Which element is likely to be chemically unreactive? krypton (8 valence electrons) rubidium (1 valence electron) silicon (4 valence electrons) magnesium (2 valence electrons) bromine (7 valence electrons)?

Electronic configuration of elements of interest are as follows:
1) Kr = [Ar] 3d10 4s2 4p6 
2) Rb = [Kr] 5s1
3) Si = [Ne] 3s2 3p2
4) Mg = [Ne] 3s2
5) Br = [Ar] 3d10 4s2 4p5

From above listed electronic configurations of different elements, it can be seen that expect Kr, all the elements has partially filled atomic orbitals. Hence, they will be reactive in nature. So, Kr is likely to be chemically nonreactive.

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).