A-level Chemistry Definitions Questions and Answers

A-level Chemistry Definitions Questions and Answers isotopes atoms of the same element with different numbers of neutrons atomic (proton) number the number of protons in the nucleus of an atom mass (nucleon) number the number of particles (protons and neutrons) in the nucleus ion a positively or negatively charged atom or (covalently bonded) group of atoms (molecular ion) relative isotopic mass the mass of an atom of an isotope compared with one twelfth of the mass of an atom of carbon-12 relative atomic mass the weighted mean mass of an an atom of an element compared with one twelfth of the mass of an atom of carbon-12 relative molecular mass (Mr) the weighted mean mass of a molecule compared with one twelfth of the mass of an atom of carbon-12 relative formula mass is the weighted mean mass of a formula unit compared with one twelfth of the mass of an atom of carbon-12 amount of substance the quantity whose unit is the mole: chemists use 'amount of substance' as a means of counting atoms the Avogadro constant is the number of atoms per m ole of the carbon-12 isotope a mole the amount of any substance containing as many particles as there are carbon atoms in exactly 12g of the carbon-12 isotope molar mass is the mass per mole of a substance: units are grams per mole empirical formula the simplest whole-number ratio of atoms of each element present in a compound molecule a small group of atoms held together by covalent bonds molar volume the volume per mole of a gas: at room temperature and pressure this is 24 litres concentration of a solution the amount of solute, in moles, dissolved per litre of solution standard solution a solution of known concentration that can be used to determine the concentration of another solution, by titration species a general term to describe any type of particle taking part in a chemical reaction stoichiometry the molar relationship between the relative quantities of substances taking part in a reaction acid a proton-donor base a proton-acceptor alkali a base that dissolves in water, forming hydroxide ions salt a compound formed from an acid where the H+ ion has been replaced by a metal ion or another positive ion cation a positively-charged ion anion a negatively-charged ion hydrated a crystalline compound that contains water molecules anhydrous a substance that contains no water molecules water of crystallisation water molecules that form an essential part of the crystalline structure of a compound oxidation loss of electrons or an increase in oxidation number reduction gain of electrons or a decrease in oxidation number reducing agent a reagent that reduces (adds electrons to) another species oxidising agent a reagent that oxidises (removes electrons from) another species