Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

The following rules are followed for writing the number of electrons in different energy levels or shells for the first eighteen elements:

1. The maximum number of electrons in each shell or orbit is determined by the formula 2n², where n is the shell number.
   K shell, n=1, no. of electrons = 2 x 1² = 2
   L shell, n=2, no. of electrons = 2 x 2² = 8
   M shell, n=3, no. of electrons = 2 x 3² = 18
2. Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.
3. The atom's outermost shell needs 2 electrons for stability (duplet rule) if it has only one shell. If it has multiple shells, it requires 8 electrons (octet rule) in the outermost shell for stability.