Here's why:

* Electron Configuration: Bromine's electron configuration is [Ar] 3d¹⁰ 4s² 4p⁵.

* Valence Electrons: Valence electrons are the electrons in the outermost energy level (shell) of an atom. In bromine's case, the outermost shell is the 4th shell (4s² 4p⁵).

* Counting Valence Electrons: The 4s subshell has 2 electrons and the 4p subshell has 5 electrons, giving a total of 2 + 5 = 7 valence electrons.