Wed January 26, 2011 By: Saloni Agrawal

In K, L,M, N shells,in the N shell it can accommodate 32 electrons. So why the outermost shell can accommodate only 8 shells?

Expert Reply
Thu January 27, 2011
Dear Student
The shells are divided into subshells also.

An atom's electron shells are filled according to the following theoretical constraints:

  • Each s subshell holds at most 2 electrons
  • Each p subshell holds at most 6 electrons
  • Each d subshell holds at most 10 electrons
  • Each f subshell holds at most 14 electrons
  • Each g subshell holds at most 18 electron
In K we have only 1 subshell that is s, so maximum number of electrons in K shell is 2
In L we have 2 subshell that is s and p, so maximum number of electrons in L shell is 2+ 6= 8
In M we have 3 subshell that is s, p and d so maximum number of electrons in M shell is 2+6+10.
In N we have s, p, d and f subshells, so maximum number of electrons in N shell is 2+6+10+14 = 32 electrons.
During filling of electrons the following rules are considered.
  1. Orbitals are filled in the order of increasing n+l;
  2. Where two orbitals have the same value of n+l, they are filled in order of increasing n.
So, we follow this order for filling the orbitals:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and 7p


We hope that clarifies your query.
Ask the Expert