Question

Ionisation enthalpies of elements of second period are given below: Ionisation enthalpy/ k cal mol^–1:

520, 899, 801, 1086, 1402, 1314, 1681, 2080.

Match the correct enthalpy with the elements and complete the graph given in figure. Also write symbols of elements with their atomic number

Answer 1

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- 

• 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 [1s²2s²] which is a stable electronic arrangement. Thus, higher energy is required to knock out the electron from fully filled 2. v-orbital. While B [1s² 2s² 2p¹] 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.

• 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 (1s² 2s² 2¹_x2p^l_y 2p¹_z) which is not present in O (1s² 2s² 2p²_x 2p¹_y 2p¹_z).
  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