’t even form covalent bond, because they’re just happy. While these characters up here, they sometimes will form covalent bonds, and when they do, they really
like to hog those electrons. Now what are the least electronegative, sometimes called very electropositive? Well these things down
here in the bottom left. These, over here, they have only, you know in the case of cesium, they have one electron to give away that would take them to a
stable state like, like xenon, or in the case of these group two elements they might have to give away two, but it’s much easier to give away two then to gain a whole bunch of them. And they’re big, they’re big atoms. So those outer most electrons are getting less attracted to the positive nucleus. So the trend in the periodic table as you go from the bottom left, to the top right, you’re getting more, more
electro, electronegative.

About James Carlton

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81 thoughts on “Electronegativity | Atomic structure and properties | AP Chemistry | Khan Academy

  1. Do have to repeat what you write like a gazillion time.
    Btw amazing videos got me through my exams.

  2. So this is why the column of 1A from Soduim to Cesium is so reactive. And that Cesium practically explodes when you throw it in water.

  3. thanks im currently revising chemistry and came across the definition of electronegativity but didnt understand it but i have now thanks to this video big thumbs up 👌

  4. What I don't like about these videos is the guy repeats himself way too many times, so much I forget what his point of explanation is. Like just shut up and right it out, stop sounding out everything you write lol.

  5. Thanks a lot man u r a proper legend! Keep doing what you do coz u r getting through my freakin O' Levels! xDD

  6. Thank you so much for this video. I have a chem test in two days about all the periodic trends and I missed one day due to being ill so i missed this lesson. I heard my teacher explain it a bit but I couldnt fully understand. This helped so much!

  7. thank you! peace out!i know oxygen for sures is highest electronegativity ! cool I was oxygen for my Halloween costume. I loved oxygen and now I love it even more! I understand that going down row 1 sublevel as you get to sublevel two , three, four etc the electronegativity gets less. The least amount of electrons a element has it is more likely to give away an electron . the elements that are closest to completing their octet will want to gain take from other elements to have a full octet. cool beans

  8. I don't understand why h2o has 6 valence electrons…. Like what the hell? And how does Li have a .98 electronegativity?

  9. How does Na have 1 electron in it's outer shell if the atomic # is 11? Isn't it supposed to have 11 electrons since protons and electrons are supposed to be balanced?

  10. Thanks so much for your videos – your explanations are well constructed, paced and they're  mentally accessible. I really appreciate all you've done in the past years!

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