COVALENT BONDS

The covalent, or shared electron pair, model of chemical bonding was first suggested by G. N. Lewis of the University of California in 1916. Lewis proposed that a sharing of two electrons by two hydrogen atoms permits each one to have a stable closed-shell electron configuration analogous to helium.

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Structural formulas of this type in which electrons are represented as dots are called Lewis structures.

The amount of energy required to dissociate a hydrogen molecule H2 to two separate hydrogen atoms is called its bond dissociation energy (or bond energy). For H2 it is quite large, being equal to 435 kJ/mol (104 kcal/mol). The main contributor to the strength of the covalent bond in H2 is the increased binding force exerted on its two electrons. Each electron in H2 ''feels'' the attractive force of two nuclei, rather than one as it would in an isolated hydrogen atom.