* Sodium (Na) has one electron in its outermost shell. It wants to lose this electron to achieve a stable electron configuration.

* Chlorine (Cl) has seven electrons in its outermost shell. It wants to gain one electron to achieve a stable electron configuration.

When sodium and chlorine bond:

* Sodium donates its one electron to chlorine.

* Sodium, having lost an electron, becomes a positively charged ion (Na+).

* Chlorine, having gained an electron, becomes a negatively charged ion (Cl-).

Therefore, after accepting the electron, chlorine has a negative charge (-1).