Q.Explain why electronegativity increases across a period from left to right but decreases on descending a group.

Electronegativity is the relative ability of a bonded atom to attract the shared electron pair of a covalent bond towards itself, and it is measured on scales such as the Pauling scale.

When two atoms form a covalent bond, they share a pair of electrons, but the sharing is rarely perfectly equal — one atom usually attracts the bonding electron density more strongly than the other. Electronegativity is defined as the tendency (or relative power) of an atom, when bonded in a molecule, to attract the shared pair of electrons towards itself.

Key points:

• It is NOT a fixed, measurable physical property like ionisation energy; it is a relative number assigned on an arbitrary scale (most commonly the Pauling scale, where fluorine, the most electronegative element, is assigned a value of about 4.0). …