Explain the following:
$(a)$ Electronegativity of elements increases on moving from left to right in the periodic table.
$(b)$ Ionisation enthalpy decreases in a group from top to bottom.

(N/A) As we move from left to right across a period,the atomic number increases,which means the nuclear charge increases while the number of shells remains the same. This leads to a stronger attraction between the nucleus and the valence electrons,causing the atomic radius to decrease and the electronegativity to increase.
$(b)$ As we move down a group,the number of shells increases,which increases the atomic radius. The valence electrons are placed at a greater distance from the nucleus,and the shielding effect of inner electrons increases. Consequently,the effective nuclear charge experienced by the valence electrons decreases,making it easier to remove an electron,which results in a decrease in ionisation enthalpy.