The theory that atoms consist of electrons orbiting in shells around the nucleus led to a simple explanation of both kinds of bonding: atoms combine to achieve highly-stable filled outer shells containing 2, 8, or 18 electrons, either by transfer of electrons from one atom to the other (ionic bond), or by the sharing of one electron from each atom so that both electrons orbit around both nuclei (covalent bond). In the coordinate bond, a variant of the covalent bond, both shared electrons are provided by one atom.
Quantum theory has now shown that electrons occupy orbitals having certain shapes and energies, and that, when atoms combine, the outer atomic orbitals are mixed to form molecular orbitals. The energy difference constitutes the bond energy – the energy required to break the bond by separating the atoms. Molecular orbitals are classified as σ (sigma) if symmetric when rotated through 180° about the line joining the nuclei, or π (pi) if antisymmetric.
The energy and length of chemical bonds, and the angles between them, may be investigated by spectroscopy and X-ray diffraction. See also hydrogen bond.
Related category CHEMICAL BONDING
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