Here's why:

* Electronegativity: Bromine is a nonmetal with a relatively high electronegativity. This means it has a strong tendency to attract electrons.

* Octet Rule: Bromine needs to gain one electron to achieve a stable octet (8 electrons) in its outer shell.

* Ion Formation: To achieve this stable configuration, bromine readily accepts an electron, forming a negatively charged ion called a bromide ion (Br⁻).