CBSE Class 10 Answered

Calcium has 20 electrons and m shell is a 3rd energy level.

Its electronic configuration is 1s²2s²2p⁶3s²3p⁶ 4s² instead of 1s²2s²2p⁶3s²3p⁶ 3d¹4s¹.

This is because of Afbau’s principle which states that electrons fill orbitals starting at the lowest available (possible) energy levels before filling higher levels (e.g. 1s before 2s and 4s before 3d). As the energy of 3d orbital is higher than 4s, the last two electrons in calcium get accomadated in 4s energy level instead of 3d.

It is because of this order in which the atomic orbitals are filled as stated in Afbau’s principle that the electronic configuration of calcium is 2, 8, 8, 2 and not 2, 8, 9, 1 or 2, 8, 10.