CBSE Class 10 Answered

According to Aufbau principle states, in the ground state of the atoms, the orbital are filled in increasing order of their energies.

That means electrons first occupy the lowest energy orbital available to them and enter in the higher energy orbital only after the lower energy orbitals are filled.

The order in which the electrons are filled is :

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f and so on

As per this principle, we can observe that, the energy of 3d orbital is more than the 4s orbital. Hence after fulfilling the 3p orbital the rest two electrons of Ca enter into 4s orbital instead of 3d orbital. Hence the electron configuration is 2,8,8,2 instead of 2,8,10.