When chlorine gains an electron from sodium, it becomes a chloride ion (Cl-). Chlorine atoms have seven valence electrons, meaning they need one more electron to fill their outer shells and become stable. Sodium atoms, on the other hand, have one valence electron, which they easily give up to other atoms. When sodium and chlorine atoms come into contact, the sodium atoms transfer their valence electrons to the chlorine atoms, forming positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). These ions are then attracted to each other by electrostatic forces, forming sodium chloride (NaCl), a stable ionic compound known as table salt.