Avogadro, Amedeo, Conte di Quaregna e Ceretto (1776-1856), Italian physicist and chemist who proposed a hypothesis that later became known as Avogadro's law. Avogadro was born in Turin and educated as a lawyer. He became interested in mathematics and physics and, after several years of study, was appointed to a professorship at the Royal College of Vercelli. From 1820 until his death, Avogadro was professor of physics at the University of Turin. Although he also conducted research on electricity and the physical properties of liquids, he is best known for his work with gases, which led him to formulate in 1811 the law that now bears his name.
Avogadro's law states that two equal volumes of gas at the same temperature and pressure contain an equal number of gas molecules. Now known to be true, this law was not universally accepted until the 1850s.

Avogadro's Number, the number of molecules that exist in one mole, or gram molecular weight, of any substance. One gram molecular weight is the weight of a substance, in grams, that is numerically equivalent to the dimensionless molecular weight of that substance (see Periodic Law). The number of molecules in one gram molecular weight has been determined to be approximately 6.0221367 × 1023 molecules, as established by various methods currently available to physical chemists.

The Avogadro number is named in honor of the Italian physicist Amedeo Avogadro, who postulated in 1811 that equal volumes of gases, at equivalent temperatures and pressures, contain the same number of molecules (see Avogadro's Law). The theory was significant in the development of chemistry, but the number itself was not calculated until the later 19th century, when the concept was extended to include not only gases but all chemicals. Volume considerations do not apply to liquids or solids, but Avogadro's number itself holds true for all substances, whatever their state.

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