The amount of a given substance in a stated unit of a mixture, solution, or ore. Common methods of stating concentration are percentage by weight or by volume, normality, or weight per unit volume (as grams per cubic centimeter or pounds per gallon). The concentration of an atom, ion, or molecule in a solution may be symbolized by the use of square brackets, as [Cl−]. For radioactivity, the concentration is usually expressed as millicuries per milliliter (mCi/mL) or millicuries per millimole (mCi/mM).