Bromine is most likely to form a bromide ion, which is an anion. This is because bromine is a halogen and has seven valence electrons. To achieve a stable electron configuration, it gains one electron, resulting in a negatively charged ion (anion) with a charge of -1.
In chemical terms, bromine (Br) gains an electron to form the bromide ion (Br–). This process is typical for halogens, which are found in Group 17 of the periodic table. The tendency to gain an electron makes bromine an electronegative element, and the resulting bromide ion is more stable than the neutral bromine atom.
So, to answer the question, bromine forms an anion, specifically the bromide ion (Br–).