Question

some materials are hard why​

Answer 1

because their molecules are densely packed

Answer 2

ANSWER:

The irregularities in the organization of atoms in material make it harder.    

EXPLANATION:

The bonding of atoms in such elements is harder. This is the factor that makes alloys harder than those of elements that are composed of pure metals.

For example:

Brass is an alloy that is comprised of copper, zinc is harder whereas pure gold is softer. Hardness in metal could be increased by raising the grain boundaries in the metal.