[Monium101]

Hello everyone! 

I have a question regarding about the molecular orbital theory, how could one predict whether a certain atoms exist, lets say for example, if I were asked to predict whether or not He2+ existed?? I understand the contents of the MO diagram where one would draw out the atomic orbitals of that particular atom but how do we determine whether those electrons occupy the σ bonding MO OR σ* antibonding MO. I know helium in this case would have 3 electrons so how would the MO diagram be depicted?? Any help is greatly appreciated!!! 

[kriggy]

http://en.wikipedia.org/wiki/Aufbau_principle
Basicaly if you draw the molecular orbital diagram then you can fill the electrons starting with those with lower energy.