CBSE Class 9 Answered

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

  Sub-shells

    Number of Electrons

1 (K-shell)

   s

     2

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)