Avogadro's Number
Avogadro's number (N_A) is the defined numerical value 6.02214076 × 10²³ mol⁻¹, representing the number of constituent particles in one mole of a substance. Since the 2019 SI redefinition, it is an exact fixed constant rather than a measured value. It links the macroscopic world of grams and liters to the microscopic world of atoms and molecules, and is one of the most fundamental constants in all of science.
Key Formula
N_A = 6.02214076 × 10²³ mol⁻¹
LaTeX: N_A = 6.02214076 \times 10^{23}\, \text{mol}^{-1}
| Symbol | Meaning | Unit |
|---|---|---|
| N_A | Avogadro's number — number of entities per mole | mol⁻¹ |
Worked Example
Problem
How many atoms are present in 2.5 mol of aluminium (Al)?
Solution
Step 1: Use the relationship: N = n × N_A. Step 2: Substitute: N = 2.5 mol × 6.022 × 10²³ mol⁻¹. Step 3: Calculate: N = 1.506 × 10²⁴ atoms.
Answer
1.506 × 10²⁴ atoms of aluminium
Historical Measurements of Avogadro's Number
| Year | Scientist | Method | Value Obtained |
|---|---|---|---|
| 1865 | Josef Loschmidt | Kinetic theory of gases | ≈ 4.0 × 10²³ |
| 1908 | Jean Perrin | Brownian motion | 6.5 × 10²³ |
| 1917 | Robert Millikan | Oil drop / X-ray | 6.064 × 10²³ |
| 1965 | Various | Crystal diffraction | 6.0220 × 10²³ |
| 2019 | BIPM redefinition | Exact SI definition | 6.02214076 × 10²³ (exact) |
Interactive Tools
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Related Terms
Mole (chemistry)
The mole is the SI base unit for the amount of substance, defined as exactly 6.02214076 × 10²³ elementary entities (atoms, molecules, ions, or other particles). It provides chemists with a practical bridge between the atomic scale and macroscopic, measurable quantities in the laboratory. One mole of any substance contains the same number of particles, making it the universal counting unit of chemistry.
Molar Mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol), and numerically equal to the substance's relative atomic or molecular mass in unified atomic mass units. It is calculated by summing the atomic masses of all atoms in one formula unit of the substance, using values from the periodic table. Molar mass is the essential conversion factor between grams (measurable in the lab) and moles (used in chemical calculations).
Stoichiometry
Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It uses balanced chemical equations to calculate the masses, volumes, or moles of substances involved in a reaction. Stoichiometry is fundamental to industrial chemistry, pharmaceutical manufacturing, and any process requiring precise control of chemical quantities.
Named after Amedeo Avogadro (1776–1856), an Italian scientist who proposed that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. The constant was named in his honour by physicist Jean Perrin in 1909.