Ok, so I know the difference between bonding and antibonding orbitals (one the electrons add destructively and the other they add constructively) but what I don’t understand is how we know which electrons will add destructively and which electrons will add destructively. For this wouldn’t we need to know the phase of the individual electrons? For example my chemistry book says that when two H atoms combine the first two electrons will go into the bonding pair…… but why! Doesn’t this mean that the electrons are in phase? And if so how do we know the first two are in phase? It then gives a theoretical example of He2 and why it doesn’t exists saying that the first two electrons go into the bonding pair and the second two go into the antibonding pair. How do we know that the third and fourth valence electrons go into the antibonding pair? My book gives some nifty looking energy level diagrams for these situations and shows how the antibonding pair is higher in energy than the bonding pair but I still don’t understanding why the electrons fall into the bonding pairs in that order. For example why couldn’t the first two go into the bonding pair and then the second two also go into the bonding pair as well?