In this example, we need to construct a molecular orbital diagram for the dihelium cation, which is HE2+. All right, so the way we do this is we're going to determine the number of valence electrons for both elements. Here we're dealing with two helium's. Remember, helium's have atomic numbers of two. We also say that helium, they each have two valence electrons, so they'd be two times four 2 * 2, which is 4 valence electrons. But then here we're going to say this plus one charge means we've lost an electron. So in total we have 3 valence electrons for the diphthelium cation.
What we do next is we construct the molecular orbital diagram based on the location of the valence electrons. If we're dealing with period one elements like we are here, then we start out with 1S. If it's period two element it's 2S, and if it's. 3 It's 3S Step 3:00. We'd follow the three principles so off off principle, Poly exclusion principle, and Hun's rule, and fill in the molecular orbitals from bottom to the top.
If we come back up here to our image here, we're dealing with three valence electrons. So we'd actually erase one of these and we have 3 total valence electrons, which we would then distribute into these molecular orbitals. So we'd start out by filling out the lowest energy one according to Aufbau principle. So go one up, one down and then we still have one more electron left that we need to fill in and we go here in the anti bonding molecular orbital.
So this is what we would say are bonding. Our molecular orbital diagram would resemble it would look like this where we would fill in totally the bonding molecular orbital that is Sigma 1S and have half build my anti bonding molecular orbital which is Sigma star 1S.