Metallic Bonding

In metals, positive metal ions are held together by electron clouds. This is known as metallic bonding.

These electrons are free to move through the structure, this is why metals conduct electricity.

This can explain the change in melting points as you go down group I.

The melting points decrease as you go down the group.

The atoms get larger as you go down the group (because they have more shells of electrons).

The group I metals all have one electron in the outside shell.

Think of this as the glue that holds the metal structure together.

If we have small atoms and a certain amount of glue,
we will have fairly big attractions between the atoms and so the melting point will be quite high.

If we have large atoms and the same amount of glue
(still one electron available from the outside shell)
there will be less attraction and so the melting point will be lower.

This can also help to explain why the group I metals are soft.
Hardness depends on the attraction between the atoms.
Lots of attraction means the metal will be hard.
None of the group I metals can be described as “hard”
but as you go down the group, they get even softer.