Chemical Bonding & Molecular Structure — 60 Terms Defined

HOMO and LUMO Chemical Bonding & Molecular Structure eV The highest occupied and lowest unoccupied molecular orbitals, which together govern a molecule's reactivity and spectroscopy. ΔE = E_LUMO − E_HOMO

Hybridisation Chemical Bonding & Molecular Structure Dimensionless The mixing of atomic orbitals on the same atom to form an equal number of new, equivalent orbitals suited to bonding.

Hydrogen Bond Chemical Bonding & Molecular Structure kJ/mol A strong dipole interaction between a hydrogen atom bonded to N, O or F and a lone pair on another such atom. X–H···Y (X, Y = N, O, F)

Incomplete Octet Chemical Bonding & Molecular Structure Dimensionless A stable bonding arrangement in which the central atom carries fewer than eight valence electrons.

Intermolecular Forces Chemical Bonding & Molecular Structure kJ/mol The comparatively weak attractions acting between separate molecules rather than within them.

Ionic Bond Chemical Bonding & Molecular Structure kJ/mol The electrostatic attraction between oppositely charged ions formed by the complete transfer of electrons from one atom to another. F ∝ q₁q₂ / r²

Ion–Dipole Interaction Chemical Bonding & Molecular Structure kJ/mol The attraction between an ion and the oppositely charged end of a polar molecule. E ∝ q μ / r²

Lattice Energy Chemical Bonding & Molecular Structure kJ/mol The energy released when one mole of an ionic solid is formed from its constituent gaseous ions. U ∝ (z⁺z⁻) / (r⁺ + r⁻)

Lewis Structure Chemical Bonding & Molecular Structure Dimensionless A diagram showing the bonding and lone pairs of a molecule using dots or lines for valence electrons.

Linear Geometry Chemical Bonding & Molecular Structure degrees A molecular shape in which the bonded atoms lie on a straight line, giving a bond angle of 180°. AX₂ or AX₂E₃

London Dispersion Forces Chemical Bonding & Molecular Structure kJ/mol Attractions between molecules caused by instantaneous fluctuations in electron distribution inducing dipoles in neighbours. E ∝ α² / r⁶

Lone Pair Chemical Bonding & Molecular Structure Dimensionless A pair of valence electrons localised on one atom and not involved in bonding.

Metallic Bond Chemical Bonding & Molecular Structure kJ/mol The attraction between a lattice of metal cations and the delocalised electrons that move freely throughout the structure.

Molecular Geometry Chemical Bonding & Molecular Structure degrees The three-dimensional arrangement of the bonded atoms in a molecule, ignoring the positions of lone pairs.

Molecular Orbital Theory Chemical Bonding & Molecular Structure Dimensionless The bonding model in which atomic orbitals combine to form molecular orbitals delocalised over the entire molecule. ψ_MO = c₁ψ_A ± c₂ψ_B

Octahedral Geometry Chemical Bonding & Molecular Structure degrees A shape with six bonded atoms at the vertices of a regular octahedron, all bond angles being 90° or 180°. AX₆

Octet Rule Chemical Bonding & Molecular Structure Dimensionless The observation that atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons.

Odd-Electron Molecule Chemical Bonding & Molecular Structure Dimensionless A species containing an unpaired electron, so that its total electron count is odd and no octet structure is possible.

Pi Bond Chemical Bonding & Molecular Structure kJ/mol A covalent bond formed by the sideways overlap of parallel p orbitals, with electron density above and below the internuclear axis.

Polar Covalent Bond Chemical Bonding & Molecular Structure D A covalent bond in which the shared electrons are unequally distributed because the bonded atoms differ in electronegativity. δ⁺A—Bδ⁻

Polarisability Chemical Bonding & Molecular Structure C·m²/V The ease with which the electron cloud of an atom, ion or molecule can be distorted by an external electric field. μ_induced = α × E

Polarising Power Chemical Bonding & Molecular Structure Dimensionless The ability of a cation to distort the electron cloud of a neighbouring anion. ∝ charge / radius²

Resonance Chemical Bonding & Molecular Structure Dimensionless The description of a molecule whose true structure is a weighted average of two or more valid Lewis structures. A ↔ B ↔ C

Resonance Energy Chemical Bonding & Molecular Structure kJ/mol The extra stability a molecule gains from electron delocalisation, measured against the most stable single Lewis structure. ΔE_res = E(hybrid) − E(best structure)