Electronegativity is the attraction of a bonded atom for the pair of electrons in a covalent bond.
One of the bonded atoms may have a greater attraction for the shared pair of electrons than the other. The result is that the shared pair of electrons is not shared evenly and the electron cloud is distorted the bond becomes polar.
If the electronegativity difference is large enough (greater than 1.8) the more electronegative atom gains control of the electrons and the bond will become ionic rather than covalent.Electronegativity is measured on the Pauling scale.
Electronegativity increases across a period and up a group. Across a period, nuclear charge increases and atomic radius decreases. Up a group, atomic radius and shielding decrease. In both cases the nuclear attraction for the bonded pair of electrons increases.
