Electronegativity

When two atoms of different elements form a covalent bond, the shared electron pair is rarely shared perfectly equally — one atom typically pulls it closer to itself. The measure of a covalently-bonded atom's ability to attract the shared electrons toward itself is called its ELECTRONEGATIVITY (EN). Because it isn't something that can be measured directly by experiment, several scientists developed their own numerical electronegativity scales over the years; the one used most widely today is Linus Pauling's, on which Pauling (in 1922) arbitrarily assigned fluorine — expected to have the single highest electronegativity of any element — the reference value of 4.0, with every other element's value determined relative to that anchor. Electronegativity represents the strength of the nucleus's attractive pull specifically on the shared (bonding) valence electrons of a covalent bond, so it depends directly on the effective nuclear charge experienced by those bonding electrons. It therefore INCREASES across a period, as effective nuclear charge steadily rises left to right, and DECREASES down a group, as the growing valence-shell size and growing cor …