General Trends in Group 13

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Thallium forms a compound with the formula T l I three, with one thallium and three iodines. If each iodine is minus one, thallium must be plus three, right? Hmm, not so fast, because by the end of this video you will see why not. Group thirteen trends come up often in JEE, so let's begin.

0:25Meet the boron family

So, group thirteen is the boron family: boron, B on screen, aluminium, A l, gallium, G a, indium, I n, and thallium, T l. All five have the valence configuration n s two, n p one. Here n is the outer shell, and the raised two and one count its s and p electrons. Okay? Boron is a non-metal, called a metalloid in some books, while aluminium to thallium are metals. Right?

0:54Atomic radius: the gallium surprise

Now, atomic radius, in picometres, the p m on screen. Boron is eighty five, aluminium one hundred and forty three, gallium one hundred and thirty five, indium one hundred and sixty seven, and thallium one hundred and seventy. Gallium sits below aluminium, so which should be bigger? You would say gallium, since radius normally grows down a group. But careful, at one hundred and thirty five, gallium is actually smaller than aluminium! The pointed sign means less than, so the order is boron, gallium, aluminium, indium, thallium. Clear? So why is gallium pulled in? It comes right after the ten three d elements, so it carries ten three d electrons, and these shield the nucleus poorly. So the extra nuclear charge pulls the valence shell in. Got it?

1:43Ionisation enthalpy: not a smooth fall

Next, the first ionisation enthalpy, in kilojoules per mole; on screen, mol to the minus one means per mole. Boron is eight hundred and one, aluminium five hundred and seventy seven, gallium five hundred and seventy nine, indium five hundred and fifty eight, and thallium five hundred and eighty nine. So, does it fall smoothly down the group? No, it does not fall smoothly. The sign opening the other way means greater than: boron first, then thallium, gallium, aluminium, and indium last. Again, the cause is poor shielding: by d electrons in gallium, and by four f and five d electrons in thallium. With me? Now, here is a classic trap. Gallium is smaller than aluminium in radius, but higher in ionisation enthalpy and in electronegativity. Okay?

2:34Melting point: gallium in your hand

Now melting points, in kelvin. Boron, a giant covalent solid, melts at two thousand four hundred and fifty three kelvin. Then it falls sharply down to gallium: aluminium nine hundred and thirty three, and gallium just three hundred and three. After gallium it rises again, with indium at four hundred and thirty and thallium at five hundred and seventy six. Right? Three hundred and three kelvin is so low that gallium melts in your hand. Yet it does not boil until two thousand six hundred and seventy six kelvin, the widest liquid range in the group, and one of the widest of all elements. What could such a liquid be used for? Yes, exactly, gallium is used in high-temperature thermometers. Nice, isn't it?

3:19Oxidation states: +1 grows down the group

Now the key trend: oxidation states. Boron and aluminium show plus three. But down the group, plus one becomes steadily more stable: aluminium, less than gallium, less than indium, less than thallium. So for thallium, which is more stable, plus one or plus three? Yes, exactly, plus one is the more stable state for thallium. So T l three plus, the raised three plus being its charge, is a strong oxidising agent: it readily takes electrons. Clear? E nought of M three plus to M, the small circle meaning standard, is the standard electrode potential, where M is the metal. Aluminium is minus one point six six, gallium minus zero point five six, indium minus zero point three four, and thallium plus one point two six volts, the only positive value. Got it?

4:11The inert pair effect

This trend has a name: the inert pair effect. In the heavier p-block elements, the n s two pair, the outer s electrons, is reluctant to take part in bonding. Okay? So the oxidation state two below the group valency becomes more stable down the group. Here three minus two gives plus one, our thallium story. Right? Why does the pair hold back? First, the filled d and f subshells shield it poorly. Second, large atoms form weaker bonds, so the energy from two extra bonds no longer pays for unpairing the s electrons. With me? It is not only group thirteen: T l plus here, P b two plus, lead, in group fourteen, and B i three plus, bismuth, in group fifteen. Clear? Now careful, here is a common trap. The plus one state of boron or aluminium is not due to the inert pair effect, which matters only for the heavy members, okay?

5:05Oxides: more basic down the group

Down the group, oxides and hydroxides become more basic. Boron's oxide, B two O three, and its hydroxide, BOH three, are acidic. The small numbers written low count atoms, and the brackets mean three O H groups. Got it? The oxides of aluminium and gallium are amphoteric, showing both acidic and basic behaviour, and those of indium and thallium are basic. Right?

5:30Boron is different

Now, boron is different from the rest. It is very small, with very high ionisation enthalpies, so it never forms a B three plus ion, and all its compounds are covalent. Clear? It has no d orbitals, so its maximum covalency is four, as in B F four minus, where the square brackets hold one ion. Aluminium can reach six, as in A l F six, three minus. Okay? In its trihalides, boron has only six valence electrons, so they are electron deficient and act as Lewis acids, which means electron pair acceptors. With me?

6:03Example 1: why Ga has higher IE than Al, and Tl than In

Now, example one. Why is the first ionisation enthalpy of gallium, five hundred and seventy nine, slightly larger than that of aluminium, five hundred and seventy seven kilojoules per mole? And why is thallium's larger than indium's? Pause the video and try it first. Done? Okay, let's check. Gallium has ten more protons than aluminium, but also ten three d electrons that shield poorly. So Z effective, the capital Z with the small e f f below it, rises. That is the effective nuclear charge, the pull the outer electron really feels. Right? Now look at thallium. Compared with indium, it also has fourteen four f electrons, which shield even worse: this is the lanthanoid contraction. Clear? So thallium's six p electron is held more tightly than indium's five p electron. In both cases, the cause is poor shielding. Got it?

7:00Example 2: oxidation state of thallium in TlI3

Now example two, our puzzle from the start. Thallium forms a compound of formula T l I three. What is the oxidation state of thallium in it? Pause the video and try it first. So, three iodines, each minus one, and thallium is plus three, right? Wait, what did we say about thallium in the plus three state? Yes, it is a strong oxidising agent, and that is the trap. E nought for T l three plus to T l plus is plus one point two five volts. That is far above E nought for I two, iodine, to I minus, at zero point five four volts. With me? So T l three plus would oxidise I minus, the iodide ion. That means thallium cannot stay as plus three next to iodide. Okay? The compound is thallium one triiodide, the Roman one meaning plus one. It is T l plus, with I three minus in square brackets, the triiodide ion. So thallium is plus one, and that solves our puzzle. Clear?

8:06Quick revision: Ga radius, inert pair effect, boron

So, quick revision. Gallium is smaller than aluminium, because its ten three d electrons shield poorly. The inert pair effect makes plus one more stable down the group, so T l three plus is a strong oxidising agent. And boron has no B three plus ion, a maximum covalency of four, and Lewis acid trihalides. Now try the practice problems on this page. All the best!