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Main content
Current time:0:00Total duration:7:35
AP.Chem:
SAP‑1 (EU)
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SAP‑1.A (LO)
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SAP‑1.A.4 (EK)
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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

the previous videos we've talked about only the first ionization energy in this video we're going to compare the first and the second ionization energies and we're going to use lithium as our example so in the previous video we already know that lithium has an atomic number of three so there are three protons in the nucleus and a neutral atom of lithium the number of electrons equals the number of protons and so we know there are three electrons in lithium here the electron configuration is 1s2 2s1 so we have two electrons in the 1s orbital so we can go ahead and put those two electrons in the 1s orbital like that and then we have one more electron and that electron is going to go into the 2's orbital like this and so that would be a very simple picture of the neutral lithium atom if we apply enough energy alright we can actually pull away this outer electron here so we can pull away that electron and we call this the first ionization energy so the first ionization energy and to pull away that electron takes approximately 520 kilojoules per mole and so once we've pulled that electron away right we we no longer have a neutral lithium atom right we would have in lithium ion because we would still have three positive charges in the nucleus but we have only two negative charges now we only have two electrons because we pulled one away so three minus two gives us plus one so this is the lithium plus 1 cation and the electron configuration would just be 1s2 because we lost we lost the the electron in the 2's orbital and so we could keep going we could apply some more energy and pull away another electron so let's say that we pull away this electron this time okay so we're taking a second electron away and so we wouldn't call this ionization energy one we would therefore call this ionization energy two because this is to take away the second electron and this value turns out to be approximately 7298 kilojoules per mole and so if we take away that second electron alright once again we still have three positive charges in the nucleus but we have we have only one negative charge now it's only one electron so this is no longer the lithium plus 1 cation this is a lithium plus - cation because three minus one is plus two so this is lithium plus two here and the electron configuration would be only one electron in a 1s orbital so 1s 1 and so we can see that there is a big difference between the first ionization energy and the second ionization energy so 520 versus 7298 so let's see if we can explain the reasoning for this extremely large difference in in ionization energies and we're going to use the three factors that we've talked about in the previous videos right so the first factor we discussed was nuclear charge which refers to the number of protons in the nucleus all right so if we look at the neutral lithium atom right three positive charges in the nucleus that positive charge is what's going to attract this electron in in magenta here all right and if we look at the lithium plus 1 cation similar situation right we still have a a three protons in the nucleus and that positive charge is what's going to be attracting this electron as well and so because of the same number of protons we have to think more about effective nuclear charge as opposed to how many protons there are in the nucleus and before we do that we have to consider the effect of electron shielding so let's talk about electron shielding next so electron shielding also called electron screening so electron shielding / screening all right so when we think about electron shielding where we're think about the inner orbital electrons here so going back to the neutral lithium atom right these these two inner shell electrons right here are going to repel this outer shell electron alright so this one is going to repel this one as well and so you could think about it as they screen the electron in magenta from feeling the full force of the positive 3 charge in the nucleus because electrons repel other electrons and so the way to calculate the effective nuclear charge right so we've done this in the in the previous videos as well the simple way of calculating effective nuclear charge is to take the number of protons right so plus 3 and from that you subtract the number of shielding electrons so in this case it would be these two these two electrons in the 1s orbital so three minus two gives us an effective nuclear charge of plus one and so the electron in magenta isn't feeling isn't feeling a nuclear charge of plus three it's really only feeling an effective nuclear charge close to positive one because the actual value is approximately 1.3 when you do the more complicated calculations and so the effect of electron shielding is is to is to decrease is to decrease the overall nuclear charge that this electron in magenta feels and so when we move over here to to this electron right so we're talking about this electron in it in magenta for the lithium plus 1 cation that really is it the same such it's not the same situation right there's there's not much electron shielding this electron over here might repel a little bit right but there are no inner shell electrons repelling this electron in magenta and because of that right the electron in magenta is going to feel this positive 3 charge much more of the full positive 3 charge of the nucleus and so therefore there's going to be a much greater attractive force holding this electron in magenta to this nucleus and therefore you have to apply more energy to pull that electron away so the effect of electron shielding tells you right the second electron is much harder to remove than the first and so we see a large increase in ionization energy from the first ionization energy to the second ionization energy the last factor that we discussed was distanced right so the distance of those electrons in magenta from that from the nucleus right so on the Left once again going back to the neutral lithium atom this electron is in the second energy level right so it's further away than this electron right this electron is in is in the first energy level in the 1s - so this distance here is smaller than the distance on the left and so since the distance is smaller right this electron in magenta feels more of an attractive force from the nucleus right once again that's Coulomb's law and so therefore there's an increased attractive force therefore you take more energy to pull that electron away so it takes much more energy to pull the second electron away than the first and so that's why we see an increase in ionization energy so distance says the fact that this electron is closer means it takes more energy to pull it away and that's another reason why this number for the second ionization energy is so much larger than the first so it takes a heck of a lot more energy to pull away your second electron and that explains why we see lithium forming a plus one cation all right because doesn't take anywhere near as much energy to pull away one electron as it does to take away - to form a lithium 2-plus and so this is one way to tell what kind of anion will form look at the ionization energies then when you see a huge jump right when you see a huge jump that Clues you in as to as to which ions are easier to form
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