Question

we know that M shell have capacity to hold 18 electron. But if we will do electronic configuration of calcium then it will be 2,8,8,2 having 20 electrons. But why it is not 2,8,9 as M shell have capacity to hold 18 electrons.

Answer 1

K shell contains   1s orbital containing 2 electrons at most.

L shell contains    2s and 2p orbitals.
    So L shell can hold  2 + 6 = 8 electrons in total.

M shell contains 3s , 3p and 3d orbitals. 
    So M shell can hold , 2 + 6 + 10 electrons = 18 electrons in total.

For Calcium having an atomic number = 20, the configuration is :

         1s2,  2s2  2p6, 3s2 3p6 4s2
         K = 2, L= 8, M = 8,  N = 2.

It is not   K = 2, L = 8, M = 10 or    K=2  L=8, M = 9 & N =1.

The reason is that there is a rule for filling electrons in the orbitals.  It is based on the energy levels of the orbitals.

We find that energy level of 3d orbital is more than that of 4s orbital. Hence only after 4s is filled, electrons are filled in 3d orbitals.  Lower energy orbitals are filled first.

This principle is called Aufbau principle.  The electron energy can be mentioned informally by adding principal quantum number n and secondary quantum number l.    So n+l decides the order of filling of orbitals as the atomic number goes up.