An outer electron or outermost electron is also known as valence electron. Only outer electrons are involved in chemical reactions.

In what way do noble gases behave differently from other chemistry ? Noble gases are unreactive because they have fully filled outer shells, that is, they have duplet or octet electronic configuration.

How do atom achieve the electronic configuration of a noble gas?

When atoms lose or gain electrons, they form ions.

An ion can either carry positive or negative charge(s).

Metals form positively-charged ions(cations), Non-metals form negatively-charged ions(anions).

Positive ions or cations are formed when atoms ~lose electrons~.

Negative ions or anions are formed when atoms ~gain electons~.