= Bond-dissociation energy
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A <bond-dissociation energy> specifies the <energy> needed for a stated dissociation into separated fragments. An <energy> from the lowest vibrational state, an <energy> from the bottom of a <potential energy> well, and a finite-temperature molar dissociation <enthalpy> require different conventions; do not silently mix them in an <electronegativity> formula.
Which fragments, in which <quantum states>, and at which <temperature>? For a neutral gaseous <molecule> with a specified two-electron <covalent bond>, the usual thermochemical convention is <homolysis>,
$$
\mathrm{A{-}B}(g)\longrightarrow\mathrm A^{\bullet}(g)+\mathrm B^{\bullet}(g).
$$
Each fragment receives one of the two bonding <Electrons>; the dots denote unpaired <Electrons>. The final fragments are isolated from each other, with their own relaxed <molecular geometry> and specified electronic <quantum states>. The <energy> cost includes this relaxation, rather than merely stretching the initial <chemical bond> while freezing everything else. For a neutral parent, <heterolysis> instead produces $\mathrm A^+(g)+\mathrm B^-(g)$, or the opposite assignment of <electric charges>; those are different final states and different <energies>. A value for gaseous neutral fragments cannot be transferred to <ions>, a <chemical solution>, or a <solid> without accounting for those changed conditions.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/bond-dissociation energy>[definition of bond-dissociation energy: molar enthalpy for homolysis of a specified molecular entity]{ref}
The distinction is useful beyond terminology. <Pauling electronegativity> compares consistently defined <bond-dissociation energies>; <photodissociation> can reveal a specified ground-state threshold; a thermal <chemical reaction> balance requires compatible <enthalpies>. A threshold is also different from an <activation energy>: it compares initial and final <quantum states>, whereas a barrier can obstruct the path between them.
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