The Mole Concept, Avogadro's Constant, and Molar Mass

Chemists count particles in enormous numbers, and the mole is the unit that makes this possible. A single drop of water contains far too many molecules to count individually, so chemistry uses a "chemist's dozen" called the mole. One mole is defined as the amount of substance that contains exactly $6.022 \times 10^{23}$ elementary entities. This number is Avogadro's constant, given the symbol $N_A$, with units of $\text{mol}^{-1}$. Just as a dozen always means twelve whether we count eggs or electrons, a mole always means $6.022 \times 10^{23}$ particles whether those particles are atoms, molecules, ions, or formula units. It is essential to state what kind of particle you are counting, because one mole of oxygen gas ($O_2$) contains one mole of molecules but two moles of oxygen atoms.

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