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Classification of Elements and Periodicity in Properties — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryClassification of Elements and Periodicity in Properties5 Marks
Question
Ionisation enthalpies of elements of 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 also write symbols of elements with their atomic number.
✓

Answer

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 simultaneous decrease in atomic radius. However, there are some exceptions given below:
  1. In spite of increased nuclear charge, the first ionisation enthalpy of B is lower than that of Be. This is due to the presence of fully filled 2s-orbital of Be $[1s^22s^2]$ which is a stable electronic arrangement. Thus, higher energy is required to knock out the electron from fully filled 2.v-orbital. While $B [1s^2 2s^2 2p^1]$ contains valence electrons in 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.
  1. The first ionisation enthalpy of N is higher than that of O though the nuclear charge of O is higher than that of N. This is due to the reason that in case of N, the electron is to be removed from a more stable, exactly half-filled electronic configuration $\big(1\text{s}^2\ 2\text{s}^2\ 2\text{p}^1_\text{x}\ 2\text{p}^1_\text{y}\ 2\text{p}^1_\text{z}\big)$ which is not present in $\text{O}\big(1\text{s}^2\ 2\text{s}^2\ 2\text{p}^2_\text{x}\ 2\text{p}^1_\text{y}\ 2\text{p}^1_\text{z}\big).$
Therefore, the first ionisation enthalpy of N is higher than that of O. The symbols of elements along with their atomic numbers are given in the following graph.

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