Chemistry › Definitions › Bonding and structure

Bonding and structure definitions

Every term the lessons in this unit define, lesson by lesson, worded as the lessons use them. Each lesson title links to its notes.

45 terms

Ionic bonding and ionic lattices

Ionic bonding: The strong electrostatic attraction between oppositely charged ions, acting in all directions through a lattice.
Ion: An atom or group of atoms that carries an overall charge because its number of electrons differs from its number of protons.
Compound ion: A group of covalently bonded atoms that carries an overall charge, such as SO₄²⁻ or NH₄⁺.
Formula unit: The group of ions given by the formula of an ionic compound, in the ratio they occur in the lattice; for CaCl₂, one Ca²⁺ and two Cl⁻.
Giant ionic lattice: A regular three-dimensional arrangement of oppositely charged ions, repeated throughout the crystal and held by electrostatic attraction in all directions.
Coordination number: The number of nearest neighbours of opposite charge that surround an ion in a lattice; six for each ion in sodium chloride.
Isoelectronic: Having the same number of electrons, and so the same electron configuration, as another atom or ion.
Ionic radius: The radius of an ion in a crystal, found from the distance between the centres of neighbouring ions.
Electron density map: A contour map, calculated from the X-ray diffraction pattern of a crystal, showing how the density of electrons varies through the structure.

Covalent bond: A shared pair of electrons between two atoms, attracted electrostatically to the nuclei of both.
Bonding pair: A pair of electrons shared between two atoms in a covalent bond.
Lone pair: A pair of outer-shell electrons on one atom that is not used in bonding.
Double bond: Two shared pairs of electrons between the same two atoms, shorter and stronger than a single bond between them.
Bond length: The distance between the nuclei of two covalently bonded atoms; for the same pair of atoms, the shorter the bond, the stronger it is.
Dative covalent bond: A covalent bond in which both electrons of the shared pair come from the same atom; also called a co-ordinate bond.
Metallic bonding: The electrostatic attraction between a lattice of positive metal ions and the delocalised electrons around them.
Delocalised electrons: Electrons that are not associated with any single atom or bond and are free to move through a structure.
Sigma (σ) bond: A covalent bond formed by end-on overlap of two orbitals, with the shared pair lying directly between the two nuclei.
Pi (π) bond: A covalent bond formed by sideways overlap of two parallel p orbitals, with electron density above and below the line between the nuclei.
Hybridisation: The mixing of an atom's s and p orbitals to give a set of identical hybrid orbitals, sp³, sp² or sp, that point along its σ bonds.

Giant structure: A structure in which the bonding extends throughout the whole crystal, with no separate molecules, as in ionic, metallic and macromolecular crystals.
Macromolecular crystal: A giant structure of atoms joined by covalent bonds in a network that extends throughout the crystal; also called giant covalent.
Molecular crystal: A regular arrangement of simple molecules held in place by intermolecular forces, with covalent bonds only within each molecule.
Intermolecular force: An attraction between separate molecules, much weaker than the covalent bonds within them.
Allotrope: One of two or more different structural forms of the same element in the same physical state, such as diamond and graphite.
Sublimation: The change of state from solid directly to gas, without passing through the liquid.
Enthalpy of fusion: The enthalpy change when one mole of a solid is converted into a liquid at its melting point.
Enthalpy of vaporisation: The enthalpy change when one mole of a liquid is converted into a gas at its boiling point.
Fullerene: A molecular allotrope of carbon in which each atom is bonded to three others in a closed cage or tube, such as C₆₀.

Electron-pair repulsion theory: The theory that the electron pairs round a central atom repel each other and move as far apart as possible, which fixes the shape of the molecule.
Bond angle: The angle between two bonds that meet at the same atom.
Wedge bond: A bond drawn as a solid wedge, showing that it points out of the plane of the paper towards the viewer.
Dash bond: A bond drawn as a hatched wedge or dashed line, showing that it points behind the plane of the paper.
Equatorial position: One of the three positions in a trigonal bipyramid that lie in a triangle round the central atom, at 120° to each other.
Axial position: One of the two positions in a trigonal bipyramid directly above and below the central atom, at 90° to the equatorial positions.
Region of electron density: A lone pair, a single bond, a double bond or a triple bond, each counted once when deciding the shape round a central atom.

Electronegativity: The power of an atom to attract the pair of electrons in a covalent bond towards itself.
Polar bond: A covalent bond between atoms of different electronegativity, in which the bonding pair is shared unequally and the atoms carry partial charges δ+ and δ−.
Permanent dipole: A separation of charge within a molecule, caused by polar bonds, that is present at all times.
Polar molecule: A molecule with an overall permanent dipole, because its bond dipoles do not cancel.
Dipole moment: A measure of the size of the overall separation of charge in a molecule; zero when the bond dipoles cancel.
Induced dipole–dipole force: An attraction between an instantaneous dipole in one molecule and the dipole it induces in a neighbouring molecule; it acts between all molecules.
Permanent dipole–dipole force: An attraction between the δ+ end of one polar molecule and the δ− end of another.
Hydrogen bond: An attraction between a lone pair on an N, O or F atom and a δ+ hydrogen atom covalently bonded to N, O or F in another molecule.
Surface tension: The energy needed to increase the surface area of a liquid, which arises because molecules at the surface are pulled inwards by the molecules below and beside them.