How many valence electrons in each of the following?

a) Br

Answer

7 valence electrons

Explanation

If we complete the table below for a bromine atom it is

Element

Nuclear
Charge

Electron
Configuration

Inner Core
Electrons

Valence
Electrons

Br

35

1s22s22p63s23p64s23d104p5

28

7

Valence electrons are those electrons in the outer most shell. In the case of bromine the outer most shell is the n = 4 shell. There are 7 electrons in the fourth shell; the 2 in the s subshell and the 5 in the p subshell. Note the 3d electrons are inner core for bromine.