Question

for example can a chromium atom pair up with a helium atom ,helium as 2 electron and chromium has 6 valence electrons??

Answer 1

Helium will never pair up with anything. It is a noble gas, therefore it's "octect" (duet in heliums case), is already satisfied. As far as your question goes though, there is no good answer. The answer depends on the equilibrium constant for that chemical reaction. You can read more about that here - http://en.wikipedia.org/wiki/Chemical_equilibrium - Hopefully that will help. If you get further into chemistry it will become much clearer.