Most periodic tables list atomic masses.
Most periodic tables list atomic masses.

Atoms are extremely small relative to the scale of human life. Scientists use a unit called a mole to describe large quantities of atoms and other small things. One mole is equal to 6.022 x 10^23 particles. This number is called Avogadro's number. These particles can be individual atoms, molecules of a compound or any other particle being observed. In order to calculate the number of atoms in a one gram sample of any substance, you must calculate the number of moles of that substance based on its molar mass. Then, you can use Avogadro's number to calculate the number of particles.

Add the atomic mass of each atom in the compound. The atomic mass of an element is the mass of one mole of that element. Atomic masses are given in atomic mass units. One atomic mass unit is equal to one gram per mole. When you add these together for a compound, you get the molar mass of that compound. For example, silica, or silicon dioxide, consists of one silicon atom and two oxygen atoms. The atomic mass of silicon is 28 grams and the atomic mass of oxygen is 16 grams. Therefore, the total mass of one mole of silicon dioxide is 60 grams.

Divide one gram by the molar mass of the compound. This gives the number of moles of the compound in a one gram sample. For example, one gram of silicon dioxide divided by 60 grams per mole gives around 0.0167 moles of silicon dioxide.

Multiply the number of moles by Avogadro's number. This yields the number of molecules in one gram. For example, 0.0167 moles of silicon dioxide times 6.022 x 10^23 equals around 1 x 10^22 molecules of silicon dioxide.

Multiply by the number of atoms in one molecule. For example, each molecule of silicon dioxide contains three atoms. Therefore, there are around 3 x 10^22 atoms in one gram of silicon dioxide.