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.

Answer

A

Explanation

a) Across the period, the nuclear charge in increases and the atomic radius decreases. As a result, the tendency of the atom of an element to attract the shared pair of electrons towards itself increases and hence the electronegatively of the element increases. e.g. electronegatively of the elements of the 2nd period increases reguarly from left to righ as follows Li(0.1), Be(1.5), N(3.0), O(3.5) and:
b) The ionisation enthalpy decreases regularly as we move from top to bottom, as explained below
i) On moving down to group from top to bottom, the atomic size increases gradually due to the addition of a new principle energy shell at each succeding element. As a result, the distance between the nucleus and the valence shell increases.
In other words, the force of attraction of the nucleus for the valence electrons decreased and hence the ionisation enthalpy should decrease.
ii) With the ionisation of new shells, the number of inner shell which shield the valence electrons from the nucleus increases. In other words, the shielding effect or the screening effect increases.
As a result, the force of attraction of the nucleus for the valence electrons further decreases and hence the ionisation enthalpy should decreases.
iii) Further, in a group from top to bottom nucleus charge increases with increase inn atomic number. As a result, the force of attraction of the nucleus for the valence electrons increass and hence the ionisation enthalpy should increase.
The combined effect of the increase in atomic size and screening effect more than compensate the effect of the increased nuclear charge. Consequently, the valance electrons become less and less firmly held by the nucleus and hence the ionisation enthalpy gradually decreases as we move down the group.