By Riti Gupta | Updated Mar 24, 2022

Counting individual atoms is impossible with a balance, so chemists use Avogadro’s number to link the microscopic world to macroscopic measurements.

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Avogadro’s Number: The Bridge Between Atoms and Moles

Avogadro’s number is 6.022 × 10²³, the quantity of particles in one mole (mol). One mole can be a collection of atoms, molecules, or ions, depending on the substance.

What Is the Mass of One Mole?

The mass of a mole—its molar mass—is expressed in grams per mole (g mol⁻¹). You find it directly on the periodic table (e.g., chlorine: 35.45 g mol⁻¹). For compounds, sum the atomic masses of all constituent atoms (water: 2 × 1.008 + 15.999 = 18.02 g mol⁻¹).

From Atoms to Grams: The Conversion Process

To convert a given number of atoms to grams:

1. Convert atoms to moles: divide by Avogadro’s number.
2. Multiply the resulting moles by the molar mass.

Example: 3 × 10²⁴ atoms of chlorine

3 × 10²⁴ atoms Cl ÷ 6.022 × 10²³ atoms mol⁻¹ = 4.98 mol Cl
4.98 mol Cl × 35.45 g mol⁻¹ = 176.6 g Cl

Thus, 3 × 10²⁴ chlorine atoms weigh 176.6 grams.

Handling Compounds: Atoms Within a Molecule

If you only know the atom count of an element that appears in a compound, first determine how many molecules of that compound you have, using the atom‑to‑molecule ratio.

Example: 5.55 × 10²³ atoms of oxygen in CO₂

5.55 × 10²³ atoms O ÷ 2 atoms O mol₂ = 2.78 × 10²³ molecules CO₂
2.78 × 10²³ molecules CO₂ ÷ 6.022 × 10²³ molecules mol⁻¹ = 0.462 mol CO₂
0.462 mol CO₂ × 44.01 g mol⁻¹ = 20.3 g CO₂

Always keep track of your units at each step to avoid errors.

References: IUPAC Green Book, iupac.org; Periodic Table, PubChem.