covalent

Meaning: Atoms in a covalent bond share one or more electron pairs, usually nonmetals bonding to nonmetals, so each atom approaches a filled valence shell. Sharing is equal when the atoms have the same electronegativity (nonpolar, as in H₂ or Cl₂) and unequal when they differ (polar, as in H–Cl), which gives partial charges δ+ and δ−. Going from single to double to triple bonds, bond length decreases and bond energy increases. In a coordinate covalent bond one atom supplies both electrons, as in NH₄⁺ or Lewis acid–base adducts.

Equation: ΔH°rxn ≈ Σ(energies of bonds broken) − Σ(energies of bonds formed), where ΔH°rxn is the reaction enthalpy change in J/mol and each bond energy is in J/mol. Breaking bonds absorbs energy; forming bonds releases it.

On the MCAT: Expect questions that rank bond polarity by electronegativity difference, compare length and strength of C–C, C=C and C≡C, draw Lewis structures and predict shape with VSEPR, or estimate ΔH from a bond-energy table. Covalent compounds are usually molecular, with lower melting points than ionic solids.

Common confusion: Covalent does not mean equal sharing. A polar bond is still covalent, and the covalent–ionic distinction is a continuum, with very large electronegativity differences behaving ionically. Also, polar bonds do not guarantee a polar molecule: CO₂ has polar bonds but no net dipole because its linear geometry cancels them.