2.3 Elements on parade: an audiovisual interlude

There are six alkali metals; lithium, sodium, potassium, rubidium, caesium and francium. They're all soft metals which can be cut with a knife. In air the elements quickly become coated with compounds that form on the metals' surface. Here for example is lithium. When we slice it you can see the metallic lustre, but the black coating quickly reappears.

Sodium is kept under oil to prevent reaction with air. Again when we cut it the metal surface can be seen but this time corrosion occurs even more quickly.

With the next alkali metal, potassium, the corrosion in air is so quick that it's hard to see the metallic lustre at all. As we go down the group the elements seem to react more quickly with air.

Now let's see another reaction of the alkali metals, the reaction with water. We'll start with lithium. The metal floats on the water and reacts with it, giving off hydrogen gas.

Now for sodium; the same sort of thing happens although the reaction is a bit more vigorous. All the alkali metals react with water in the same way. Let's see an equation for the reaction. Hydrogen gas is produced and the metal dissolves to give an aqueous cation with a single positive charge.

Now for potassium; this time you'll see a flame. The heat given out by the reaction is produced so quickly that the hydrogen gas catches fire. It burns with a lilac flame.

The next element is rubidium. this time we put a safety screen between us and the reaction. You can see that things gradually become more terrifying as we go down the group.

Let's try caesium, our fifth alkali metal.

Here are the four halogen elements, fluorine, chlorine, bromine and iodine. Fluorine on the left is almost colourless. Next comes chlorine which is greenish yellow. Fluorine and chlorine are both gases at room temperature. But bromine is a liquid and iodine is a solid; even so, they're both quite volatile. You can see the coloured vapours - orange-red above the liquid bromine and purple above the solid iodine.

Whether solid, liquid or gas, the halogen elements consist of diatomic molecules; F2, Cl2, Br2 and I2.

Fluorine reacts, often ferociously, with almost everything else. A stream of fluorine instantly sets iron wool on fire and it does the same thing to charcoal. Now lets see the reaction with hydrogen. There's hydrogen in the balloon. A jet of fluorine gas pierces the balloon and explodes with the hydrogen inside. Let's see it again in slow motion. Now you can see the fireball more clearly.

Now the reaction between hydrogen and chlorine. I light the hydrogen at the jet and lower it into a jar of chlorine. The hydrogen carries on burning but with a pale blue flame.

When the halogens react with hydrogen, the hydrogen halides are produced; HF, HCl, HBr and HI. In each compound the halogen has a valency of one. All four compounds are gases.

They're also very soluble in water. Here's water swallowing a jar of hydrogen chloride.

Now let's see how the halogens react with aluminium. When aluminium powder meets fluorine gas the reaction's over in a flash.

To get chlorine to react with aluminium, I'll heat some foil in a stream of the gas. The white fumes are aluminium trichloride.

With bromine I just drop aluminium foil into the liquid. There's a short delay but normal service is soon resumed. Once the reaction gets going it quickly takes on the appearance of the pit of hell. As with the chlorine reaction there's white smoke; that's aluminium tribromide.

The reaction with iodine is just as spectacular. A little warming helps to get things moving, but once the reaction's off it needs no further assistance.

The products of all four of these reactions are white solids and they're all trihalides, AlF3, AlCl3, AlBr3 and AlI3. Now aluminium is a trivalent element so in these compounds the halogens are all showing a valency of one. That valency of one is also satisfied when the halogen atoms bind to one another in the diatomic molecules with which I started this sequence.

This emphasis on the formulae of compounds; on the ratios in which atoms combine and what Mendeleev knew as the valencies of the elements lies at the heart of the periodic law. Without it the law would often seem ridiculous. For example look at these elements, silicon and carbon. They lie at the top of Group IV.

One form of carbon is diamond. Now you may have read somewhere that diamonds are forever. After you've seen this, you'll never believe that again. (a diamond is heated and placed in liquid oxygen)

So when diamonds burn they form a gas. As you can see it's quite heavy. Now we could have burned silicon in the same way but the product wouldn't have been a gas it would have been this. This is the oxide of silicon. It's some sand that I picked up on a Norfolk beach. So when carbon and silicon are burnt in oxygen the products look completely different; they have completely different structures too but they both have the same kind of formula. They're both dioxides. In both compounds there are two oxygen atoms for every carbon or silicon atom. It was this similarity in the formulae of the highest oxides that led Mendeleev to put carbon and silicon in the same group.

It's not difficult to bring a little colour to the chemistry of vanadium. If I dissolve sodium vanadate in dilute acid I get this solution. It's yellow because it contains the ion (VO2)+. I'm going to shake this solution with zinc amalgam which is made by stirring up zinc with mercury. The first new product has a rich blue colour. This is the ion VO2+. More shaking gives us the bottle green tripositive ion V3+. There's one more stage and a lot of shaking to go. Eventually the lavender colour of the aqueous dipositive ion V2+ appears. Here's a shot of the starting material and our three products. Chemistry doesn't come much prettier than this.

In this section we give you the chance to look more closely at the lanthanide elements and their chemistry. For now let’s shrink the main table.

First of all, the lanthanides are all metals. Here's a selection. Here’s lanthanum … samarium … dysprosium.

If it's heated in chlorine dysprosium forms a trichloride, and all the other lanthanide metals do the same. Chlorine oxidizes all of them to the +3 oxidation state. But the simplest way of reaching this oxidation state is to dissolve the metals in dilute acid.

Here's praseodymium. Hydrogen is evolved, and if the reaction is done in air, then whatever the lanthanide metal, the final product is the aqueous tripositive ion. In this case it's green.

Compounds or ions in other oxidation states are much less stable. For instance the dipositive aqueous ion of samarium can be made. It's blood red. But it's quickly oxidized by water or hydrogen ions to the pale yellow tripositive ion. The highest known oxidation state of any lanthanide element is +4. The most stable example is cerium(IV) which occurs in cerium dioxide. This is almost colourless. Sulfuric acid converts the dioxide to a sulfate of cerium(IV). It consists of orange crystals. When the orange sulfate is dissolved in water, the orange tetrapositive aqueous ion is formed. But even cerium(IV) species are easily reduced to the +3 state. We'll use hydrogen peroxide as the reducing agent and add it to the orange aqueous ion. Oxygen is evolved as cerium(IV) is reduced. The colourless product is the aqueous tripositive ion of cerium.

So for all of the lanthanides the +3 state is very stable to oxidation or reduction. the lanthanide series which begins with lanthanum and ends with ytterbium is a set of unusually similar elements. Take a little time to study the pictures and information that we've provided on the different metals. And as you examine the reactions with acid ask yourself how these elements differ from the reactions that you studied earlier in the transition metal program.

The aqueous tripositive ion of uranium is an intense claret colour. When air is bubbled through it, oxidation occurs. A green solution of uranium(IV) is formed. Let’s take some of this uranium(IV) and add nitric acid. On heating the solution gradually turns yellow. This is a solution of the uranyl ion; uranium in oxidation state six. So these are the common oxidation states of uranium in aqueous solution. Yellow uranium(VI), green uranium(IV) and claret-coloured uranium(III).

Here's the key step in nuclear fuel reprocessing. A mixture of uranium and plutonium in kerosene oil has been shaken with hydroxylamine and nitric acid in water. Now it's all settling out. Before shaking, the kerosene solution at the top was green. Now it's yellow. That's because it now contains only uranium as yellow uranyl dinitrate. The plutonium has moved into the lower aqueous layer as blue plutonium trinitrate. When the bottom layer is run off you have a solution of the blue, aqueous, tripositive ion of plutonium