before we get into electron affinity let's really quickly review ionization energy and let's start with a neutral lithium atom with an electron configuration of 1 S2 2s1 a lithium atom has three protons in the nucleus so a positive three charge two electrons in the 1 s orbital so here are the two electrons in the 1 s orbital or our core electrons and one electron in a 2s orbital this is our outermost electron our veence electron the veence electron is shielded from the full positive three charge of the nucleus by the presence of the core electrons so like charges repel and these core electrons repel this outer electron and shield it from the full positive3 charge but there still is an attractive force between the positively charged nucleus and this outermost electron so opposite charges attract and our outermost electron still feels a p from the nucleus therefore since the outer electron is attracted to the nucleus it takes energy to completely pull away this veence electron from the neutral atom so if we pull away that outermost electron we lose our veence electron and we're left with a lithium ion with a positive charge positive one charge because we still have three protons but only two electrons now so overall A+ one charge since this ionization takes energy to rip away the electron the energy the ionization energy is positive and it's measured in KJ per mole let's compare that with electron affinity 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 2s 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 1 S2 2 S2 so still three positive charges in the nucleus two electrons in the 1 s orbital but now we've added an electron so we have four electrons total two in the 2s 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 Shield from the full positive3 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 2s orbital so this electron is 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 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 electron to a neutral lithium atom it turns out to be -60 K per mole 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 annion 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 1 S2 2s2 and 2 P6 so there's a total of 2 + 2 + 6 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 annion here so 1 S2 2s2 2p6 we 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 would be an S orbital and there'd be one electron in that orbital so here is let's say this is the electron that we just added so we we have to try to add an electron to our neon atom but if you think about uh the effect Ive nuclear charge that this electron in magenta feels right 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 10 and our shielding electrons would be 10 as well so those 10 electrons Shield this added electron from the full positive charge of from the full positive 10 charge of the nucleus and for a quick Cal calculation this tells us that the effect of 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 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 we wrote something out here if we said all right I'm trying to go from NE I'm trying to add an electron to Neon to turn it into an anion instead of giving off energy this process would take energy so we would have to force we 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 gas masses are unreactive this anine that we intended isn't going to stay around for long so it takes energy to force this onto our neutral neon atom so you could say that the electron affinity is positive here it takes energy but usually you don't see positive values for electron affinity for this sort of situation at least most textbooks I've looked at would just say the electron affinity of neon is zero since I believe it is hard to measure the actual value of this here we have have the elements in the second period on the periodic table and let's look at their electron affinities we've already seen that adding an electron to a neutral atom of lithium gives off 60 KJ per mole next we have burum with a zero value for the electron affinity that means it actually takes energy so this number is actually positive and it takes energy to add an electron to a neutral atom of burum so if we think about going from a neutral atom of burum to form the buril anion we look at electron configurations neutral atom of burum is 1 S2 2s2 and so to form the negatively charged burum annion it would be 1 S2 2s2 and to add the extra electron it must go into a 2p orbital which is of higher energy and so this is actually the same thing or very similar to Neon which we just discussed for neon the electron configuration was 1 S2 2s to 2p6 and to add an extra electron we had to go to the third energy level we had to open up a new shell and the electron that we added was effectively screened from the full nuclear charge by these other electrons and a similar thing happens here for the burum annion to add this extra electron we have to open up a higher energy p orbital this electron is on average further away from the nucleus and is effectively shielded from the full positive charge of the nucleus and therefore there's no affinity for this added electron so there's no affinity for this electron so it takes energy to form the burum anion and that's why we see this zero value here for burum burum has no value for electron affinity or it's actually a very positive value and we just say it has a zero value next let's look at Boron here so this gives off -27 K per mole and we can see a little bit of a trend here as we go from Boron to carbon to oxygen to Florine so as we go across across the period on the periodic table more energy is given off and therefore Florine has the most affinity for an electron so as we go across a period we get an increase in the electron affinity so the negative sign just means that energy is given off so we're really just looking at the magnitude more energy is given off when you add an electron to a neutral atom of Florine than if you add an electron to a neutral atom of oxygen and we can explain this General Trend in terms of effective nuclear charge as we go across our period we also have an increase in the effective nuclear charge and if the added electron is feeling more of a pull from the nucleus which is what we mean here by at by increased effective nuclear charge more energy will be given off off when we add that electron so this idea explains the general Trend we see for electron affinity as you go across a period we get an increase in the electron affinity we've already talked about brillium as an exception neon as an exception but what about nitrogen in here we can see that nitrogen doesn't really have an affinity for an electron and you'll see many different values for this one depending on which textbook you're looking in but if we look at some electron configurations really quickly we can try to explain this so for nitrogen the electron configuration is 1 S2 2 S2 and then 2 P3 so if we draw out our orbitals let's just say this is the 2s orbital and then these are the 3 2p orbital so we'll just do these electrons here two electrons in the 2s orbital and then we have three electrons in the P orbitals so let's draw those in there if we try to add an electron to a neutral nitrogen atom we're adding an electron to one of these orbitals which already has an electron in it so adding an electron to one of these orbitals right the added electron would be repelled by the electron that was in there to start with and this is the reason that you usually see in textbooks for the fact that this does not follow the trend nitrogen doesn't have an affinity for one added electron so after going through all of that it's obvious that electron affinity is a little more complicated than ionization energy and ionization energy with a pretty clear Trend and it was a little easier to explain why for electron affinity going across a period on the periodic table we see a little bit of a trend but there are many exceptions to this and perhaps our explanations are a little bit too simplistic to explain actually what's going on but across a period you see you do see a little bit of a trend going down a group is much harder you see more inconsistencies and it's not really even worth going over a general trend for that