Question

Explain the variation in ionisation enthalpies across a period and down the group.

Answer 1

Variation in ionisation enthalpies: Ionisation enthalpy increases with increases in atomic number along the period. Some elements show irregularities in this trend due to the type of electron to be removed and the extra stability of the exactly half-filled and completely filled electronic configurations.
Variation along the period: In the second period Li to Be, the ionisation enthalpy increases due to increased nuclear charge and smaller atomic radius of Be as compared that of Li. For Be to B, although the nuclear charge of B is more than that of being, yet the ionisation enthalpy of B is lower than that of Be. For B to C to N, the first ionisation enthalpy of these elements keeps on increasing due to progressively increasing nuclear charge and decreasing atomic radius. For N to O, the first ionisation enthalpy of oxygen is lower than that of N although the nuclear charge of O is higher than that of nitrogen. For O to F to Ne, the first ionisation enthalpy increases from O to F to Ne because of the increasing nuclear charge. Similar variations in the first ionisation enthalpies of the elements of the third period have been observed.