In organic chemistry, students learn about molecular orbitals. Molecular orbitals are combinations of atomic orbitals that result in the overall orbitals for the molecules. The number of atomic orbitals we begin with must be equal to the number of molecular orbitals that are created as a result.
Atomic orbitals can come together in constructive and destructive ways. When atomic orbitals come together constructively, it results in creation of a molecular orbital with a lower energy, bonding orbital. When atomic orbitals come together destructively, it results in a molecular orbital with higher energy, anti-bonding molecular orbital. We signify the anti bonding interaction by the change of color and the nodal plane (a place where electrons can not be found). Many reactions can be explained though the molecular orbital theory. When two molecules react with one another, we figure out the molecular orbitals for both. We then, find HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital). When the HOMO of one molecule overlaps favorably with the LUMO of another, the result is a more stable molecule and therefore this reaction is predicted to happen. However, if the HOMO and LUMO do not interact favorably, the reaction is not likely to happen and the bond will not form.
Some of the examples of molecular orbital problems are two show formation of H2. The atomic orbitals for each H atom is 1s (sphere). Therefore they can come together to make two new molecular orbitals, one bonding and one anti-boding. When we distribute electrons in molecular orbitals, we always start from the bottom (most stable orbital) and go to higher as needed. We need to remember that each molecular orbital can contain a maximum of 2 electrons.
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