Ionisation enthalpies of elements of the second period are given below: Ionisation enthalpy/ kcal mol–1: 520, 899, 801, 1086, 1402, 1314, 1681, 2080. Match the correct enthalpy with the elements and complete the graph given in Fig. 3.1. Also, write symbols of elements with their atomic number.
Ionisation enthalpies of elements of the second period are given below: Ionisation enthalpy/ kcal mol–1: 520, 899, 801, 1086, 1402, 1314, 1681, 2080. Match the correct enthalpy with the elements and complete the graph given in Fig. 3.1. Also, write symbols of elements with their atomic number.

Solution:

N has a higher first ionisation enthalpy than O, despite the fact that O has a higher nuclear charge. This is because the electron in N must be removed from a more stable, exactly half-filled electronic configuration (1s2 2s2 21x2ply 2p1z) that does not exist in O.
Therefore, the first ionization enthalpy of N is higher than that of O. The symbols of elements along with their atomic numbers are given in the following graph

To match the correct enthalpy with the elements and to complete the graph, the following points are taken into consideration. As we move from left to right across a period, the ionization enthalpy keeps on increasing due to increased nuclear charge and a simultaneous decrease in atomic radius. However, there are some exceptions given below-

Despite having a higher nuclear charge, first ionization enthalpy of B is lower than Be’s. The presence of a fully filled 2s-orbital of Be [1s22s2], which is a stable electronic arrangement, accounts for this. Thus, higher energy is required to knock out the electron from fully filled 2.v-orbital. In contrast, B [1s2 2s2 2p1] has valence electrons in the 2s and 2p orbitals. It can easily lose its one e– from 2p-orbital in order to achieve noble gas configuration. Thus, first ionisation enthalpy of B is lower than that of Be.
Since the electrons in 2s-orbital are more tightly held by the nucleus than those present in 2p-orbital, therefore, ionisation enthalpy of B is lower than that of Be.