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Chemical Bonding & Molecular Structure

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Chemical Bonding & Molecular Structure

HOMO and LUMO

The highest occupied and lowest unoccupied molecular orbitals, which together govern a molecule's reactivity and spectroscopy.

ΔE = E_LUMO − E_HOMO eV
Chemical Bonding & Molecular Structure

Hybridisation

The mixing of atomic orbitals on the same atom to form an equal number of new, equivalent orbitals suited to bonding.

Dimensionless
Chemical Bonding & Molecular Structure

Hydrogen Bond

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) kJ/mol
Chemical Bonding & Molecular Structure

Incomplete Octet

A stable bonding arrangement in which the central atom carries fewer than eight valence electrons.

Dimensionless
Chemical Bonding & Molecular Structure

Intermolecular Forces

The comparatively weak attractions acting between separate molecules rather than within them.

kJ/mol
Chemical Bonding & Molecular Structure

Ionic Bond

The electrostatic attraction between oppositely charged ions formed by the complete transfer of electrons from one atom to another.

F ∝ q₁q₂ / r² kJ/mol
Chemical Bonding & Molecular Structure

Ion–Dipole Interaction

The attraction between an ion and the oppositely charged end of a polar molecule.

E ∝ q μ / r² kJ/mol
Chemical Bonding & Molecular Structure

Lattice Energy

The energy released when one mole of an ionic solid is formed from its constituent gaseous ions.

U ∝ (z⁺z⁻) / (r⁺ + r⁻) kJ/mol
Chemical Bonding & Molecular Structure

Lewis Structure

A diagram showing the bonding and lone pairs of a molecule using dots or lines for valence electrons.

Dimensionless
Chemical Bonding & Molecular Structure

Linear Geometry

A molecular shape in which the bonded atoms lie on a straight line, giving a bond angle of 180°.

AX₂ or AX₂E₃ degrees
Chemical Bonding & Molecular Structure

London Dispersion Forces

Attractions between molecules caused by instantaneous fluctuations in electron distribution inducing dipoles in neighbours.

E ∝ α² / r⁶ kJ/mol
Chemical Bonding & Molecular Structure

Lone Pair

A pair of valence electrons localised on one atom and not involved in bonding.

Dimensionless
Chemical Bonding & Molecular Structure

Metallic Bond

The attraction between a lattice of metal cations and the delocalised electrons that move freely throughout the structure.

kJ/mol
Chemical Bonding & Molecular Structure

Molecular Geometry

The three-dimensional arrangement of the bonded atoms in a molecule, ignoring the positions of lone pairs.

degrees
Chemical Bonding & Molecular Structure

Molecular Orbital Theory

The bonding model in which atomic orbitals combine to form molecular orbitals delocalised over the entire molecule.

ψ_MO = c₁ψ_A ± c₂ψ_B Dimensionless
Chemical Bonding & Molecular Structure

Octahedral Geometry

A shape with six bonded atoms at the vertices of a regular octahedron, all bond angles being 90° or 180°.

AX₆ degrees
Chemical Bonding & Molecular Structure

Octet Rule

The observation that atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons.

Dimensionless
Chemical Bonding & Molecular Structure

Odd-Electron Molecule

A species containing an unpaired electron, so that its total electron count is odd and no octet structure is possible.

Dimensionless
Chemical Bonding & Molecular Structure

Pi Bond

A covalent bond formed by the sideways overlap of parallel p orbitals, with electron density above and below the internuclear axis.

kJ/mol
Chemical Bonding & Molecular Structure

Polar Covalent Bond

A covalent bond in which the shared electrons are unequally distributed because the bonded atoms differ in electronegativity.

δ⁺A—Bδ⁻ D
Chemical Bonding & Molecular Structure

Polarisability

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 C·m²/V
Chemical Bonding & Molecular Structure

Polarising Power

The ability of a cation to distort the electron cloud of a neighbouring anion.

∝ charge / radius² Dimensionless
Chemical Bonding & Molecular Structure

Resonance

The description of a molecule whose true structure is a weighted average of two or more valid Lewis structures.

A ↔ B ↔ C Dimensionless
Chemical Bonding & Molecular Structure

Resonance Energy

The extra stability a molecule gains from electron delocalisation, measured against the most stable single Lewis structure.

ΔE_res = E(hybrid) − E(best structure) kJ/mol