OCR A Level chemistry definitions

• Isotope


  Atoms of the same element with the same number of electrons and protons but a different numbers of neutrons and different masses
• Relative Isotopic mass


  The mass of an isotope relative to 1/12 the mass of carbon-12
• Relative atomic mass


  The weighted mean mass of an atom relative to 1/12 the mass of carbon-12
• Orbital


  A region of space in the atom that can hold up to two electrons
• First Ionisation Energy


  The amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+
• Alkali


  A water-soluble base that releases OH- ions in aqueous solution
• Base


  A proton acceptor
• Acid


  A substance that releases H+ ions in solution (a proton donor)
• Strong acid


  An acid that fully dissociates in solution
• Weak acid


  An acid that partially dissociates in solution
• Ionic bonding


  The electrostatic attraction between oppositely charged ions
• Covalent bonding


  The electrostatic attraction between the nuclei of the bonding atoms and the shared pair of electrons
• Dative covalent (coordinate) bonding


  A covalent bond where both electrons have been donated by one atom
• Electronegativity


  The ability of a bonding atom to attract a shared pair of electrons in a covalent bond
• Polar molecule


  A molecule that contains polar bonds (due to differences in electronegativities of bonding atom) and is unsymmetrical so the permanent dipoles don't cancel out (there's an overall dipole)
• Hydrogen bonding


  Intermolecular bonding between molecules containing N, O, or F (NB not C) and the H atom of -NH, OH or HF
• Metallic bonding


  The electrostatic attraction between metal cations and delocalised electrons
• Periodic Table


  The arrangement of elements by increasing proton number in periods showing periodicity and groups having similar properties
• Periodicity


  Repeating trends in physical and chemical properties across different periods of the periodic table
• Disproportionation reaction


  Oxidation and reduction of the same element
• Activation energy


  The minimum amount of energy required for a reaction to take place
• Standard enthalpy change of formation

  The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions
• Standard enthalpy change of combustion

  The enthalpy change when one mole of a substance is burnt completely under standard conditions
• Standard enthalpy change of neutralisation

  The enthalpy change when an acid is neutralised by a base to form a salt and one mole of water, under standard conditions
• Average bond enthalpy


  The breaking of one mole of bonds in gaseous molecules
• Homologous series


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


  Carbon chains are straight, branched or ringed (non-aromatic)
• Alicyclic


  Aliphatic with non-aromatic rings
• Aromatic


  Containing a benzene ring
• Structural isomer


  Molecules with the same molecular formula but different structural formulae
• Homolytic bond fission


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


  One bonding atom receives both electrons from the bonded pair
• Radical


  A species with an unpaired electron
• Pi bond


  The sideways overlap of adjacent p-orbitals above and below the plane of the sigma bond
• Stereoisomers


  Compounds with the same structural formula but a different arrangement of the atoms in space
• E/Z isomerism


  An example of stereoisomerism arising due to the restricted rotation of the pi bond and the requirement of two different groups attached to each carbon atom of the C-C bond
• Cis-trans isomerism


  E/Z isomerism when both carbon atoms of the C-C bond have one of the same groups [often hydrogen]
• Electrophile


  An electron pair acceptor
• Nucleophile


  An electron pair donor
• Homogeneous catalyst


  A catalyst that is in the same physical state as the reactants