Elementary entities in the context of "Complex (chemistry)"

In chemistry, the amount of substance (symbol n) in a given sample of matter is defined as a ratio (n = N/N_A) between the number of elementary entities (N) and the Avogadro constant (N_A). The unit of amount of substance in the International System of Units is the mole (symbol: mol), a base unit. Since 2019, the mole has been defined such that the value of the Avogadro constant N_A is exactly 6.02214076×10 mol, defining a macroscopic unit convenient for use in laboratory-scale chemistry. The elementary entities are usually molecules, atoms, ions, or ion pairs of a specified kind. The particular substance sampled may be specified using a subscript or in parentheses, e.g., the amount of sodium chloride (NaCl) could be denoted as n_NaCl or n(NaCl). Sometimes, the amount of substance is referred to as the chemical amount or, informally, as the "number of moles" in a given sample of matter. The latter term is deprecated by the IUPAC because, for a substance X, the correct meaning of "number of moles" is n(X)/mol. The amount of substance in a sample can be calculated from measured quantities, such as mass or volume, given the molar mass of the substance or the molar volume of an ideal gas at a given temperature and pressure.