chemical bonds

Meaning: Atoms bond because the bonded arrangement has lower potential energy than separated atoms. Ionic bonds form by electron transfer, usually between a metal and a nonmetal, giving cations and anions held by electrostatic attraction. Covalent bonds form by sharing electron pairs between nonmetals. If the sharing is unequal, the bond is polar covalent, with partial charges (δ+ and δ−) on the atoms. Metallic bonding is a lattice of cations in a sea of delocalized electrons, which explains conductivity and malleability.

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

On the MCAT: You may be asked to rank bonds by polarity using electronegativity difference (a large difference suggests ionic, a moderate one polar covalent, near zero nonpolar covalent). You may compare bond length, bond energy and bond order: a higher bond order (single → double → triple) means a shorter, stronger bond. Passages may also ask you to predict properties, such as high melting points and conduction when molten or dissolved for ionic solids. Questions may also use Lewis structures, formal charge, or ΔH estimated from bond energies.

Common confusion: Ionic and covalent are ends of a continuum, not separate categories. Also, intermolecular forces (hydrogen bonds, dipole–dipole, London dispersion) are not chemical bonds. They are much weaker, and they are what change in boiling or melting, not the covalent bonds inside the molecule.