How many electrons are in the outer shell of a bromine atom?

• A. 5
• B. 7
• C. 3
• D. 9

Answer: (B)

Valence electrons in bromine’s outer shell determine how many electrons sit in the fourth energy level. Bromine is in group 17, so its outermost shell has seven electrons. Its electron configuration ends with 4s2 4p5, meaning the fourth shell contains 2 electrons in the s subshell and 5 in the p subshell, totaling seven valence electrons. This seven-electron outer shell is typical for halogens, which are one electron short of a full octet and tend to gain one electron to reach eight. The other numbers don’t fit because they would miscount the s and p electrons in the outer shell or exceed the shell’s capacity.