Question

A piece of chalk can be broken down into small pieces by hammering but a iron piece can't why

Answer 1

this is because chalk is brittle and has bits of air spaces between itself.
iron is hard and it's molecules are tightly packed.

Answer 2

Chalk pieces can be broken down into small pieces because they had no strong bonds i.e. weak forces of attraction or vander waal force of attraction.
But metals like iron has strong force of attraction
think so it will help you