Q.The elements Li, B, C, Be and N have the electronegativities 1.0, 2.0, 2.5, 1.5 and 3.0, respectively on the Pauling scale.

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).

• Electronegativity generally increases across a period (left to right) and decreases down a group, following broadly the same trend as ionisation energy and electron affinity, for the same reasons (increasing nuclear charge / decreasing atomic size across a period; increasing atomic size and shielding down a group).

• The difference in electronegativity between two bonded atoms determines the polarity of the bond: a large difference gives an ionic or highly polar covalent bond, while a small or zero difference gives a non-polar (or purely covalent) bond.