Foundation reading 4 of 17 · ~6 min

Count particles in moles

Your goal: Explain a mole as a fixed particle count and name the particles being counted.

A focused reading from The Mole and Molar Mass. Reading time is a Chem Help estimate.

Optional prerequisite review

Review if useful. These suggestions never lock a step.

Why chemists count in moles

A single water molecule has a mass of about 3 × 10⁻²³ g, far too small for any balance. A drop of water contains roughly 10²¹ molecules, far too many to count.

Chemists solve both problems at once by working with a huge, fixed-size group of particles called the mole, in the same way a stationery shop counts paper in reams rather than sheets.

Reactions happen particle by particle, so their recipes are written in particles; laboratory work happens gram by gram. The mole is the translation between those two languages.

Whenever a problem gives you grams but asks about particles (or the reverse), expect the mole to appear somewhere in the middle.

What a mole actually is

Since the 2019 revision of the SI, the mole is defined exactly: one mole contains exactly 6.02214076 × 10²³ elementary entities. That number is the fixed numerical value of the Avogadro constant, N_A, expressed in mol⁻¹.

In this course you will normally round it to 6.022 × 10²³ mol⁻¹.

The definition says nothing about which particles they are, so you must always state the entity: one mole of CO₂ means 6.022 × 10²³ CO₂ molecules, while one mole of oxygen atoms means 6.022 × 10²³ O atoms.

1 mol = 6.022 × 10²³ entities (atoms, molecules, formula units, ions, electrons)

The mole is a count, not a mass and not a volume. Comparing one mole of lead with one mole of helium compares equal numbers of atoms, not equal masses.

Sources and further reading

Chemistry 2e, Section 3.1: Formula Mass and the Mole Concept
Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, et al. · OpenStax, Rice University · Chapter 3.1 · pp. Section 3.1 (web edition)
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Recorded attribution / license claim (applicability unverified): Adapted from "Chemistry 2e" by OpenStax, Rice University, licensed CC BY 4.0. Changes were made.
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SI base unit: mole (mol)
Bureau International des Poids et Mesures (BIPM) · SI base units
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Definitions of SI Base Units
National Institute of Standards and Technology (NIST)
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Chem Help Question Bank, Unit 1 Stoichiometry, topic "Definition of the mole/Mole conversions"
Chem Help · Chem Help
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