Explain the rule according to which electrons are filled in various energy levels.

Distribution of electrons into various orbits was suggested by Bohr and Bury

1. The maximum possible number of electrons in a particular shell is given by the formula 2n², where n is the serial number of that shell.
   First shell or K shell has (2 x 1²) = 2 electrons
   Second shell or L shell has (2 x 2²) = 8 electrons
   Third shell or M shell has (2 x 3²) = 18 electrons
   Fourth shell or N shell has (2 x 4²) = 32 electrons
   and so on.
2. Electrons are not accommodated in a given shell unless the inner shells are filled. In other words the shells are filled in stepwise manner.
3. The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more than 8 electrons.
4. The outermost shell of a chemically stable atom can accommodate a maximum of 8 electrons, except Hydrogen and Helium atoms, which have only one shell, and can thus accommodate a maximum of 2 electrons only.