When a sodium atom reacts with a bromine atom, the sodium atom loses an electron to the bromine atom, forming a sodium ion and a bromide ion. The electron that is transferred from the sodium atom to the bromine atom fills the outermost electron shell of the bromine atom, making it a stable ion.

$$Na(g) + Br_2(g) \longrightarrow Na^+(g) + 2Br^{-}(g)$$

In this reaction, the sodium atom is oxidized because it loses an electron, while the bromine atom is reduced because it gains an electron.