Here's why:

* Electron Configuration: Bromine's electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁵.

* Outermost Shell: The outermost shell is the 4th shell (n=4).

* Valence Electrons: The electrons in the outermost shell are called valence electrons. In bromine's case, there are 2 electrons in the 4s subshell and 5 electrons in the 4p subshell, totaling 7 valence electrons.