For most elements, a complete outer most shell consists of eight electrons. The tendency of atoms to acquire an outer most shell of eight is called octet rule (from the Greek word 'octo' means eight).
A few elements tend to acquire an outer most shell of any two electrons, like the lightest noble gas, helium. This tendency is called the duplet rule.
Atoms of different elements tend to acquire their octet (or
duplet) in the outermost shell in different ways. They need to
- lose electrons
- or gain electrons
- or share electrons