For Bromine the atomic number is 35. so number of protons = 35
In a neutral atom the number of protons = the number of electrons. 35 protons = 35 electrons.
The atomic mass is the sum of the number of protons and the number of neutrons (as electrons weigh almost nothing) so
Protons + neutrons = Atomic mass
atomic mass of br is 80
35+n=80 to solve for n subtract 35 from both sides.
35−35+n=80−35 which gives
n = 45.
Therefore no of neutrons=45.
All you need to do to find electronic configuration is work your way across the periodic table filling the orbitals as you go
The full version of this is: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5.
Bromine is of halogen group and hence have 7 valence electrons hence valency is 7. Also from the electronic configuration we can see that outermost shell has 7 valence electron from 4s2 4p5 configuration.
Read it carefully and try to understand everything carefully.Hope this will help you.