Question

A piece of chalk can be broken easily but iron can not why

Answer 1

That's because the atoms(smmalest particles of an element) are loosely arranged in a chalk piece whereas in a piece of iron the atoms are very tightly packed, and hence the iron piece is rigid(hard).
hence it cannot be broken..

Answer 2

The inter particular force keeps the particles together. The strength of this force of attraction is lesser in chalk, hence it could be easily broken into small pieces  by hammering. But the strength of inter molecular force of attraction is very strong in iron  therefore it is not possible to break them into small pieces by hammering.