There are six alkali metals:
Lithium, sodium, potassium,
rubidium, cesium and francium.
They are all soft metals which can be cut with a knife.
In air the elements quickly become coated with compounds
that form on the metal's 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, a 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 in 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 and 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 cesium, our fifth alkali metal.
