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.