Absorption spectroscopy: Generally, spectroscopy that relies upon measuring the radiation absorbed by a sample as it is excited to higher energy levels.
Absorption spectroscopy: Generally, spectroscopy that relies upon measuring the radiation absorbed by a sample as it is excited to higher energy levels.
Achiral: Lacking chirality. i.e. having no stereocentre.
Acidic Proton: A hydrogen atom on a molecule which is (most) likely to be attacked and removed by a base.
Acidity Constant: Ka , the equilibrium constant for the dissociation of an acid in water. This is a measure of the extent of dissociation of an acid into its conjugate base and a proton. The greater the value of Ka, the stronger the acid.
Activation Energy: (ΔG‡) The minimum energy required in order for reacting species to form an activated complex.
Activity: An effective concentration adjusted to take into account the non-ideality of the solution.
Acyl Group: A carbonyl containing group.
Addition: A type of reaction, the simplest example of which is two species reacting to leave one product only.
Adiabatic barrier: A barrier that does not allow the transfer of heat through it.
Aldehyde: Molecule/functional group of generic type RHC=O, where R is a carbon chain of some sort. See here for a picture.
Aliphatic: As opposed to aromatic, applies to carbon chains.
Alkane: A simple saturated hydrocarbon chain.
Alkene: A carbon-carbon double bond, or a molecule containing a carbon-carbon double bond.
Alkyl Group: A hydrocarbon group e.g. methyl (CH3), ethyl (C2H5) etc.
Allotrope: An element which has forms occurring with different structures is said to have allotropes, eg. Diamond and Graphite are allotropes of Carbon.
Allylic: The allylic position on an organic molecule is as shown here;
ie. it has an adjacent (carbon-carbon) double bond.
Ambidentate: A ligand which can bond to the metal ion center through different atoms, but by only one atom at any time, is called ambidentate.
Amide: A molecule containing the following functional group: (CO)NR2 – where R = C or H.
Amine: A molecule containing the following functional group: R3N – where R = C or H. Also possible is R4N+ where the lone pair of the nitrogen is donated into a single bond.
Amphiprotic: A substance is said to be amphiprotic if it can act as an acid or a base, ie. it can act as a H+ donor or acceptor.
Angstrom (Å): A non-SI unit of measurement of length equal to 10-10m.
Anharmonic: Deviation of the motion of vibrating atoms from the purely harmonic. It arises because the potential curve for two bonded atoms is not truly parabolic.
Anion: A negatively charged ion.
Anisotropic: Not equal in all directions. i.e. having some angular dependence.
Antibonding orbital: A molecular orbital, produced by the overlap of atomic orbitals, which lies at higher energy than the atomic orbitals which composed it. Occupation of such an orbital destabilises two atoms close to each other, causing them to move apart.
Antiperiplanar: See the appropriate page in elimination.
Aromatic: A cyclic system containing (4n+2) π electrons. Aromaticity imparts extra stability to a molecule. See Huckel’s Rule.
Arrhenius Equation: 
. Used for obtaining values of the activation energy of a reaction from values of the rate constant.
Aryl Group: An aromatic containing group.
Associative Mechanism: The mechanism for the substitution of ligands of a complex where the rate determining step is the addition of the entering ligand to the complex, where the coordination number of the metal ion is increased.
Atomic Number: The total number of protons and neutrons in the nucleus of an atom, ie: the mass of the atom.
Atomic Orbital: The wavefunction which defines the properties (location etc) of an electron in an atom. We speak of an electron that obeys such a wavefunction as occupying the relevant orbital.
Autoprotolysis constant: When two like molecules exchange a proton, the product of the concentrations of the protonated and deprotonated compounds is the autoprotolysis constant, given the symbol, Kw.
Axial: In conformational analysis, a bond positioned vertically with respect to the plane of the molecule.