AK LECTURES - Atomic Radius, Ionization Energy, Electronegativity and Electron Affinity
Start studying Atomic radius, electronegativity, ionization energy, and electron affinity (McCombs). Size of cation in relation to Atom. Cation is smaller than. The ionization energy is defined as the amount of energy that is required to pull an electron off of an atom. The ionization energy increases as you go from left to . If you think about it, electronegativity depends not only on how strongly to expect that there should be any one-to-one correlation between the EN and the When it comes to electron affinity, chlorine has a higher value than.
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.
Let's compare that with electron affinity.
Electron affinity: period trend (video) | Khan Academy
So, in electron affinity, let's say we're starting with our neutral lithium atom again, but this time, instead of taking an electron away, we are adding an electron. So, we would add an electron to the two s orbital. So we started off with three electrons in the neutral lithium atom, and we're adding one more.
So the electron configuration for the lithium ion would be one s two, two s two. So still three positive charges in the nucleus, two electrons in the one s orbital, but now we've added an electron, so we have four electrons total, two in the two s orbital.
So let me highlight the electron that we added in magenta. So this is the electron that we added to a neutral lithium atom. And this electron, we know, is shielded from the full positive three charge of the nucleus by our two core electrons in here, right? So like charges repel. It's also going to be repelled a little bit by this electron, that's also in the two s orbital.
So this electron's going to repel this one as well. But, there is an attractive force between our positively charged nucleus and our negative charge on the electron. So this electron that we added still feels an attractive force that's pulling on it from the nucleus. And so, if you add this fourth electron, energy is given off. And since energy is given off, this is going to have a negative value for the electron affinity. For adding an electron to a neutral lithium atom, it turns out to be kiloJoules per mol.
So energy is released when an electron is added, and that is because the electron that we added is still able to be attracted to the charged nucleus. So if the nucleus has an attraction for the added electron, you're going to get a negative value for the electron affinity. Or that's one way to measure electron affinity.
Note that the lithium anion is larger than the neutral lithium atom. It's just hard to represent it here with those diagrams. So as long as the added electron feels an attractive force from the nucleus, energy is given off. Let's look at one more comparison between ionization energy and electron affinity.
In ionization energy, since the outer electron here is attracted to the nucleus, we have to work hard to pull that electron away. It takes energy for us to rip away that electron. And since it takes us energy, we have to do work, and the energy is positive, in terms of ionization energy.
But for electron affinity, since the electron that we're adding is attracted to the positive charge of the nucleus, we don't have to force this, we don't have to do any work. Energy is given off in this process, and that's why it's a negative value for the electron affinity.
Electron affinities don't have to be negative. For some atoms, there's actually no attraction for an extra electron. Let's take neon, for example. Neon has an electron configuration of one s two, two s two, and two p six.
So there's a total of two plus two plus six, or 10 electrons, and a positive 10 charge in the nucleus for a neutral neon atom. So let's say this is our nucleus here, with a positive 10 charge, 10 protons.
And then we have our 10 electrons here, surrounding our nucleus. So this is our neutral neon atom. If we try to add an electron, so here let's add an electron. So we still have our 10 protons in the nucleus. We still have our 10 electrons, which would now be our core electrons. To add a new electron, this would be the neon anion here, so one s two, two s two, two p six. We've filled the second energy level. To add an electron, we must go to a new energy level.
So it would be the third energy level, it would be an s orbital, and there would be one electron in that orbital. So, here is, let's say this is the electron that we just added.
So we have to try to add an electron to our neon atom. But if you think about the effective nuclear charge that this electron in magenta feels, alright, so the effective nuclear charge, that's equal to the atomic number, or the number of protons, and from that, you subtract the number of shielding electrons.
Since we have 10 protons in the nucleus, this would be And our shielding electrons would be 10, as well. So those 10 electrons shield this added electron from the full positive 10 charge of the nucleus. And for a quick calculation, this tells us that the effective nuclear charge is zero.
And this is, you know, simplifying things a little bit, but you can think about this outer electron that we tried to add, of not having any attraction for the nucleus.
These 10 electrons shield it completely from the positive 10 charge. And since there's no attraction for this electron, energy is not given off in this process.
Actually, it would take energy to force an electron onto neon. So if you wrote something out here, and if we said, alright, I'm trying to go from, I'm trying to add an electron to neon, to turn it into an anion.
Electron affinity: period trend
Instead of giving off energy, this process would take energy. So we would have to force, we would have to try to force this to occur. So it takes energy to force an electron on a neutral atom of neon.
And we say that neon has no affinity for an electron. So it's unreactive, it's a noble gas, and this is one way to explain why noble gases are unreactive.
This anion that we intended isn't going to stay around for long.