Overview

Transcript

0:00Introduction

Hello everyone! Okay, let us start with two statements, written in the JEE Main style. Statement one says that across a period, the first ionisation enthalpy generally increases. Statement two says that magnesium has a higher first ionisation enthalpy than aluminium, the element right after it. Aluminium comes after magnesium, so these two cannot both be true, right? Hold that thought; example one will settle it. Five chapters on periodicity are done, and now it is time to test yourself. This chapter gives you three sets of mixed problems in the JEE Main style, so let us see how to use them. Let's begin.

0:44How this chapter works

So, what do JEE Main questions on periodicity ask for? There are three types. First, the correct order of a property, for example the order of radii. Okay? Second, the odd one out in a set. And third, the truth of paired statements, where you decide which of the statements are true. Right? Quick question for you. Our two statements from the start: which of these three types is that? Paired statements, yes, exactly! Now look at the three sets. Set one is easy, with eight solved problems; Set two is medium, with eight; and Set three is hard, with six. Clear? Each set then ends with practice problems for you to try on your own. Got it? And the real JEE Main questions, from twenty twenty one to twenty twenty six? You will find them at the end of Chapters one to five. The problems here are original, written in the same style. With me?

1:43Trend first, then the exceptions

Now, look at this graph from Chapter three, for the first twenty elements. Up the side is delta i H one, the first ionisation enthalpy: the Greek capital delta, a small i for ionisation, H for enthalpy, and a small one for the first electron. It is in kilojoules per mole; the small raised minus one after mol means per mole. Along the bottom is capital Z, the atomic number. Across a period, the value generally rises: see the climb from lithium to neon. Okay? Down a group, it generally falls. Look at lithium, sodium and potassium, at the bottom of each drop: each one is lower than the one before. Right? But look closely at that climb from lithium to neon. It is not smooth. Where does it dip? At boron, and at oxygen, the point just after nitrogen, very good! Boron is where the outer electron first goes into a p subshell, so the first dip comes where s changes to p. That p electron is shielded by the s pair, so it comes off easily. Clear? And at oxygen, nitrogen's p three becomes p four; the small raised numbers count the p electrons. Nitrogen's half-filled p three is extra stable, and in oxygen's p four, the paired electron is repelled. So here is the trap: the trend breaks exactly at these spots, and many order questions test them. Got it?

3:08Radii: a quick one from Set 2

Now, a quick one on radii, from Set two. The small raised signs on screen show the charges on these ions. So tell me, which is larger, sodium plus or magnesium two plus? And which is larger, O two minus or S two minus? Let us take the first pair. Sodium has eleven electrons and loses one; magnesium has twelve and loses two. So both ions have ten electrons, which makes them isoelectronic. Right? For isoelectronic species, the radius falls as the nuclear charge rises. The atomic number, Z, is eleven against twelve, so magnesium two plus is pulled in tighter. The double arrow means so: sodium plus is the larger ion. Clear? Now take the second pair. Sulfur has the higher nuclear charge, sixteen against eight. So by the same rule, S two minus is the smaller one, right? No, no, wait, that is the trap! Count the electrons first. O two minus has eight plus two, that is ten; S two minus has sixteen plus two, that is eighteen. They are not isoelectronic, so that rule does not apply here. Okay? Instead, use the group trend. Sulfur sits below oxygen in group sixteen, with one more shell. So S two minus is the larger ion. Got it?

4:34Example 1: Mg or Al, higher first ionisation enthalpy

Now, example one, from Set one, and it settles our opening puzzle. Which has the higher first ionisation enthalpy, magnesium or aluminium? Pause the video, and try it first. Done? Okay, let's check. Aluminium comes right after magnesium in period three, with one more proton. Across a period the value rises, so aluminium is higher, and statement two is false, right? Oh no, careful, that is the trap! Write the configurations first. Magnesium is neon core, three s two; the neon in square brackets stands for the electron arrangement of neon. Aluminium is neon core, three s two, three p one. Clear? So magnesium must lose a penetrating three s electron, one that gets close to the nucleus. But aluminium loses its three p electron, shielded by the three s pair, so it comes off more easily. It is the same s to p dip we saw at boron. With me? So for delta i H one, magnesium is greater than aluminium, and statement two is true. And statement one says generally increases; that word generally leaves room for exceptions. So both statements are true, and our puzzle is solved. In paired statements, read every word.

5:55Example 2: group from successive ionisation enthalpies

Okay? Now example two, a step harder: a medium problem from Set two. The successive ionisation enthalpies of an element are eight hundred and one, then two thousand four hundred and twenty seven, then three thousand six hundred and sixty, and then twenty five thousand and twenty six, all in kilojoules per mole. Successive means one electron after another. Identify its group. Pause the video, and try it first. Done? Okay, let's check. From eight hundred and one to two thousand four hundred and twenty seven, the value grows about three times! So the big jump comes after the first electron, and this is group one, right? No, no, wait, that is the trap! Every next value is larger anyway, because each electron is removed from a more positive ion. Two thousand four hundred and twenty seven minus eight hundred and one equals one thousand six hundred and twenty six. Three thousand six hundred and sixty minus two thousand four hundred and twenty seven equals one thousand two hundred and thirty three. These are normal rises. Okay? Now look at the last rise. Twenty five thousand and twenty six minus three thousand six hundred and sixty equals twenty one thousand three hundred and sixty six. Compare that with the rises before! This is the very large jump: the fourth electron must come from the noble-gas core. With me? So the jump comes after the third electron, and the double arrow means so: three valence electrons. And three valence electrons means group three, right? Oh no, careful, that is the second trap! Groups one and two are the s block, and the p block starts at group thirteen. So for a main-group element, three valence electrons means group thirteen, not group three. In fact, these are the values of boron, at the top of group thirteen. So find the jump, count the electrons before it, and then name the group carefully. Got it?

7:59Quick revision: question types, exceptions, count electrons

So, quick revision. Three question types: order, odd one out, and paired statements, so read every word. Use the trend first, then check the exceptions, where s changes to p, and where p three becomes p four. And count the electrons: are the ions isoelectronic or not, and how many come off before the big jump? Now open Set one, and begin. All the best!