Amedeo Avogadro

Lore & Background

Lorenzo Romano Amedeo Carlo Avogadro, Count of Quaregna and Cerreto, was an Italian scientist born in Turin to a noble family of the Kingdom of Sardinia. He graduated in ecclesiastical law at age twenty and initially practiced law, but soon dedicated himself to physics and mathematics, then called positive philosophy. He began teaching these subjects at a liceo in Vercelli in 1809. In 1811, he published an essay containing his hypothesis that equal volumes of gases under the same temperature and pressure contain equal numbers of molecules, now known as Avogadro's law. He became a professor of physics at the University of Turin in 1820 but lost his chair in 1823 due to his involvement in a revolutionary movement. He was recalled to the university in 1833 and taught for another twenty years. Avogadro also held posts in statistics, meteorology, and weights and measures, introducing the metric system into Piedmont, and served on the Royal Superior Council on Public Instruction. His private life was sober and religious; he married Felicita Mazzé and had six children. He died on 9 July 1856. In his honor, the number of elementary entities per mole of a substance is called the Avogadro constant, one of the seven defining constants of the SI system.

Reader's Guide

Avogadro’s law established that equal volumes of any gas, under identical temperature and pressure, contain the same number of molecules. This principle allowed chemists to calculate relative molecular masses by comparing the masses of equal gas volumes. A key advance was his clear separation of the concepts of atoms and molecules—he proposed that gases consist of molecules, which themselves are composed of smaller units (what he called “elementary molecules,” equivalent to modern atoms). This distinction resolved widespread confusion, as contemporaries like John Dalton had not considered molecules as aggregates. Despite its importance, the hypothesis was largely ignored by the scientific community at the time; André-Marie Ampère proposed a similar idea three years later, but acceptance remained slow. Avogadro’s work also extended to determining gas densities and the composition of organic compounds. The Avogadro constant—the number of elementary entities per mole of a substance—is named in his honor and is one of the seven defining constants of the SI system, used to precisely compute the outcomes of chemical reactions. Beyond theoretical chemistry, Avogadro served in roles related to statistics, meteorology, and weights and measures, and he introduced the metric system into Piedmont.

Did You Know?

• Avogadro graduated in ecclesiastical law at age 20 before turning to physics and mathematics.
• Avogadro introduced the metric system into Piedmont.