%IC = (1 − e^(−0.25 × ΔEN²)) × 100
No bond is 100% ionic or 100% covalent — most lie on a spectrum. Percent ionic character (%IC) quantifies where a bond falls. Pauling's empirical equation: %IC = (1 − e^(−0.25ΔEN²)) × 100 estimates ionic character from the electronegativity difference. Alternatively, %IC = (μ_observed / μ_ionic) × 100, where μ_ionic = e × d (charge × distance = 4.803 D·Å × bond length in Å). HF has ΔEN = 1.78, giving %IC ≈ 55% — despite being called a covalent molecule! NaCl has ΔEN = 2.23, giving %IC ≈ 74%. The 50% ionic character threshold roughly corresponds to ΔEN ≈ 1.7, the traditional cutoff between ionic and covalent bonds, though this boundary is somewhat arbitrary.