The Lanthanoid Contraction and Its Consequences

Transcript

0:00Introduction

Hello everyone! Let me start with a small puzzle. Zirconium and hafnium are in the same group, and hafnium sits one full row below zirconium. One row down means one more shell, so hafnium should be clearly bigger, right? But the radius of zirconium is one hundred and sixty picometres, and the radius of hafnium is one hundred and fifty nine picometres. Hafnium is even a little smaller! So where did all that extra size go? Hold that thought, because by the end of this video you will know. This idea comes up often in JEE, so let's begin.

0:38What is the lanthanoid contraction?

So, what is the lanthanoid contraction? Look at the series from L a, lanthanum, to L u, lutetium. As we go along, the atoms get smaller overall, and their three plus ions, written L n three plus on screen, where L n stands for any lanthanoid, get smaller very steadily, step after step. This fall in the atomic and ionic radii is called the lanthanoid contraction. For the atoms the fall is not perfectly regular, so keep the word steadily for the ions. Okay? Now some numbers. L a three plus is the lanthanum ion; the small three plus written up top is its charge. Its radius is about one hundred and six picometres. The wavy equals sign means about, and p m means picometres. Follow the arrow to L u three plus, the lutetium ion, at about eighty five picometres. So the ion really does shrink across the series.

1:33The cause: poor shielding by 4f electrons

Right? Now, why does this happen? From one element to the next, Z, the atomic number, goes up by one. And the new electron goes into the four f subshell. So every step adds one more electron. Then the atom should get bigger, shouldn't it? No, and this is exactly where students go wrong. Electrons in the four f subshell shield very poorly. The f orbitals have a diffuse, spread out shape, so a four f electron does a poor job of screening the nucleus, from the other four f electrons, and from the outer electrons. With me? So what happens? The nuclear charge goes up by one at each step, but the extra shielding is small. So the effective nuclear charge, the pull the outer electrons really feel, rises. The electrons are pulled in, and the size shrinks, step after step. Clear?

2:26Five consequences of the contraction

Alright, now the consequences, and the course lists five of them. Number one: the four d and five d elements of the same group have almost the same radii. The lanthanoid contraction almost exactly cancels the extra size we expected from the new shell. And almost the same size means very similar chemistry. Got it? Now think about the lanthanoids themselves. Neighbouring lanthanoids differ in size only a little. So tell me, will they be easy to separate from one another? No, they will not, and that is number two: the lanthanoids are hard to separate. Nearly the same size means almost the same chemistry. Okay? Next, the hydroxides. On screen, L a O H three is lanthanum hydroxide, and L u O H three is lutetium hydroxide. The small three after the bracket means three O H groups. Which one is the stronger base? Yes, it is lanthanum hydroxide! Here, L n stands for any lanthanoid, and the basicity of L n O H three decreases from lanthanum to lutetium. The smaller lutetium ion polarises more strongly, so its hydroxide is more covalent, and less basic. Clear? Number four is about yttrium. Y three plus, the yttrium ion, resembles the heavy lanthanoids, the ones near the end of the series. Its size is almost equal to that of H o three plus, the holmium ion; there is our wavy equals sign again. With me? And number five: the five d elements have high ionisation enthalpies and high densities. This too is a result of the same contraction. Right?

4:10Example 1: order Ce3+, La3+, Pm3+, Yb3+ by ionic radius

Let's try one from the course. Arrange these four ions in increasing ionic radius: C e three plus, cerium; L a three plus, lanthanum; P m three plus, promethium; and Y b three plus, ytterbium. Pause the video and try it first. Done? Okay, let's check. First, the atomic numbers. Lanthanum is fifty seven, cerium is fifty eight, promethium is sixty one, and ytterbium is seventy. Okay? Now, the question says increasing radius. So we just write them in increasing atomic number, lanthanum, cerium, promethium, ytterbium, right? Oh no, careful, that is the trap! Here is the trick: in this series, the higher the atomic number, the smaller the three plus ion. That is the lanthanoid contraction. So the order of radius runs opposite to the order of atomic number. Got it? So the smallest is ytterbium, with the highest atomic number. Y b three plus is less than P m three plus, which is less than C e three plus, which is less than L a three plus. Each pointed sign on screen means less than, so the radius grows from left to right. Clear?

5:25Example 2: why Zr and Hf are so hard to separate

Now, remember our puzzle from the start? Let's finish it with a course question. Why are zirconium and hafnium so difficult to separate? Pause the video and try it first. Done? Okay, let's check. Z r, zirconium, is a four d element, and H f, hafnium, is a five d element in the same group, one row lower. So normally, hafnium should be bigger. With me? Now, from what we learnt today, tell me, what cancels that extra size of hafnium? Yes, exactly, the lanthanoid contraction! Zirconium is one hundred and sixty picometres, and hafnium is one hundred and fifty nine picometres. They are just one picometre apart. So our puzzle is solved: the contraction cancelled the extra size of the new shell. Okay? And nearly equal sizes mean almost the same chemistry; the double arrow on screen means so, or implies. Zirconium and hafnium behave almost like twins, and that is why they are so difficult to separate. So the answer is nearly equal sizes. Right?

6:36Quick revision: the cause and the main consequences

So, quick revision. The lanthanoid contraction: the radii fall from lanthanum to lutetium, very steadily for the three plus ions. The cause: poor shielding by four f electrons. And the effects: zirconium and hafnium of nearly the same size, the lanthanoids hard to separate, the basicity of the hydroxides falling, and yttrium three plus close to holmium three plus. Now go and try the practice problem on this page. You will find it easy. All the best!