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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.