Question

Why does the rusting of iron make it thinner?

Answer 1

The oxide layer chips off.

Explanation:

When iron rusts, the part that's exposed to the air is reacting with oxygen in the air in this reaction:

`4Fe + 3O_2 -> 2Fe_2O_3`

This process of returning metals to their ore states is called corrosion.

Usually, when metals corrode, a thin layer of the product of the corrosion reaction (`Fe_2O_3` for iron) forms at the surface of the metal and protects the inside.

But for iron, this oxide layer (`Fe_2O_3`) chips off very easily.
When it chips off, some un-corroded inner parts are exposed; these parts another oxide layer.
So, eventually, the amount of iron will become less and less because new oxide layers keep chipping off.