Successive Ionisation Enthalpies

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Sodium and magnesium sit side by side in the periodic table, and magnesium has one more proton in its nucleus. So magnesium should hold its electrons a little more tightly, right? Now look at the energy needed to pull out the second electron. For magnesium, it is one thousand four hundred and fifty one kilojoules per mole. For sodium, it is four thousand five hundred and sixty two, more than three times as much! So why does sodium hold on so hard to its second electron? Today we will find out, with the idea of successive ionisation enthalpies. Questions on this come up often in JEE, so let's begin.

0:44One electron at a time

Alright, let's start with the first ionisation enthalpy. Take a gaseous atom, called X; the small g in brackets means gas. The arrow shows one electron being pulled out, the e minus on screen, and what remains is the ion X plus, with a small raised plus sign. The energy needed is delta i H one: capital Greek delta, a subscript i for ionisation, capital H for enthalpy, and a subscript one for the first electron. Okay? But we do not have to stop at one electron. From X plus, we can pull out a second electron and get X two plus, with a small raised two plus. That energy is delta i H two, the second ionisation enthalpy. Right? And a third electron, taken from X two plus, gives X three plus. That energy is delta i H three, the third ionisation enthalpy. Clear? These values, taken one after another, are called successive ionisation enthalpies. The k J mol to the power minus one on screen means kilojoules per mole, the unit for all of them. Got it?

1:57Each electron costs more than the last

Now, a question for you. The first electron is gone, and what is left is a positive ion. So tell me, will the second electron be easier to remove, or harder? Harder, yes, exactly! Read the line with me. Delta i H one is less than delta i H two, which is less than delta i H three; the less than signs show the order, and the three dots mean the pattern goes on. And notice the word always on screen. Every next value is larger. With me? And the reason is simple: each electron is removed from a more positive ion. The more positive the ion, the harder it pulls on the electrons that remain. So every next electron costs more energy.

2:42The very large jump

Okay? Now, the values always rise, but do they rise by similar amounts? Think about this. We keep pulling out the outer electrons, the valence electrons. What happens once they are all gone? Yes, now we must dig into the core! Once the valence electrons are gone, the ion has the configuration of a noble gas, and the next electron must come from that noble-gas core. Core electrons sit much closer to the nucleus, so at this point the value takes a very large jump. Clear? So here is the key idea. Count the electrons removed before the jump. That count is the number of valence electrons. Right? And the valence electrons tell us the group. The double arrow on screen means gives, and each small arrow means goes with group. So one valence electron means group one, and two mean group two, and the dots mean the list goes on. But for three, careful, because three valence electrons means group thirteen, not group three. Students lose easy marks on this. Got it?

3:48Example 1: group from 738, 1451, 7733 kJ per mol

Now, example one. An element has its first three ionisation enthalpies as seven hundred and thirty eight, then one thousand four hundred and fifty one, and then seven thousand seven hundred and thirty three, all in kilojoules per mole. To which group does it belong? Pause the video and try it first. Done? Okay, let's check. The first value is seven hundred and thirty eight, and the second is one thousand four hundred and fifty one. It nearly doubles! So the big jump comes after the first electron, and this is group one, right? No, no, wait, that is exactly the trap! Every next value is larger anyway, so some rise is normal. One thousand four hundred and fifty one minus seven hundred and thirty eight equals seven hundred and thirteen, and that is just a normal rise, okay? Now the next step. Seven thousand seven hundred and thirty three minus one thousand four hundred and fifty one equals six thousand two hundred and eighty two. Compare that with the seven hundred and thirteen we had before! So this is the very large jump. Clear? So the jump comes after two electrons. Two electrons were removed before it, so the element has two valence electrons. With me? And two valence electrons means group two. In fact, these are the values of magnesium. Remember its second value, one thousand four hundred and fifty one, because we need it again in a minute. Very good!

5:28Example 2: second ionisation enthalpy of Na vs Mg

Now a slightly harder one, and it is our puzzle from the start. Why is the second ionisation enthalpy of sodium, four thousand five hundred and sixty two, far larger than that of magnesium, which is one thousand four hundred and fifty one kilojoules per mole? Pause the video and try it. Here is a hint: write the configuration of each ion. Done? Okay, let's check, starting with sodium. Sodium, written with the symbol N a, is neon core, three s one. The neon in square brackets stands for the electron arrangement of neon, and the small raised one means one electron in the three s subshell. Remove that electron, and sodium plus is just the neon core, a noble-gas configuration. Okay? So now tell me, where must the second electron of sodium come from? From the neon core, exactly! Sodium plus has no electron left outside the neon core, so the second electron must come from the core itself. That is why the value is so large, four thousand five hundred and sixty two. Right? Now magnesium, written M g. Magnesium is neon core, three s two, with two electrons outside the core. Remove one, and magnesium plus is neon core, three s one. It still has a three s electron. Got it? So the second electron of magnesium is still a three s electron, a valence electron, not a core electron. That is why its value is only one thousand four hundred and fifty one. So our puzzle is solved: what matters is where the electron comes from, the valence shell or the core. Clear? And see how this fits our rule. The big jump for sodium comes after one electron, so it is in group one. For magnesium, it comes after two, so it is in group two, just as in example one. Very good!

7:26Quick revision: rising values, core jump, valence and group

So, quick revision. Successive ionisation enthalpies always increase, because each electron is pulled from a more positive ion. There is a very large jump when the next electron must come from the noble-gas core. And the electrons removed before the jump are the valence electrons, which give the group, such as group one, group two or group thirteen. Now go and try the practice problem on this page. Find the jump, and the group follows. All the best!