What causes the anomalies in the ionisation energies of elements like oxygen and nitrogen?

The anomalies in the ionisation energies of elements like oxygen and nitrogen are caused by electron configuration and electron pairing.

The ionisation energy of an element is the energy required to remove an electron from a gaseous atom or ion. Generally, ionisation energy increases across a period from left to right and decreases down a group. However, there are some exceptions to this trend, such as in the case of oxygen and nitrogen.

Nitrogen has a higher first ionisation energy than oxygen, which seems counterintuitive as we move from left to right across the periodic table. This anomaly is due to the electron configuration of these elements. Nitrogen has a half-filled 2p subshell (2p3 configuration), which is a particularly stable arrangement. This stability means that it requires more energy to remove an electron from nitrogen than from oxygen.

On the other hand, oxygen has a 2p4 configuration, with one of the 2p orbitals containing two paired electrons. Paired electrons in the same orbital experience a repulsion effect, which makes it easier to remove one of them. Therefore, the first ionisation energy of oxygen is lower than that of nitrogen.

Another anomaly occurs in the second ionisation energies of oxygen and nitrogen. The second ionisation energy of oxygen is lower than that of nitrogen. This is because, after losing one electron, oxygen has a half-filled 2p subshell, which is a stable configuration. Therefore, it requires less energy to remove a second electron from oxygen than from nitrogen.

In summary, the anomalies in the ionisation energies of elements like oxygen and nitrogen are due to the stability of half-filled subshells and the repulsion effect of paired electrons in the same orbital. Understanding these anomalies helps us to better understand the behaviour of these elements and their chemical properties.