ROYAL SOCIETY OF CHEMISTRY DATA BOOK RECOMMENDED SYMBOLS FOR PHYSICAL QUANTITIES The recommended symbols for a number of physical quantities are given below, together with the appropriate SI unit. For practical reasons, multiplying prefixes are often used with the SI unit – thus, mol dm ­3 is used for concentration rather than mol m ­3 . Because there is no compulsion on the choice of symbols (and because the same letter may be recommended for several different quantities), a physical quantity symbol should always be defined precisely the first time it is used. The following notes explain some of the conventions followed when using physical quantities, symbols for physical quantities, units, and symbols for units. Physical quantity = numerical value x unit This definition of a physical quantity as the product of a numerical value (sometimes called the measure) and a unit leads to the following conventions for labelling table headings and graph axes, and for substituting in formulae. The equation can be rearranged Physical quantity / unit = numerical value Tables of data have column headings in the form (physical quantity / unit). Quantity Symbol SI unit activation energy E J mol ­1 activity (radioactive) A Bq, s ­1 atomic number Z – Avogadro constant L, NA mol Boltzmann constant k J K ­1 Bragg angle θ rad charge number of ion z – concentration of substance B [B] mol m ­3 degree of dissociation α – density ρ kg m electric charge Q C electric current I A electric dipole moment μ C m electric potential V V electric potential difference V V electromotive force E V
­1 ­3 ROYAL SOCIETY OF CHEMISTRY DATA BOOK Page 1 of 3 www.chemsoc.org/networks/learnnet/data.htm Quantity Symbol SI unit electron mass me kg elementary (electron) charge e C enthalpy H J mol ­1 enthalpy change ΔH J mol entropy S J K mol equilibrium constant K as appropriate Faraday constant F C mol ­1 frequency ν, f Hz, s ­1 Gibbs free energy G J mol ­1 half­life (radioactive or reaction) T½, t½ s internal energy U, E J length l m magnetic field strength H A m mass m kg mass number A – mass of electron me kg mass of neutron mn kg mass of proton mp kg molar volume Vm m 3 mol ­1 molar mass of a compound B MB kg mol ­1 molar mass of an element A kg mol molecular mass m kg mole fraction of substance B xB – neutron number N – number of molecules N – Planck constant h J s pressure p, P Pa (= N m ­2 ) quantum number (principal) n – radius r m rate constant of (n + 1) order reaction k, kr m 3n mol ­3n s ­1 relative atomic mass of an element (atomic weight) Ar –
­1 ­1 ­1 ­1 ­1 th ROYAL SOCIETY OF CHEMISTRY DATA BOOK Page 2 of 3 www.chemsoc.org/networks/learnnet/data.htm Quantity Symbol SI unit relative molecular mass of a substance (molecular weight) Mr – resistance R Ω specific heat capacity c J kg ­1 K ­1 speed v m s ­1 speed of electromagnetic waves (light) in vacuum c m s ­1 temperature, common (Celsius) θ, t o temperature, thermodynamic (absolute) T K temperature difference θ K time t s velocity v m s ­1 volume V m 3 wavelength m wavenumber λ v work W J
C m ­1 ROYAL SOCIETY OF CHEMISTRY DATA BOOK Page 3 of 3 www.chemsoc.org/networks/learnnet/data.htm