i. The d-orbitals containing unpaired electrons may overlap to form covalent bonds which are responsible for the hardness.
ii. As the number of unpaired electrons increases, the number of covalent bonds and the strength of the metallic bonds increases. The increase in the number of covalent bonds result in increase in the strength and hardness of metal.
iii. Cr, Mo and W have maximum number of unpaired d-electrons which makes them very hard due to increase in the number of covalent bonds.
iv. Zn, Cd, and Hg on the other hand do not have unpaired d-electrons. Hence, they are not very hard.
