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Current time:0:00Total duration:13:06
AP.Chem:
SAP‑2 (EU)
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SAP‑2.A (LO)
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SAP‑2.A.2 (EK)
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SAP‑2.A.3 (EK)

Video transcript

before we get into electron affinity let's really quickly review ionization energy let's start with a neutral lithium atom with an electron configuration of 1s2 2s1 a lithium atom has three protons in the nucleus so a positive three charge two electrons in the 1s orbital so here are the two electrons in the 1s orbital or our core electrons and one electron in a 2's orbital this is our outermost electron our valence electron the valence electron is shielded from the full positive 3 charge of the nucleus by the presence of the core electrons the like charges repel and these core electrons repel this outer electron and shield it from the full positive 3 charge but there still is an attractive force between the positively charged nucleus and this outermost electrons so opposite charges attract and our outermost electrons still feels a pull from the nucleus therefore since the outer electron is attracted to the nucleus it takes energy to completely pull away this valence electron from the neutral atom so if we pull away the outermost electron we lose our valence 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 plus 1 charge since this ionization takes energy to rip away the electron the energy the ionization energy is positive and it's measured in kilojoules 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 2's orbital so we start off with 3 electrons in the neutral lithium atom we're adding one more so the electron configuration for the lithium ion would be 1s2 2s2 so still three positive charges in the nucleus two electrons in the 1s orbital but now we've added an electron so we four electrons total 2 and the 2 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 3 charge of the nucleus by our two core electrons in here and so like charges repel it's also going to be repelled a little bit by this electron that's also in the 2's 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 steal 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 negative 60 kilojoules 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 anion has 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 look at one more comparison between ionization energy and electron affinity and 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 as 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 me on for example neon has an electron configuration of 1s2 2s2 and 2p6 so there's a total of 2 plus 2 plus 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 anion here so 1s2 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 the effective nuclear charge that this electron in magenta feels our answer 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 are shielding electrons would be 10 as well so those 10 electrons shield this added electron from the full positive charge of wonderful 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 not having any attraction for the nucleus these ten electrons shield it completely from the positive ten 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 on to neon so if we wrote something out here if we said all right I'm trying to go from me 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 gases are unreactive this anion that we intended isn't going to stay around for long so it takes energy to force this on to 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 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 kilojoules per mole next we have beryllium with a zero value for the electron affinity that means it actually takes energies this number is actually positive and it takes energy to add an electron to a neutral atom of beryllium so to think about going from a neutral atom of beryllium to form the beryllium anion we looked at electron configurations neutral atom of beryllium is 1s2 2s2 and so to form the negatively charged beryllium anion it would be 1s2 2s2 and to add the extra electron and must go into a 2p orbital which is of higher energy and so this is actually the same thing or very - neon which we just discussed for neon the electron configuration was 1s2 2s2 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 beryllium anion 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 as 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 beryllium anion and that's why we see this zero value here for beryllium beryllium has no value for electron of 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 negative 27 kilojoules per mole and we can see a little bit of a trend here as we go from boron to carbon to oxygen to fluorine so as we go across across the period on the periodic table more energy is given off and therefore fluorine has the most affinity for and electron so as we go across a period we get an increase in the electron affinity so the negative sign this 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 fluorine then 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 pole from the nucleus which is what we mean here by at by increased effective nuclear charge more energy will be given off when we add that electron so this idea explains the general trend we see for electron that's in see as you go across a period we get an increase in the electron affinity we've already talked about beryllium 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 1s2 2s2 and then 2p 3 so if we draw out our orbitals let's just say this is the 2's orbital and then these are the 3 2p orbitals so we'll just do these electrons here 2 electrons in the 2's orbital and then we have 3 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 an 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