<P> Theoretical chemistry requires quantities from core physics, such as time, volume, temperature, and pressure . But the highly quantitative nature of physical chemistry, in a more specialized way than core physics, uses molar amounts of substance rather than simply counting numbers; this leads to the specialized definitions in this article . Core physics itself rarely uses the mole, except in areas overlapping thermodynamics and chemistry . </P> <P> Entity refers to the type of particle / s in question, such as atoms, molecules, complexes, radicals, ions, electrons etc . </P> <P> Conventionally for concentrations and activities, square brackets () are used around the chemical molecular formula . For an arbitrary atom, generic letters in upright non-bold typeface such as A, B, R, X or Y etc. are often used . </P> <P> No standard symbols are used for the following quantities, as specifically applied to a substance: </P>

What do brackets in an chemical equation mean