Here's why:

* Bromine's position: Bromine is in Group 17 (also known as VIIA) of the periodic table.

* Valence electrons: Group 17 elements have 7 valence electrons.

* Octet rule: Atoms tend to gain, lose, or share electrons to achieve a stable configuration of 8 valence electrons (like the noble gases).

Therefore, bromine needs one more electron to complete its octet and achieve a stable electronic configuration. It achieves this by forming a single covalent bond or gaining an electron to form an anion (Br⁻).