The number of atoms in one mole of a substance is equal to the

Analyze the question: It asks what the number of atoms (or particles) in one mole of a substance equals.

Explain the concept: A mole is the amount of a substance that contains the same number of elementary particles (atoms, molecules, or ions) as there are atoms in exactly 12 g of carbon-12. That fixed number is the Avogadro constant (Avogadro number):

N_A = 6.02 × 10²³ particles per mole

Check each option:

• A. Mass number: This is the number of protons plus neutrons in one atom's nucleus. It varies from element to element, so it is not a constant count of particles in a mole.
• B. Oxidation number: This is the charge an atom appears to have in a compound. It has nothing to do with counting particles in a mole.
• C. Atomic number: This is the number of protons in an atom. It differs for each element, so it is not correct either.
• D. Avogadro number: This is 6.02 × 10²³, the number of particles in one mole of any substance. ✔

Example: 1 mole of sodium (23 g) contains 6.02 × 10²³ sodium atoms. 1 mole of carbon (12 g) also contains 6.02 × 10²³ carbon atoms. The mass differs but the number of atoms is the same.

Exam tip: Whenever you see "number of particles in one mole," think Avogadro. Remember the link: 1 mole = 6.02 × 10²³ particles = molar mass in grams.