Given below are the plots of first ionization enthalpy (Δi H) of elem

1.	Given below are the plots of first ionization enthalpy (Δi H) of elements of the second period (Fig. 1) and Δi H of alkali metals as a function of Z (Fig. 2). Give reasons for the following:(a) First ionization enthalpy generally increases as we go across a period and decreases as we descend a group.(b) Ionization enthalpy of boron is slightly less than that of beryllium.(c) Ionization enthalpy of nitrogen is more than that of oxygen.
Answer» (a) First ionization enthalpy generally increases as we go across a period due to increase in nuclear charge and decreases as we descend a group due to increase in atomic size. (b) Ionization enthalpy of boron is slightly less than of beryllium because in boron, 2s-orbital is complete and its valence electron (2p¹) is shielded by the 2s electrons so less energy is required to remove the 2p electron from a boron atom than is required to remove the 2s electron from a beryllium atom. (c) Ionization enthalpy of nitrogen is more than that of oxygen because in nitrogen, half – filled p-orbital (2p³) is present which is more stable than 2p⁴of O-atom, so more energy is required to remove electron from 2p orbital of nitrogen than that of oxygen.

Given below are the plots of first ionization enthalpy (Δi H) of elements of the second period (Fig. 1) and Δi H of alkali metals as a function of Z (Fig. 2). Give reasons for the following:(a) First ionization enthalpy generally increases as we go across a period and decreases as we descend a group.(b) Ionization enthalpy of boron is slightly less than that of beryllium.(c) Ionization enthalpy of nitrogen is more than that of oxygen.

Answer»

(a) First ionization enthalpy generally increases as we go across a period due to increase in nuclear charge and decreases as we descend a group due to increase in atomic size.

(b) Ionization enthalpy of boron is slightly less than of beryllium because in boron, 2s-orbital is complete and its valence electron (2p¹) is shielded by the 2s electrons so less energy is required to remove the 2p electron from a boron atom than is required to remove the 2s electron from a beryllium atom.

(c) Ionization enthalpy of nitrogen is more than that of oxygen because in nitrogen, half – filled p-orbital (2p³) is present which is more stable than 2p⁴of O-atom, so more energy is required to remove electron from 2p orbital of nitrogen than that of oxygen.

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