Explain distribution of electrons in different shells with examples.

Asked by singhaivk05 | 27th Jan, 2015, 06:59: PM

Expert Answer:

Electrons tend to arrange themselves around nuclei so that they have the lowest possible energy. They would all like to get into the lowest energy level, sometimes called the K-shell, but are prevented from doing so by some rules that pop up in quantum mechanics. You can see how electrons are arranged in a particular atom by taking a look at our Periodic Table of Elements.

Example: The electronic configuration of Oxygen is 1s22s22p4


Thus, there are four shells in an atom namely K, L, M, N. In these shells, electrons are added in a systematical manner with the formula 2N2.

Energy Level


    Number of Electrons

1 (K-shell)



2 (L-shell)

   s, p

     8 (2 in s, 6 in p)

3 (M-shell)

   s, p, d

     18 (2 in s, 6 in p, 10 in d)

4 (N-shell)

   s, p, d, f

     32 (2 in s, 6 in p, 10 in d, 14 in f)

The rules of placing electrons within shells is known as the Aufbau principle. These rules are:

  1. Electrons are placed in the lowest energetically available subshell.
  2. An orbital can hold at most 2 electrons.
  3. If two or more energetically equivalent orbitals are available (example, p and d) then electrons should be spread out before they are paired up (Hund’s rule).

Answered by Prachi Sawant | 28th Jan, 2015, 11:28: AM