Explain the following
a) Electronegatively of elements increase on moving from left to right in the periodic table.
b) Ionisation enthalpy decrease in a group from top to bottom.

Explanation

(a) The electronegativity generally increases on moving across a period from left to right (e.g., from Li to F in the second period). This is due to decrease in atomic size and increase in effective nuclear charge. As a result of increase in effective nuclear charge, the attraction for the outer electrons and the nucleus increases in a period and therefore, electronegativity also increases. (b) On moving down a group, there is a gradual decrease in ionisation enthalpy. The decrease in ionization enthalpy down a group can be explained in terms of net effect of the following factors :
(i) In going from top to bottom in a group, the nuclear charge increases.
(ii) There is a gradual increase in atomic size due to an additional main energy shell (n).
(iii) There is increase in shielding effect on the outermost electron due to increase in the number of inner electrons. The effect of increase in atomic size and the shielding effect is much more than the effect of increase in nuclear charge. As a result, the electron becomes less and less firmly held to the nucleus as we move down the group. Hence, there is a gradual decrease in the ionisation enthalpies in a group.