Average Bond Energies

Important Notes

• Bond energies are averages over many compounds, so estimates of ΔH are approximations; enthalpies of formation give more accurate values when available.
• ΔH ≈ Σ D(bonds broken) − Σ D(bonds formed). Breaking bonds costs energy (positive); forming bonds releases it.
• For the same pair of atoms, more bonds = stronger and shorter: C—C 345 kJ/mol (1.54 Å) → C=C 611 (1.33 Å) → C≡C 837 (1.20 Å).
• Length Basis / Context distinguishes tabulated averages from lengths for a named molecule or crystal. Numbered references identify the source of each added length. A molecule-specific length is not a universal average and is not necessarily measured in the same species as the average bond energy.
• Lengths vary with molecular environment, phase, temperature and measurement convention. For example, the O—I entry is the vibrationally averaged HOI length (1.9941 Å); its inferred equilibrium length is 1.9874 Å. The P—S entry describes the longer ring bonds, not the shorter terminal bonds in the same ion.
• This table is a general reference; averaged bond energies vary somewhat from one compound to another.

Verification references: OpenStax Chemistry 2e, §7.5 Strengths of Ionic and Covalent Bonds (Tables 7.2 and 7.3); CRC Handbook of Chemistry and Physics, 2007, and Olmsted & Williams, Chemistry, 5th ed. (2011), via Chempendix; [1] NIST CCCBDB, experimental geometries and diatomic lengths (SRD 101); [1] NIST CCCBDB, experimental geometries in the experiment/theory comparison; [1] NIST CCCBDB, experimental geometries in the experiment/theory comparison; [1] NIST CCCBDB, experimental geometry of trans-diazene; [1] NIST CCCBDB, experimental data for silicon tetrabromide; [1] NIST CCCBDB, experimental coordinates of phosphorus tribromide; [2] Synthesis and properties of phosphanyl triphenylsilanes and pentaphenyldisilanes (2025); [3] Mitzel and Losehand, Dimethyl-, Disilyl- and Digermylsulfide (2004); [4] Kolonits and Hargittai, Molecular Structure of Silicon Tetraiodide (1998); [5] Aminophosphine structure study, Dalton Transactions (2015), author manuscript; [6] Rotter, Matching Phosphorus and Chalcogens (2009), chapter II, section 2.1; [7] Ketelaar and Palmer, Electron Diffraction of Nitrosyl Chloride and Bromide (1937), pp. 2629-2633; [8] Andrew R. Barron, Rice University, Halides of Phosphorous, Table 1; [9] Hirota, Electron Diffraction Investigation of Sulfur Monochloride and Monobromide (1958), Table III; [10] Ozeki and Saito, Submillimeter-wave spectra of hypoiodous acid (2004).