covalent bonding

Meaning: A covalent bond forms when two atoms share valence electron pairs, which lowers the energy of the system compared with the separated atoms. It usually occurs between nonmetals. Equal sharing (Cl₂) gives a nonpolar bond. Unequal sharing (HCl) puts electron density nearer the more electronegative atom, giving a polar bond with partial charges δ+ and δ−. A double bond is one sigma plus one pi bond; a triple bond is one sigma plus two pi. In a coordinate covalent bond, one atom supplies both shared electrons.

Equation: ΔH°rxn ≈ Σ(bond energies broken) − Σ(bond energies formed), where ΔH°rxn is the reaction enthalpy change (J/mol) and each bond energy is the energy needed to break that bond (J/mol).

On the MCAT: Expect rankings of bond length, strength or polarity. From single to double to triple, length decreases and bond energy increases. Other tasks include Lewis structures and formal charge, estimating ΔH from a bond-energy table, and contrasting molecular compounds (low melting points) with ionic solids.

Common confusion: Students treat covalent and ionic as two sharp categories, or assume sharing is equal. Bonding runs along a continuum set by electronegativity difference, and most bonds between different elements are polar covalent. Breaking a bond absorbs energy; forming one releases it.