The mole is the unit of measurement in the International System of Units (SI) for amount of substance. It is defined as the amount of a chemical substance that contains as many elementary entities, e.g., atoms, molecules, ions, electrons, or photons/ This number is expressed by the Avogadro constant, which has a value of $$6.022140857 \times 10^{23} \;mol^{-1}$$. The mole is one of the base units of the SI, and has the unit symbol mol.