definitions

• Acid


  A proton (hydrogen ion) donor
• Acid–base indicator


  A substance which changes colour when the pH in an acid–base titration changes rapidly
• Acid dissociation constant, Ka


  The equilibrium constant for a weak acid
• Activation energy


  The minimum energy particles must have when they collide to allow a reaction to occur
• Alkali


  A soluble base
• Allotrope


  Different forms of the same element. For example, diamond and graphite are allotropes of carbon
• Alpha radiation (α)


  The He2+ nuclei emitted when radioactive atoms decay
• Amphoteric oxide


  An oxide which can react with an acid as well as with an alkali
• Anion


  A negatively charged ion
• Atomic number (Z)


  The number of protons in the nucleus of the atom
• Atomic orbital


  A region of space outside the nucleus where there is a good probability of finding one or two specific types of electron. Orbitals can be s, p, d or f
• Atomic radius/covalent radius


  Half the distance between the nuclei of two atoms of the same type in a covalent bond
• Avogadro constant


  The number of atoms, ions or molecules in a mole of atoms, ions or molecules
• Avogadro's law


  Equal volumes of gases contain the same number of molecules
• Base


  A proton (hydrogen ion) acceptor
• Beta radiation (β)


  Electrons given off from the nucleus when a radioactive isotope decays
• Bidentate


  Ligands which can form two co-ordinate bonds with the central transition element ion
• Boltzmann distribution curve


  A graph of the fraction of particles with particular energies against the energy. It has a characteristic humped curve which 'tails' off gradually
• Bond energy


  The energy needed to break a mole of a particular bond in one mole of gaseous molecules (under standard conditions)
• Bond polarisation


  A covalent bond where the bonding electrons are not shared equally due to differences in electronegativity. Shown by an arrow and the signs δ+ and δ-
• Born–Haber cycle


  An enthalpy cycle used to calculate lattice energy
• Boyle's law


  The volume of a gas is inversely proportional to its pressure
• Brønsted–Lowry theory


  An acid is a proton donor. A base is a proton acceptor
• Buffer solution


  A solution that minimises changes in pH when acids or alkalis are added to the solution
• Catalyst


  A substance that increases the rate of reaction without change to itself
• Cation


  A positively charged ion
• Charles' law


  The volume of a gas is directly proportional to the temperature in kelvins
• Closed system


  A system in which matter or energy is not gained or lost
• Common ion effect


  The reduction in the solubility of a sparingly soluble salt by adding another salt which has an ion in common
• Complex ion


  An ion containing a central transition element bonded to ligands by co-ordinate bonds
• Compound


  A substance containing atoms from two or more different elements bonded together
• Condensation


  The change in state when a vapour changes to a liquid
• Conjugate pair


  An acid and base on each side of an equation which differ from each other by a H+ ion
• Co-ordinate bond


  A covalent bond where both the electrons are provided by the same atom
• Co-ordination number


  The number of co-ordinate bonds formed by ligands to the central transition element ion in a complex
• Covalent bond (single)


  A bond formed by the sharing of a pair of electrons between two atoms
• Covalent radius


  See atomic radius
• Dative covalent bond


  See Co-ordinate bond
• Degenerate orbitals


  Atomic orbitals at the same energy level
• Delocalised electrons


  Electrons which are free to move between three or more adjacent atoms