Now, this unit, again, is about periodic trends. And we've looked at trends in shielding, in effective nuclear charge, in radius, in ionization energy. Our next trend that we're going to look at is electron affinity. So we're going to define electron affinity and look at the basic trends of electron affinity in terms of where it's located on the periodic table. Let's begin by defining it. It is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion. Okay, it's a negative of. Now, I have seen this defined without the negative of part. Basically, what it says is this. We're going to take an atom. The atom is going to be in this gaseous state. That atom is going to accept an electron. Okay? Now, if that little atom really wants the electron, when it takes in that electron, the atom with its, that is now an ion, will lower in energy and give off the excess. So there'll be a giving off of energy. If something really wants an electron, we say it has an affinity for that electron. You know, we could say that in terms of other things, I have a good affinity for money or gold or diamonds. You know, we really like it, we want it, and we're willing to pay for it. Well, that's what the electron affinity is playing off of here. If an atom wants that electron and it gives it a favorable electron configuration, it would provide energy out, it would give energy out, and pay for that electron. Now, it's a negative of because we want it to have a positive value if it has a good affinity. So if we, if something has a high electron affinity, has a big positive affinity, that means it gave off a exothermic, it gave off a lot of energy. So let's put down some more words here. So it's the energy associated with this reaction. So, we have not yet talked about delta H, this is kind of a dangerous little thing that we see right here, but this is the energy change of that reaction. There's going to be some kind of energy change associated with that reaction. We're going to call it Y. Okay? Electron affinity is the negative of that, so it'd be a negative Y, so opposite sign Y, basically. So if an electron really wants to, or if an atom really wants that electron and it's very favorable for it to have that electron, it will give off energy. And the amount of energy it gives off is a negative value. So electron affinity then would be a positive value. So things with high positive electron affinities really want to accept that electron. Now, if an atom does not want an electron, you'd have to force it on. Okay, you would have to force that electron on. And pushing it on would require energy, and that would be a positive value associated with that. And then the electron affinity would be, in that case, a negative value. And what we need to understand is that electron affinity can be positive or it can be negative. A positive value means it is favorable to take on that electron. It has a high affinity for that electron. And large values of E sub A, I mean EA, electron affinity, means that atom wants the electron. Now, we can probably look at a periodic table and imagine who's going to have the highest electron affinity. Which one of those atoms want the electron the most? But let's just look at the trends, okay? The trend is, an increase in the tendency to accept an electron from the left to the right across the periodic table. Okay? This shows up as very large positive values of electron affinity for our halogens. Our halogens are the ones that have the highest affinity for an electron. And if you think about their electron configuration, they are all one away from being a noble gas configuration. Okay? Electron affinery, affinities are generally, generally speaking, lower for metals than those for nonmetals, because our metals don't tend to want to accept an electron. Metals want to get rid of electrons. Okay, so this shouldn't surprise us. We have over on the nonmetal side, we tend to want to gain electrons. When you're on the metal side, you want to lose electrons. That trend doesn't expect, does not surprise us. But let's look at this. And this thing is all over the place. But it goes up and down, and up and down, and up and down. But let's see, up here we definitely see that our halogens have the highest electron affinity. Okay? We have it increasing as a general trend, let's follow lithium to, mm, we don't have the halogens on there because, I mean, not the halogens, they don't have the noble gases on there, because they don't have much of an affinity for electrons, do they? They like the way they are. But, here we're starting at lithium, and working our way across, and finishing at fluorine. There is a general increase as we move from left to right. But boy, it goes up and down and up and down a lot. We have nitrogen way down here, as a nonmetal. Why would nitrogen as a nonmetal have an electron affinity sitting here, at basically zero? Well, let's think about nitrogen for just a moment. Electron configuration of nitrogen is helium, 2s2, 2p3. Is that not already a half-filled p subshell? So if we put an electron on and say yeah, okay, I'll take it, because it is an electron and I am moving, I am a nonmetal, I'm moving towards becoming a noble gas. It's moving me in the right direction. But, meh, I'm pretty happy the way I am. I've got three, I've got a half-filled p subshell, and that's a good thing. So nitrogen and any of the elements that are under nitrogen on the periodic table, are going to have that half-filled subshell. But the only thing I really can say with certainty as I look at the electron affinity trend is, with very happy certainty, I can say that the halogens are all the highest electron affinity. I can also say, generally speaking, that as we go down, this is going down a group, the halogen group. It is decreasing as we move down, except, of course, fluorine is not following suit. Fluorine would need to be up here to really follow suit with that. So even if up and down doesn't work really well, but we can certainly say with, with all confidence that our halogens have the greatest electron affinity. Okay, so that is our last trend on the period table. What we're going to be seeing in our next learning objectives is how those trends play out with the properties of our elements. But with this learning objective, we have defined electron affinity, make sure you know the definition. It's a little convoluted with the opposite sign of what the energy is. But we've got that definition and we have a general look at the trend of electron affinity, when we look at the periodic table.
