# How do chemists count atoms and molecules?

Chemists use the property of mass equivalence, and have introduced the concept of $\text{moles}$. If we have a mole of substance we have $6.022 \times {10}^{23}$ individual particles of that substance.
For a given atom, we know the mass of $6.022 \times {10}^{23}$ individual atoms, and this mass is given on the Periodic Table, which should always be made available in every exam. It is up to you to learn how to use it.