The concentration of 1 mole per litre; 1 mole being Avogadro's number (6.02 x 10²³) of molecules, which is equivalent in mass to the sum of the atomic masses (in grams) of the constituent atoms; e.g. 1 M NaCl is a concentration of 58.45 g/l (Na = 22.99, Cl = 35.46). Sub-molar concentrations are: millimolar (mM) 10^-3 mol/l; micromolar (μM) 10^-6mol/l; nanomolar (nM) 10^-9 mol/l; picomolar (pM) 10^-12 mol/l; femtomolar (fM) 10^-15 mol/l; attomolar (aM) 10^-18 mol/l; zeptomolar (zM) 10^-21 mol/l.