in this video let's look at the periodic trends for ionization energy so for this period as we go across from lithium all the way over to Neon so as we go this way across our periodic table we can see in general there's an increase in the ionization energy so lithium is positive 520 KJ per mole burum goes up to 900 K per mole and then again in general we see this increase in ionization energies going over to Neon so going across a period is an increase in the ionization energy and that's because as we go across our period there's an increase in the effective nuclear charge so increase in Z effective and remember the formula for that is the effective nuclear charge is equal to um the actual number of protons which is z and from that we subtract s which is the average number number of inner electrons shielding our outer electrons so let's examine this in more detail looking at lithium and brillium lithium has atomic number three so three protons in the nucleus so a positive3 charge and lithium's electron configuration we know is 1 S2 2s1 so two electrons in our 1s orbital and one electron in the 2s orbital brillium has one more proton and one more electron so one more proton in the nucleus so a plus4 charge and for burum the electron configuration is 1 S2 2s2 so two electrons in the 1 s orbital and then two electrons in the 2s orbital let's calculate the effective nuclear charge for both of these and first we'll start with lithium so for lithium lithium has a plus three charge in the nucleus so the effective nuclear charge is equal to positive3 and from that we subtract the average number of inner electrons shielding our outer electrons in this case we have these two inner or core electrons that are shielding our outer electron our veence electron from this full positive three charge so we know that like charges repel so this electron is going to repel this electron a little bit and this electron repels this electron and these two inner core electrons of lithium have a shielding effect they protect the outer electron from the full positive3 charge so there's two shielding electrons so for a quick effective nuclear charge calculation positive3 minus 2 gives us a value of + one for the effective nuclear charge so it's like this outer electron of lithium is feeling a nuclear charge of + one which pulls it toward the nucleus right so there's an attractive force between the outer electron and uh and our nucleus now the actual calculation for this um Z does um S I should say does not have to be an integer the actual value for lithium is approximately 1.3 but our quick crude calculation tells us positive 1 let's do the same calculation for burum so the effective nuclear charge for burum is equal to the the number of protons right which for burum is pos4 and from that we subtract the number of inner electrons that are shielding the outer electrons so it's a similar situation we have two inner electrons that are shielding this outer electron right they're repelling this outer electron shielding the outer electron from the full positive four charge of the nucleus so we say there are two inner electrons so the effect of nuclear charge is pos4 minus 2 giving us giving us an effective nuclear charge of positive2 in reality the effective nuclear charge is approximately 1.9 and that's because burum has another electron in its 2s orbital over here which does affect this electron a little bit repels it a little bit and so it actually decreases the effect of nuclear charge to about 1.9 but again for a quick calculation positive2 works so the outer electron for Brum let's just choose this one again is feeling an effective nuclear charge of pos2 which means that it's going to be pulled closer to the nucleus there's a greater attractive force on this outer electron for burum as compared to this outer electron for lithium the effective nuclear charge is only + one for this outer electron and because of this the burum atom is smaller right the 2s orbital gets smaller and the atom itself is smaller brillium is smaller than lithium so this outer electron here let me switch colors again this outer electron for burum is closer to the nucleus than the outer electron for lithium it feels a greater attractive force and therefore it takes more energy to pull this electron away from the neutral burum atom and that's the reason for the higher ionization energy so burum has an ization energy of positive 900 K per mole compared to lithiums of 520 K per mole so it has to do with the effective nuclear charge so far we've compared lithium and burum and we saw that the ionization energy went from positive 520 KJ per mole to 900 K per mole and we said that was because of the increased effective nuclear charge for burum but as we go from brillium to Boron there's still still an increased effective nuclear charge but notice our ionization energy goes from 900 KJ per mole for burum to only 800 KJ per mole for Boron so there's a slight decrease in the ionization energy and let's look at the electron configuration of boron to see if we can explain that boron has five electrons so the electron configuration is 1 S2 2s2 and 2p1 so that fifth electron of boron goes into a 2p orbital and the 2 p orbital is higher in energy than a 2s orbital which means the electron the 2p orbital is on average further away from the nucleus than the two electrons in the 2s orbital so if we just sketch this out really quickly let's say that's my 2s orbital I have two electrons in there and this one electron in the 2p orbital is on average further away from the nucleus so those two electrons in the 2s orbital actually can repel this electron in the 2p orbital so there's a little bit extra shielding there of the 2p electron from the full attraction of the nucleus right so even though we have five protons in the nucleus and a positive five charge for Boron the fact that these 2s electrons add a little bit of extra shielding means it's easier to pull this electron away so it turns out to be a little bit easier to pull this electron the 2p orbital away due to to these 2s electrons and that's the reason for this slight decrease in ionization energy as we go from Boron to carbon we see an increase in ionization energy from carbon to nitrogen an increase in ionization energy again we attribute that to increased effective nuclear charge but when we go from nitrogen to oxygen we see a slight decrease again from about 1400 K per mole down to about 1,300 K per mole for oxygen so let's see if we can explain that by writing out some electron configurations for nitrogen and oxygen nitrogen has seven electrons to think about so its electron configuration is 1 S2 2 S2 and 2 P3 so that takes care of all seven electrons for oxygen we have another electron so 1 S2 2 S2 2p4 is the electron configuration for oxygen let's just draw using orbital notation the 2s orbital and the 2p orbital so for nitrogen here's our 2s orbital we have two electrons in there so let's draw in our two electrons and for our 2 p orbitals we have three electrons so here are the two P orbitals and let's draw in our three electrons using orbital notation let's do the same thing for oxygen so there's the 2s orbital for oxygen which is full so we'll sketch in those two electrons and we have four electrons in the two P orbitals so let me draw in the two P orbitals there's one electron there's two there's three and notice what happens when we add the fourth electron we're adding it to an orbital that already has an electron in it so when I add that fourth electron to the 2p orbital it's repelled by the electron that's already there which means it's easier to remove one of those electrons so El Rons have like charges and like charges repel and so that's the reason for this slight decrease in ionization energy so it turns out to be a little bit easier to remove um an electron from an oxygen atom than nitrogen due to this repulsion in this 2p orbital from there on we see our our general Trend again the ionization energy for Florine is up to 1681 and then again for neon we see an increase in the ion is 8 energy due to the increased effective nuclear charge