Electronic Structure of Atom

Electrons move around the nucleus of an atom in shells (energy levels). Each shell (energy level) can only hold a certain number of electrons.

For the first 20 elements in the Periodic Table:

The 1^st shell can hold a maximum of 2 electrons; the 2^nd shell can hold a maximum of 8 electrons; the 3^rd shell can hold a maximum of 8 electrons.

Note: This rule applies only to the first 20 elements in the Periodic Table. The 3^rd shell can actually hold a maximum of 18 electrons.