Here's why:
* Bromine's electron configuration: [Ar] 3d10 4s2 4p5
* Subshells: The electron configuration tells us the order in which electrons fill subshells.
* Outermost: The highest principal quantum number (n=4) indicates the outermost shell. Within that shell, the 4p subshell is the highest energy level and therefore the outermost subshell.
