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How many valence electrons are in an atom of bromine?
Bromine atom has 7 .
For a main group element, a valence electron is an electron that has the highest principal quantum number, n.
The of Br is 1s² 2s²2p⁶ 3s²3p⁶ 4s²3d¹⁰4p⁵.
The electrons with the highest quantum number are the 4s²4p⁵ electrons. Thus, Br has seven .
Of course, we could have predicted this by looking at .
Br is in Group 17, and all in Group 17 have seven valence electrons.