It sounds like they are elements on the Periodic Table of elements.
The charge on chromium in the complex Cr(NH)Br is 3.
Cr3br
The electron configuration of bromine is: [Ar]4s23d104p5.
[Ar] 4s2 3d10 4p5
The element with electron configuration [Ar] 3d10 4s2 4p5 is Bromine (Br), which has an atomic number of 35. It belongs to the halogen group in the periodic table.
[Ar] 3d5 4s1
[Ar] 4s2 3d10 4p5
In CrBr (chromium monobromide) Cr would have an oxidation number of +1. This compound is not known perhaps you meant CrBr3, where chromium has an oxidation number of +3
The electron configuration of a chromium (Cr) 3 ion is Ar 3d3 4s0.
79-Br is more common because its abudance is 50.69%. 81-Br's abudance is 49.31% making it less common. Abudance is the rate at which the isotope is found in nature. In this case out of every 100 Bromine atoms you found 51 would be 79-Br and 41 would be 81-Br.
A half filled sub-shell is more stable than other partially filled shells. As an example use Chromium. It should by logic have the following: Sc = [Ar]4s23d1 Ti = [Ar]4s23d2 V = [Ar]4s23d3 Cr should in theory be Cr = [Ar]4s23d4 but it is not. Half filled shells are more stable so the configuration is Cr = [Ar]4s13d5 two half filled sub levels and then Manganese is Mn = [Ar]4s23d5 one full and one half filled level.
Ar and Cr will not bond. Ar is a noble gas and is already in a stable electron configuration, so it does not readily bond. Cr has a half-filled d orbital, making it stable and less likely to form bonds. Mg, P, and Cl are more likely to form bonds based on their electron configurations.