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) On moving left to right in a period, tendency of an atom to attract the shared electron pair towards itself increases due to increasing `Z_(eff)`. So, electronegativity of elements increase on moving from left to right in a period.
(b) On moving top to bottom in a group, size increases due to addition of extra shells. So, attraction of nucleus outermost electron decreases. So, ionisation enthalpy decrease in a group from top to bottom.