My apologies for these fundamental questions...
I have recently learned that according to molecular orbital theory (MOT), anti-bonding sigma* 1s shells exist (in terms of potential energy) between bonding sigma 1s and 2s orbitals. This means in other words that the anti-bonding sigma* 1s orbitals are filled before the bonding 2s orbitals. One example I read used this model to illustrate the fact that He2 does not readily exist, explaining that placing electrons in the anti-bonding sigma* 1s orbitals negates the effect of the electrons first placed in the bonding 1s orbitals.
My first question, concerning He2, is: given that the change in energy from non-bonding atomic orbitals of He atoms to the bonding and anti-bonding 1s sigma orbital is the same, is not the net energy the same for bonding the same then compared to non-bonding? In other words, if putting electrons in the anti-bonding sigma* 1s orbitals results in as much negative effect as the positive effect of putting electrons in the bonding sigma orbitals; given that there is an equal number in both is this not the same net energy effect as not bonding at all?
I understand that this would not be ideal for bonding, but surely if that were true, shouldn't there be an equal likelihood of finding bonding and non-bonding He?
My second question is more far-out and perhaps more indicative of my essential lack-of-grasp of the concept of molecular bonding in general... : Why can't an two electrons (one from each He atom) form one bonding sigma 1s orbital, and two others (again one from each) from another bonding sigma 1s orbital? (which would be ideal if it were possible!) Is this the same illogical question as saying why can't two 1s atomic orbitals exist?
Thank you for you time.