In the reaction between bromine and sodium, a bromine atom gains an electron and forms the bromide ion, br–.

what happens to bromine?

After the bromine atom gains an electron and becomes a negatively charged ion, Br-, it is attracted to the Na atom, the sodium atom which as a result of the sodium atom giving its 1 valence electron to bromine, becomes a positively charged ion.

Both the sodium ion and bromine ion are attracted to each other and form an ionic bond. The electrostatic attraction between the ions allows for an ionic bond to form after donation of valence electrons from one atom to another. Ionic structures are usually ionic lattices.