Explain the variation of electronegativity across the period and down the group

Variation of electronegativity across the period

Electronegativity increases across a particular period from alkali metal to halogen

Reason

Across the period an additional electron is added on the same energy level and a proton is added to the nucleus; both nuclear attraction and screen effect increase. But an increase in nuclear charge exceeds an increase in the screening effect leading to  a progressive increase in effective nuclear charge that exerts a contracting effect on the outer electron shells.

Variation of electronegativity down the group

Electronegativity decreases

Reasons

• An increase in atomic size reduces the nuclear attraction on the outermost electron shell
• From one element to another down the group there is addition of a complete electron shell the electrons shield the outer electrons from the nuclear charge.

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