bond-dissociation energy
/ BDE /
Every chemical bond is a grip of a certain strength, and snapping it costs a certain amount of energy — just as it takes more effort to pull apart a strong magnet than a weak one. The bond-dissociation energy is exactly that price: the energy needed to break one specific bond, cleanly splitting it so each atom keeps one of the shared electrons, with everything in the gas phase. A bigger number means a stronger, harder-to-break bond.
More precisely, the bond-dissociation energy (BDE) is the energy required to break a particular bond by homolysis, the even split that gives two radicals each carrying one electron. It is usually reported in kilojoules or kilocalories per mole. Because breaking a bond always costs energy, BDEs are positive; conversely, forming that same bond releases the same energy. Two related ideas go hand in hand with bond strength: a stronger bond is usually a shorter one (the atoms are pulled closer), and bond strength rises from single to double to triple bonds (more shared electron pairs grip harder). For example, the carbon-carbon bond gets stronger and shorter as it goes from single (C-C) to double (C=C) to triple (C triple-bond C).
Bond-dissociation energies matter because comparing them tells you whether a reaction will release or absorb energy overall. A reaction is exothermic when the bonds formed in the products are collectively stronger (more total BDE) than the bonds broken in the reactants. BDEs also explain reactivity patterns: a weaker bond breaks more readily, which is why the weak O-O bond of peroxides makes them good radical initiators. An honest caveat: a tabulated bond energy is an average over many molecules, and the true cost of breaking the 'same' bond shifts depending on the rest of the molecule around it.
The C-C single bond runs about 350 kJ/mol, the C=C double bond about 610, and the C triple-bond C about 835 — each added pi bond strengthens (and shortens) the link, though never quite by the full amount of the first.
More shared pairs mean a stronger, shorter bond — but the second and third pi bonds add less than the first sigma.
Tabulated bond energies are averages; the real cost of breaking a given bond depends on the surrounding molecule. Stronger bonds tend to be shorter, but bond strength and bond length are not the identical thing.