Properties and Uses of the Lanthanoids

Transcript

0:00Introduction

Hello everyone! Let us start with a small puzzle. The lanthanoids have one characteristic charge on their ions, plus three. Many of these plus three ions are coloured. But lanthanum three plus and lutetium three plus are colourless. Same charge, so why are these two different? Keep that question in mind; we will settle it today. We will also see how reactive these metals are, and where they are used, even in the flint of a lighter. This comes up often in JEE, so let's begin.

0:32Reactive metals, much like calcium

So, let us start with the metals themselves. The lanthanoids are silvery metals, and they are reactive; the earlier members are similar to calcium in reactivity, while the later ones behave more like aluminium. Keep calcium in mind; it will guide us through this whole section. Okay? Now let me ask you this. When calcium meets water, bubbles come out. Which gas is that? Yes, it is hydrogen! And the lanthanoids do the same. With water or with dilute acids, they liberate H two, hydrogen gas; the small two written low after the H means two atoms in each molecule. Right? Heat them in oxygen and they burn to L n two O three, the oxide. Here L n, a capital L with a small n, is the general symbol for any lanthanoid, okay? So there are two metal atoms for every three oxygen atoms. With me? They even combine with N two, nitrogen gas, to give L n N, a nitride. And with C, carbon, they give carbides. Clear? They also combine with S, sulphur, and with the halogens, giving the halides L n X three, where capital X stands for any halogen. And their hydroxides, L n O H three, are basic. The brackets around O H, with the small three outside, mean three hydroxide groups for each metal. Okay?

1:56Why their compounds are mainly ionic

Next, the compounds of the lanthanoids are mainly ionic. Their ion is L n three plus, where the three plus up top is the charge. But compounds of d-block ions with the same charge, M three plus, are often covalent; M here stands for a d-block metal. So what makes the lanthanoid ion different? It is the size, yes! L n three plus is a large ion. Its charge is spread over a big volume, so it has low polarising power: it hardly distorts the electron cloud of the anion. So the bonds stay ionic. Clear?

2:32Colour and magnetism of the Ln³⁺ ions

Now let us come to colour, and to our puzzle. Many L n three plus ions are coloured, because of f to f transitions. The little dash between the two letters f means an electron jumps from one f orbital to another, taking up some of the light. Okay? So tell me, for which ions is such a jump impossible? Yes, it is the two extremes! With f zero, there is no f electron to jump; the small number up top counts the f electrons. With f fourteen, all fourteen places are full, so there is nowhere to jump. So lanthanum three plus, f zero, and lutetium three plus, f fourteen, are colourless. That settles our puzzle. Right? Now let us look at magnetism. Paramagnetic means the ion has unpaired electrons, so a magnet attracts it. In f zero there are no f electrons, and in f fourteen all of them are paired. So all the ions except f zero and f fourteen are paramagnetic. With me? Now, here is a trap that comes up often. Gadolinium three plus is four f seven: seven electrons in the f subshell of the fourth shell. That is half-filled, and half-filled means extra stable. So such a stable ion should be diamagnetic, right? That is exactly the trap, so be careful. Half-filled means one electron in each of the seven f orbitals, so seven unpaired electrons. So gadolinium three plus is paramagnetic. Stable does not mean diamagnetic. Got it?

4:07Uses of the lanthanoids

Finally, let us look at the uses. The first is misch metal, an alloy of about ninety five percent lanthanoid metals and five percent iron; the percent sign means parts per hundred. Okay? It also contains traces of sulphur, carbon, calcium and aluminium, the four symbols on screen. Misch metal goes into magnesium alloys, which are used to make bullets, shells, and lighter flints. So the lighter flint from the start of the video really does use these metals. With me? Next, the oxides of the lanthanoids are used as catalysts in petroleum cracking. And they are used as phosphors in T V screens. Got it?

4:49Example 1: which Ln3+ ions are colourless and diamagnetic

Now Example one, an easy one, and it takes us back to our puzzle. Which of lanthanum three plus, cerium three plus, gadolinium three plus and lutetium three plus are colourless and diamagnetic? Pause the video and try it first. Done? Okay, let's check. First, count the f electrons, using the configurations from earlier in this chapter. Lanthanum three plus is four f zero, cerium three plus is four f one, gadolinium three plus is four f seven, and lutetium three plus is four f fourteen. Okay? Four f one has one unpaired electron, and four f seven has seven. So cerium three plus and gadolinium three plus are paramagnetic, and they drop out. That leaves the f zero and f fourteen ions, with no unpaired electrons and no f to f transition. So the answer is lanthanum three plus and lutetium three plus, colourless and diamagnetic, the same two ions as in our puzzle. Clear?

5:53Example 2: lanthanoid metal with water, nitrogen and carbon

Now Example two, a little harder. What happens when a lanthanoid metal is heated with, part a, water, part b, nitrogen, and part c, carbon? Pause the video and try it first. Done? Okay, let's check. The key idea is simple: the lanthanoids behave like very reactive metals, similar to calcium. With me? So with water, in part a, they give the hydroxide, L n O H three, and hydrogen gas, H two, is set free. Okay? Now let us take part b, nitrogen. Nitrogen gas is quite unreactive. So with nitrogen there is no reaction, right? That is the trap, so be careful. These metals are reactive enough to combine even with nitrogen, and they give L n N, a nitride. Right? And in part c, with carbon, they form carbides. So we get the hydroxide and hydrogen, then a nitride, then carbides, exactly what a very reactive metal does. Got it?

6:58Quick revision: reactivity, colour, magnetism and uses

So, quick revision. First, these metals are reactive like calcium: hydrogen from water, the oxide L n two O three, the nitride, the carbides and the halides. Second, the large L n three plus ion gives mainly ionic compounds, and the f zero and f fourteen ions are colourless and diamagnetic. And third, misch metal goes into magnesium alloys, the oxides are catalysts, and they are used as phosphors. Now go and try the practice problem on this page. All the best!