How do you find the protons, neutrons, and electrons for a Bromine (#1^-#) anion with a mass number of 80?
The atomic number is the number of Protons add one for the number of electrons and 80 - 35 is the number of neutrons.
The atomic number is the number of protons. This is the number that never changes for an element (or else the chemical properties will change!).
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. But Bromine anion with a charge of -1 has one extra electron so 35 +1 = 36 electrons.
36 electrons is the number of electrons in the stable inert gas Krypton. (That's why ions are more stable than atoms and atoms tend to form ions.)
The atomic mass is the sum of the number of protons and the number of neutrons (as electrons weigh almost nothing) so
Protons + neutrons = Mass putting in known values
n = 45