Here's why:
* Electron Configuration: Bromine's electron configuration is [Ar] 3d10 4s2 4p5.
* Outermost Shell: The outermost shell is the fourth shell (n=4).
* Valence Electrons: The electrons in the outermost shell are called valence electrons. In this case, there are 2 electrons in the 4s subshell and 5 electrons in the 4p subshell, totaling 7 valence electrons.
