Question

Why are metals like chromium and molybdenum very hard?

Answer 1

• The d-orbitals containing unpaired electrons may overlap to form covalent bonds which are responsible for the hardness.
  
• 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 results in an increase in the strength and hardness of the metal.
• $Cr$ and $Mo$ have a maximum number of unpaired d-electrons which makes them very hard due to an increase in the number of covalent bonds.