ΔEN determines ionic vs covalent character.
Electronegativity (EN) measures an atom's ability to attract shared electrons in a bond. The Pauling scale ranges from 0.79 (Cs) to 3.98 (F). The difference in EN between two bonded atoms determines bond character: ΔEN < 0.5 = nonpolar covalent, 0.5–1.7 = polar covalent, > 1.7 = ionic. Pauling's equation estimates percent ionic character: %IC = (1 − e^(−0.25×ΔEN²)) × 100. In polar covalent bonds, the more electronegative atom carries a partial negative charge (δ−) creating a dipole moment. Trends: EN increases left→right across a period (increasing Zeff) and decreases down a group (increasing atomic radius). Electronegativity is central to predicting molecular polarity, reactivity, acid strength, and intermolecular forces.