In this video, we'll create a molecular orbital diagram for a diatomic ion composed of atoms from the second row of the periodic table. Our problem reads: create a molecular orbital diagram for peroxide, which is O2-. We can start with the orbital diagrams for the two oxygen atoms. The electron configuration of oxygen is 1s2 2s2 2p4, so our orbital diagrams just need the 1s, 2s, and 2p subshells. In molecular orbital theory, we're combining atomic orbitals from multiple atoms to produce molecular orbitals that span across those atoms. We can combine the 1s orbitals from both of our two oxygen atoms to produce two new molecular orbitals. These orbitals are designated the Sigma 1s bonding and Sigma 1s antibonding molecular orbitals. The bonding molecular orbital is lower in energy than the antibonding molecular orbital, and the antibonding molecular orbital is designated with an asterisk. We can also combine the 2s orbitals from both of our oxygen atoms to produce the Sigma 2s bonding and Sigma 2s antibonding molecular orbitals. When we combine the three 2p orbitals from both oxygen atoms, we create six new molecular orbitals: 1 Sigma 2p bonding, 1 Sigma 2p antibonding, 2 Pi 2p bonding, and 2 Pi 2p antibonding molecular orbitals. Whether the Pi 2p or Sigma 2p bonding orbitals are lower in energy depends on the atoms in the molecule or ion. For B2, C2, and N2, the Pi 2p bonding molecular orbitals are lower in energy than the Sigma 2p bonding molecular orbitals. But for O2, F2, and Ne2, the Sigma 2p bonding molecular orbitals are lower in energy. Let's first fill in our molecular orbitals for the O2 molecule, then we'll adjust for the charge on peroxide. There are a total of 16 electrons from our two oxygen atoms, so we'll add these 16 electrons to the molecular orbitals. We start filling the molecular orbitals with the lowest energy, so we'll put two electrons in the Sigma 1s bonding molecular orbital, then two electrons go into the Sigma 1s antibonding molecular orbital. Then we fill our Sigma 2s bonding and antibonding molecular orbitals, and then the Sigma 2p bonding molecular orbital. The 2 Pi 2p bonding molecular orbitals are degenerate, meaning they have the same energy. When we fill degenerate orbitals, we fill them with one electron in each orbital first, and those electrons have parallel spins. Before we double up our electrons, we have two electrons left to add, so we have one electron each in the Pi 2p antibonding molecular orbitals. This is the molecular orbital diagram for O2. To adjust this diagram for peroxide, which is O2-, we need to add two additional electrons because of the 2- charge. So we'll put those two electrons in the Pi 2p antibonding molecular orbitals. In creating the molecular orbitals, we used up the atomic orbitals, which no longer exist. This is our molecular orbital diagram for peroxide. Now let's put this into Aleks's drawing tool. The box allows us to enter a molecular orbital. We'll start by entering molecular orbitals with the lowest energy. So our lowest energy orbital is the Sigma 1s bonding molecular orbital, and then our next lowest energy orbital is the Sigma 1s antibonding molecular orbital. We use the asterisk or star to indicate that it's an antibonding molecular orbital. And then we add our Sigma 2s bonding molecular orbital. We have to use subscripts for the 2s. And then Sigma 2s antibonding molecular orbital. For diatomic molecules with oxygen, fluorine, and neon, the Sigma 2p bonding molecular orbitals are lower in energy than the Pi 2p bonding molecular orbitals. So we create our Sigma 2p bonding molecular orbital. And then we need our Pi 2p bonding molecular orbitals. There are two Pi 2p bonding molecular orbitals. We can add a second. Next come our Pi 2p antibonding molecular orbitals. Add a second orbital. And lastly, we have our Sigma 2p antibonding molecular orbital. Now that we have all of our orbitals, we fill them from the lowest energy to the highest energy in our molecular orbital diagram. We had electrons all the way up to the Pi 2p antibonding molecular orbital. Two electrons go into each orbital, and they have opposite spins. This should be our correct molecular orbital diagram for the peroxide ion.