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) As a we move from left to right across a period the nuclear charge increases and the atomic radius decreases . A a result, the tendency of the atom of a element to attract shared pair of electrons towards itself increases and hence the electronegativity of the element increases.
For example electronegativity of the elements of 2nd period increases regularly from left to right as follows: `Li (1-0) , Be (1.5) , B( 2.0)` C (2.5) N (3.0) , O (3.5) ` and F (4.0)
The ionization enthalpy decreases regularly as we move from top to bottom as explained below .
( b) (i) On moving down a group from top to bottom the atomic size increases gradually to the addition of a new principal energy shell at each succeeding 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 decreases and hence the ionization enthalpy should decrease.
(ii) With the addition 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 ionization enthalpy should decrease.
(iii) Further as we move from top to bottom in a group nuclear charge increases with increases in atomic number .As a result the force of attraction of the nucleus of the valence electrons increases and hence the ionization enthalpy should increase.
The combined effect of the increase in atomic size and screening effect more than compensates the effect of the increased nuclear charge. Consequently the valence electrons becomes less and less firmly held by the nucleus and hence the ionization enthalpy gradually decreases as we move down the group.