The correct answer is Br-, the bromide ion. It has 35 protons
in the nucleus (or it wouldn't be bromine) but it has the extra
electron giving it a full outer shell, with 36 electrons, the same
as Krypton which is a noble gas. Since it has one more
electron than it has protons, it has a net charge of -1.
The electron configuration is just the way the electrons in an
atom are arranged in the various quantized shells, or orbitals
or energy levels or whatever you want to call them, around
the nucleus.
Hope this helps.
