mole

A mole of a pure compound (all the same molecule species) is a number of grams that (roughly) matches the number of nucleons (protons and neutrons) in the molecule, i.e., a mole of atomic hydrogen (having a nucleus of one proton) is roughly one gram, of atomic helium is roughly 4 grams, etc. The SI standard mole is a fixed number of particles, the number termed Avogadro's number, currently defined to be 6.02214076 × 10²³ particles. The unit is convenient for chemists, who can relate grams to elements and molecular formulae, and can be convenient in calculations for allowing smaller numbers to be used than for particle counts. Earlier in its history, Avogadro's number and the mole were pinned to the standard gram along with some selected element isotope. However, though the number is useful in chemistry, a definition based upon nucleons-per-gram is not stable (such a value varies by element, isotope, molecular structure, pressure, temperature) and an exact number is considered more useful: a rounded (to the nearest multiple of 10¹⁵) value well-within the range of such varying physical values has been adopted for inclusion in SI.