In the reaction between bromine and sodium, a bromine atom gains an electron. what ion is formed? is the bromine oxidized. or is it reduced?

, It is reduced.

Explanation:

Electronic configuration of sodium:

Sodium atom will loose one electron to gain noble gas configuration and form sodium cation with +1 charge.

Electronic configuration of bromine:

Bromine atom will gain one electron to gain noble gas configuration and form bromide ion with -1 charge.

Oxidation reaction is defined as the reaction in which a substance looses its electrons. The oxidation state of the substance increases.

Reduction reaction is defined as the reaction in which a substance gains electrons. The oxidation state of the substance decreases.