What happens to the Cl atom when it becomes part of a stable molecule NaCl?

It looses an electron

Explanation:

We must remember that sodium chloride is an ionic compound. In an ionic compound, a metal is often bonded to a non metal.

The metal looses electron(s) and the nonmetal accepts electron(s). The metal forms its cation while the non metal forms its anion.

Thus an ionic compound is basically a pair of ions held by electrostatic interaction.

Thus, when chlorine becomes part of sodium chloride, it gains one electron from sodium and becomes Cl^-

< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >