Band Gap: This is the difference in energy between bands formed from the overlap of orbitals of different symmetry, though it is typically taken to be the energy difference between the highest fully occupied and the lowest fully unoccupied bands.

Band: A band is formed when there is extended overlap of molecular orbitals in a solid, leading to many levels which have small separations, and which can be considered as continuous.

Basis Set: The set of atomic orbitals chosen to make up the molecular orbitals from overlap considerations. The choice of basis set is not unique for a given molecule.

Benzene Ring: C6H6. A hexagonal aromatic molecule. Also the group C6H5 when it is part of another molecule.

Benzylic: The benzylic position on a molecule is shown here;

Binary mixture: A mixture that has only two constituents.

Black body: A theoretical ideal emitter of electromagnetic radiation

Boat Form: A high energy conformational form of cyclohexane. So-called because it resemblesa boat.

Boltzmann distribution: A formula for calculating the populations of different energy states at a given temperature.

Bond Order: This can be expressed for a given species as the number of electrons in bonding orbitals minus the number of electrons in antibonding orbitals, divided by two.

Bonding orbital: A molecular orbital, produced by the overlap of atomic orbitals, which lies at lower energy than the atomic orbitals which composed it. Occupation of such an orbital stabilises two atoms close to each other, forming a bond between them.

Born-Haber Cycle: This is a thermochemical cycle which describes the formation of an ionic solid in terms of the formation of the gaseous ions from the standard states of the species involved, and from which the lattice energy may be calculated.

Born-Lande Equation: This is an expression which gives the lattice energy of an ionic solid in terms of the charges on the ions and their ionic radii.

Born-Oppenheimer approximation: An approximation commonly made in quantum mechanics, which considers that since nuclei are so much more massive than electrons, they must move much more slowly. Hence the motions of the two can be separated (the nuclei can be considered as stationary points around which the electrons move).

Boundary conditions: Constraints that a wavefunction must fulfill if it is to be an acceptable physical solution of the Schrodinger equation. (Some mathematically acceptable solutions are ruled out because of the physical results they imply.) Quantisation arises as a result of the necessity to conform to boundary conditions.

Boundary surface: The surface which encloses the region in which an electron will be found 90% of the time. These surfaces are used to graphically represent the shape of an atomic or molecular orbital.

Bragg’s Law: This relates the separation of two crystal planes, d, to the wavelength of the probing x-ray radiation, λ, and the angle of scattering of the x-rays by those planes, θ: the relationship is λ = 2dsinθ.

Branched Chain: A hydrocarbon with more than two terminal carbon atoms. i.e. a non-linear hydrocarbon.

Bronsted Acid: A compound which can act as a H+ donor.

Bronsted Base: A compound which can act as a H+ acceptor.

Buckminster Fullerene: See Fullerene.

Buffer Solution: A solution which has the ability to oppose changes in pH when small quantities of acid or base are added to it.

Bu: Short for butyl, denotes a C4H9 group. Variants on this are;