The electron configuration of BROMINE is 1s² 2s²2p⁶ 3s²3p⁶ 4s²3d¹⁰4p⁵, wherein the electrons with the highest quantum number are the 4s²4p⁵ electrons. Thus, Br has seven valence electrons.
By also searching for BROMINE on the Periodic Table, we could see from there that it belongs to Group 17 (in which all elements have seven valence electrons.)
Now that we know the number of valence electrons in all, we then apply the rule that inorder to achieve stability, the valence shell must have eight electrons.
7 + x = 8
x = 8 - 7
x = 1
Thus, BROMINE needs only ONE valence electron to have a full valence shell.