12. Molecular Shapes & Valence Bond Theory
Molecular Orbital Theory
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This example, we need to construct the molecular orbital diagram for the dihelium cation, which is HE 2+. 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 2 heliums. Remember, heliums have atomic numbers of 2. We'd also say that helium, they each have 2 valence electrons, so that'd be 2 times 4 2 times 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 dithium 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 1 elements like we are here, then we'd start out with 1s. If it's period 2 element, it's 2 s, and if it's period 3, it's 3 s. Step 3, we follow the three principles, so Aufbau principle, Pauli exclusion principle, and Hund'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 3 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 1 up, 1 down, and then we still have one more electron left that we need to fill in, and it would go here in the anti bonding molecular orbital. So this is what we would say our bonding our molecular orbital diagram would resemble. It would look like this. K, where we would fill in totally the bonding molecular orbital that is sigma 1s, and have half filled my anti bonding molecular orbital, which is sigma star 1s.
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