This unit explains why atoms combine and how the shapes of molecules arise. The Kössel-Lewis approach relates bonding to the attainment of a stable noble-gas configuration: electron transfer gives ionic (electrovalent) bonds while electron sharing gives covalent bonds, both governed by the octet rule. Lewis dot structures, formal charge and the limitations of the octet rule (incomplete octet, odd-electron molecules and the expanded octet) are discussed. The stability of an ionic bond is measured by lattice enthalpy rather than by octet attainment alone. Bond parameters — bond length, bond angle, bond enthalpy and bond order — are defined alongside resonance (canonical structures and the resonance hybrid) and bond polarity expressed through dipole moment. The valence shell electron pair repulsion (VSEPR) theory predicts molecular geometry from electron-pair repulsions, where lone pair-lone pair repulsion exceeds lone pair-bond pair, which exceeds bond pair-bond pair. Valence bond theory explains bonding through orbital overlap, distinguishing sigma and pi bonds and introducing hybridisation (sp, sp\(^2\), sp\(^3\), sp\(^3\)d, sp\(^3\)d\(^2\)) to account for geometries such as those of methane, ammonia and water. Molecular orbital theory builds bonding and antibonding orbitals by the linear combination of atomic orbitals, giving bond order, stability and magnetic behaviour (explaining why dioxygen is paramagnetic). The unit closes with hydrogen bonding.
Kössel-Lewis approach and octet ruleIonic bond and lattice enthalpyBond parameters and resonanceDipole moment and bond polarityVSEPR theory and molecular shapesValence bond theory and hybridisationMolecular orbital theoryHydrogen bonding
Key terms
Octet rule
Atoms combine by transferring or sharing valence electrons to attain eight electrons in their valence shell, a stable noble-gas configuration.
Lattice enthalpy
The energy required to completely separate one mole of a solid ionic compound into its gaseous ions.
Formal charge
The difference between the valence electrons of a free atom and the electrons assigned to it in a Lewis structure.
Resonance
The representation of a molecule by several canonical structures of comparable energy whose hybrid describes the real, lower-energy molecule.
Hybridisation
The intermixing of atomic orbitals of slightly different energies to form an equal number of equivalent orbitals of identical energy and shape.
Bond order
In molecular orbital theory, one half the difference between the numbers of bonding and antibonding electrons, \(\tfrac12(N_b-N_a)\).
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According to the octet rule, atoms combine to attain how many electrons in their valence shell?
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