(a) Use orbital diagrams to illustrate what happens when an oxygen atom gains two electrons. (b) Why does \(\mathrm{O}^{3-}\) not exist?

The atomic number of an oxygen atom is 8. This means that an oxygen atom has 8 electrons in its various orbitals. The electron configuration of an oxygen atom can be determined by following the Aufbau principle, which states that electrons fill the lowest available energy levels first. As a result, the electron configuration of an oxygen atom is: \[1s^{2} 2s^{2} 2p^{4}\]

An oxygen atom has 8 electrons, distributed in the 1s, 2s and 2p orbitals. The 2p orbitals have 4 electrons, so when an oxygen atom gains two electrons, these additional electrons will be added to the 2p orbitals. The orbital diagram for an oxygen atom is: 1s: ↑↓ 2s: ↑↓ 2p: ↑↑↑ When an oxygen atom gains two electrons, the orbital diagram becomes: 1s: ↑↓ 2s: ↑↓ 2p: ↑↓↑↓↑ As a result, the electron configuration for an oxygen atom that has gained two electrons is \(1s^{2} 2s^{2} 2p^{6}\), also written as \[\mathrm{O}^{2-}: [He] 2s^{2} 2p^{6}\]

In order for the oxygen ion \(\mathrm{O}^{3-}\) to exist, it would need to gain three electrons, which would require electrons to occupy the 3s orbital. However, before an electron occupies a higher energy level, the lower energy level must be completely filled. In the case of the oxygen atom, there are no available spaces in the 2p orbitals after gaining two electrons, and adding one more electron to the 3s orbital would significantly increase the energy of the system. The \(\mathrm{O}^{3-}\) ion would be highly unstable due to the increased electron-electron repulsion and higher energy state. Thus, it is very unlikely for the oxygen ion \(\mathrm{O}^{3-}\) to exist because it would not be energetically favorable.