Chemistry · Ch 1 — Basic Concepts of Chemistry and Chemical Calculations
Avogadro Number
Avogadro Number
The number of elementary entities present in exactly one mole of any substance -- 6.022 × 10²³ -- is called the Avogadro number (or Avogadro constant), named after the Italian physicist Amedeo Avogadro, who proposed that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. Note carefully that the Avogadro number is a pure count and does not carry any unit.
Why the mole matters in reactions. In any chemical reaction, atoms and molecules react in a fixed, specific numerical ratio given by the balanced equation -- but that ratio is stated in terms of individual particles, which are impossible to count directly. Consider:
- Reaction 1: C + O₂ → CO₂ (one carbon atom + one oxygen molecule → one CO₂ molecule)
- Reaction 2: CH₄ + 2O₂ → CO₂ + 2H₂O (one methane molecule + two oxygen molecules → one CO₂ molecule + two water molecules) …
Worked out. A short biography box. Avogadro is known for Avogadro's hypothesis -- that equal volumes of all gases under the same temperature and pressure contain equal numbers of molecules. He did not himself determine the numerical value of the constant later named in his honour, but his hypothesis led to its eventual determination as 6.022 × 10²³. Rudolf Clausius, through the kinetic theory of gases, later provided supporting evidence for Avogadro's l …