The right side is 臭 (foul-smelling) - and bromine is famous for its harsh, choking red fumes. The water radical on the left marks it as one of the liquid elements, so the character almost spells out 'the smelly liquid.'
The water radical in its three-drop left form, from 水. Among the chemical elements it marks those that are liquid at room temperature - bromine is one of only two. It groups 溴 with other liquid-element characters.
Supplies the sound, read as xiù here. It doubles as a meaning hint: this element gives off a sharp, foul odor, matching the 'smell' sense this element carries elsewhere.
