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Portrait of Amedeo Avogadro

Science · Chemistry

Amedeo Avogadro

1776–1856 · Italian (Piedmontese)

Physical chemistry; molecular theory of gases

Amedeo Avogadro, Count of Quaregna and Cerreto, was a Turin lawyer turned professor of mathematical physics. In 1811 he proposed that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, and distinguished molecules from atoms, allowing true molecular weights to be worked out. Largely ignored in his lifetime, his hypothesis was championed by Stanislao Cannizzaro at the 1860 Karlsruhe Congress, and the Avogadro constant is named in his honour.

How to spot them

  • ▸Avogadro's law (hypothesis): equal volumes of gases contain equal numbers of molecules
  • ▸Avogadro constant, about 6.022 × 10^23 per mole
  • ▸Count from Turin; professor of mathematical physics
  • ▸Revived by Cannizzaro at the Karlsruhe Congress of 1860

Key discoveries & works

Hypothesis that equal volumes of gases contain equal numbers of molecules (1811)

At the same temperature and pressure; published in the Journal de physique

Distinction between 'elementary molecules' (atoms) and compound molecules (1811)

Reconciled Gay-Lussac's law of combining volumes with Dalton's atoms, implying diatomic gases such as O2 and H2

Fisica dei corpi ponderabili (1837–1841)

Four-volume treatise on the physics of matter

Number of particles in one mole of substance (named after him) (named 20th century)

The constant now fixed exactly at 6.02214076 × 10^23 per mole in the 2019 SI

Key ideas

  • Gas volumes reflect numbers of molecules, so gas densities give relative molecular weights
  • Elementary gases such as hydrogen and oxygen exist as molecules of more than one atom

Quick-fire facts

  • •Born and died in Turin, then in the Kingdom of Sardinia
  • •Trained and practised as a lawyer (ecclesiastical law) before turning to science
  • •Became professor of mathematical physics at the University of Turin
  • •His hypothesis was little accepted until after his death; Cannizzaro's Sunto (1858) and the Karlsruhe Congress (1860) won chemists over
  • •The name 'Avogadro's number' was popularised by the French physicist Jean Perrin
  • •Mole Day is celebrated by chemists on 23 October, from 6:02 am to 6:02 pm

Drill it — 6 cards

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Sources