The mole is the SI unit for the amount of substance. It is a fundamental unit in chemistry used to express the quantity of a chemical substance. One mole of any substance contains exactly $6.022 \times 10^{23}$ elementary entities (atoms, molecules, ions, electrons, etc.). This number is known as Avogadro's number ($N_A$). The mole provides a convenient way to convert between the number of particles (which is extremely large) and the macroscopic mass of a substance. The mass of one mole of a substance is called its molar mass, and it is numerically equal to the substance's atomic or molecular weight in grams per mole ($\text{g/mol}$).