A chlorine atom has 7 valence electrons in its third shell.

What will it do when forming an ion?

I will take due molecule of it

Explanation:

because it does not have a complete shell

The chlorine atom will accept a lone pair of electron from another element in other to complete its outermost shell

Explanation:

The chlorine atom will accept a lone pair of electron from another element in other to complete its outermost shell.

Chlorine being an unstable element will accept a lone pair of electron from another element like sodium to form a negatively charged ion (acceptor) thereby becoming stable. Sodium being the donor will easily donate its lone pair of electron to chlorine through covalent bonding thereby becoming the donor and forming a positively charged ion (Na+).

Note that for an element to be stable, such element may have a obey the duplet or octet rule I. e must possess 2 electrons or 8 electrons in its outermost shell.