Why different metals have different hardness?

There are some metals hard which indicates the presence of covalent bonds. This happens because transition metals have unpaired d-electrons. Higher the number of unpaired electrons present in the transition metals, more is the number of covalent bonds formed by them. This further increases the hardness of the metal and its strength.
Metals like chromium (Cr), tungsten (W) and molybdenum (Mo) have maximum number of unpaired d-electrons. Thus, these transition metals are very hard. While metals like zinc (Zn), cadmium (Cd) and mercury (Hg) are not very hard as they do not possess unpaired d-electrons.