As we move from left to right in a period, the size of the atoms decreases due to the increase in the effective nuclear charges on the outermost electron. As a result of which electronegativity of elements increases on moving from left to right in the periodic table.
(b) As we go down the group the atomic size increases which results in the increase in the distance of the electrons in the outermost shell as a result of which effective nuclear charge decreases. This results in the decrease of ionisation enthalpy.
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