A-Level Definitions

• Isotopes


  Atoms with the same number of protons but different numbers of neutrons
• Relative isotopic mass


  The mass of an atom of an isotope compared with 1/12 of the mass of an atom of carbon-12
• Relative atomic mass


  The weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of carbon-12
• Empirical formula


  The simplest whole number ratio of atoms of each element present in a compound
• Ionic bonding


  The electrostatic attraction between oppositely charged ions
• Covalent bonding


  The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
• Dative covalent bond

  A shared pair of electrons which has been provided by one of the bonding atoms only
• Average bond enthalpy


  Average enthalpy change required to break one mole of a specified type of bond in a gaseous molecule
• Electronegativity


  The ability of an atom to attract the bonding electrons in a covalent bond
• First ionisation energy


  The removal of 1 mole of electrons from 1 mole of gaseous atoms
• Metallic bonding


  Strong electrostatic attraction between cations and delocalised electrons
• Giant covalent lattice


  A 3D structure of atoms, bonded together by strong covalent bonds

  
• Disproportionation


  The oxidation and reduction of the same element in a redox reaction
• Standard conditions

  100kPa and 298K and concentration of 1 mol dm-3 (for reactions with aqueous solutions)
• Enthalpy change of formation

  The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• Enthalpy change of combustion


  The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.
• Enthalpy change of neutralisation

  The enthalpy change when 1 mole of water molecules are formed from a neutralisation reaction under standard conditions
• Catalyst


  The substance that speeds up the rate of a chemical reaction without being used up by taking an alternative pathway with lower activation energy
• Homogenous catalyst


  A catalyst that is in the same state as all the reactants and products in a reaction system
• Heterogenous catalyst


  A catalyst that exists in a different physical state than the reaction it catalyses
• Dynamic equilibrium (features)


  In a closed system where the rate of forward reaction is equal to the rate of the reverse reaction and the concentration of the reactants and products do not change
• Homologous series


  A series of organic compounds with the same functional group but with each successive member differing by CH2
• Structural isomers


  Compounds with the same molecular formula but different structural formula
• Homolytic fission


  each bonding atom receives one electron from the bonded pair, forming two radicals
• Heterolytic fission


  one bonding atoms receives both electrons from the bonded pair
• Radical


  A species with an unpaired electron
• Curly arrow


  Shows the movement of a pair of electrons from an area of high electron density to an area of low electron density
• Sigma bond (σ bond)


  Overlap of orbitals directly between the bonding atoms
• Pi bond


  Sideways overlap of adjacent p-orbitals above and below the bonding C atoms
• Stereoisomer


  Compounds with the same structural formula but with a different arrangement of the atoms in space.
• Electrophile


  Electron pair acceptor
• Nucleophile


  Electron pair donor
• Infrared Radiation causes...


  ...covalent bonds to vibrate more and absorb energy
• Rate determining step


  the slowest step in a reaction mechanism
• Half-life


  The average time taken for half the reactant to be used up
• Brønsted-Lowry acid


  Species that donates a proton
• Brønsted-Lowry base


  Species that accepts a proton
• Monobasic, Dibasic and Tribasic acid

  Mono- donates one proton per molecule.Di- donates two protons per molecule.Tri- donates three protons per molecule.
• Buffer solution


  A system that minimises pH changes on the addition of small amounts of an acid or a base
• Lattice enthalpy


  The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions