covalent bonds
Summary
A covalent bond forms when two atoms share one or more pairs of electrons, usually between nonmetals. The most testable point: going from single to double to triple bonds between the same atoms, the bond gets shorter and stronger (higher bond energy).
Detail
Meaning: Atoms share electron pairs so each can reach a stable valence shell, usually an octet (hydrogen takes two; boron and expanded octets are exceptions). Sharing is equal in a nonpolar bond (Cl₂) and unequal in a polar bond (H–Cl), where the more electronegative atom bears a partial negative charge. In a coordinate covalent bond, one atom donates both electrons. A single bond is one sigma bond; a double bond is sigma plus pi; a triple bond is sigma plus two pi.
Equation: ΔH°rxn ≈ Σ(bond energies of bonds broken) − Σ(bond energies of bonds formed), where ΔH°rxn is the enthalpy change in J/mol and each bond energy is a positive value in J/mol.
On the MCAT: Rank C–C, C=C, and C≡C by length (triple shortest) and strength (triple strongest), or estimate ΔH from a bond-energy table. Predict bond polarity from electronegativity, then use VSEPR geometry to decide whether bond dipoles cancel (CO₂ nonpolar, H₂O polar).
Common confusion: Polar bonds do not guarantee a polar molecule. Also, melting or boiling a molecular substance overcomes intermolecular forces, not its covalent bonds; network solids such as diamond are the exception, because covalent bonds run through the whole solid.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
- Principles of Modern Chemistry
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.