Question

Why are metals like chromium and molybdenum very hard?

Answer 1

Hint: Chromium and molybdenum belong to the block of the periodic table having atomic number 24 and 42 respectively. Both the transition metals belong to the same group, group 6 having maximum number of unpaired electrons in the 3d and 4s subshell for chromium and 4d, 5s subshells in molybdenum.

Complete step by step answer:
The d orbitals having unpaired electrons overlap to form covalent bonds which results in hardness of the metal. The increase in the number of covalent bonds due to increase in number of unpaired electrons result in an increased bond strength.
Due to the large number(six) of unpaired electrons, their interaction with other metals is strong. This results in covalent bonding by sharing of unpaired electrons of these metals with other metals. This increases the bond strength and results in hardness of the metal.
When added to cast irons and steel molybdenum enhances its strength, toughness, elevated temperature strength, corrosion resistance strength and hardenability. Chromium is the third hardest element found after carbon and boron. However, it is extremely brittle and rarely used in pure form.

Hence metals like chromium and molybdenum are very hard.

Note:
Chromium- molybdenum is an alloy used in oil and gas energy construction. These are used because they produce creep resistant steel capable of withstanding temperatures. They work so well together due to their combined properties. Molybdenum gives higher working temperatures and chromium results in exceptional oxidation and corrosion resistance.