![]() Ignoring the core electrons is pretty common if you recall, in atomic electron configurations we might write 2s 22p 4 instead of 1s 22s 22p 4 for oxygen we were ignoring the core. They are buried a little deeper in the atom, and they don't play a very important role in bonding. That's because if there are any 2s electrons, then those 1s electrons are really core electrons, not valence. Most of the time, we aren't going to see both the σ 1s and the σ 2s displayed in the diagram. These spherical orbitals would combine very much like 1s orbitals, and we would get a similar diagram, only at a slightly higher energy level. The next lowest set of atomic orbitals is the 2s level. Note that we have not added any electrons to that molecular orbital energy diagram yet, but when we do, we will just fill them in from the bottom up, just like we would an atomic orbital energy diagram. Altogether, the picture says that the 1s orbital on one atom and the 1s orbital on the other atom can combine in two different ways, producing the lower-energy, bonding σ 1sand the higher-energy, antibonding σ 1s*. The sides of the diagram just refer back to where those molecular orbitals came from, with dotted lines to guide you from one place to another. The order of energy so far is σ 1s, σ 1s*. It is analogous to the atomic orbital energy diagram (which goes 1s, 2s, 2p, 3s.). The middle of the diagram is just the molecular orbital energy diagram. What we see here is a molecular orbital interaction diagram. So, in a molecule, the lowest-energy molecular orbitals would be the ones formed from the lowest-energy atomic orbitals, the 1s orbitals. There are some departures from that rule, sometimes, but that's the simplest place to start. To a great extent, the order of molecular orbitals in energy can be considered to follow from the order of the atomic orbitals from which they are constructed. Just as we think of there being a progression of atomic orbitals from lowest energy to highest (1s, 2s, 2p, 3s.), we can organize these molecular orbitals by order of their energy. ![]() ![]() ![]() So far, we have looked at the ways in which pairs of atomic orbitals could combine to form molecular orbitals - to form bonds. ![]()
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