Although he did not use the word 'atom', Avogadro was one of the first to consider that perhaps particles were composed of molecules and that molecules in their turn were composed of the simpler atoms.
The Count of Quaregna and Cerrato, otherwise known as Amedeo Avogadro, was born on 9 August 1776 in Turin, Italy into an aristocratic family. Count Filippo Avogadro, his father, was a distinguished Piedmontese Lawyer, Senator (1768), Advocate General (1777), and Senate President (1799). Amedeo, a worthy son of this worthy father, got a baccalaureate in Jurisprudence at just sixteen (1792), and later, at the age of twenty, became a Doctor of Ecclesiastical Law. As expected, he joined the family profession and was a successful lawyer for the next five years. He was even made the Secretary to the Prefecture of the Eridano Department in 1801. Then, diverted by his long-standing interests in Science and Mathematics, he began to spend more time in studying these subjects, and, in 1803, intrigued by the research of Alessandro Volta, also looked into the subject of electricity in association with Felice, his brother. The two of them made such progress in their scientific endeavors, that the very next year they both received nominations to Turin's Royal Academy of Sciences. From here on the Law Profession was sidelined - although he remained involved in Public Affairs - and his principal occupation hereafter was to be science. He became, in 1806, a science instructor at the Royal College of the Provinces, and a Professor of Positive Philosophy at the Royal College of Vericelli, in 1809.
It was while at Vericelli, in 1811, that Avogadro wrote the hypothesis that is now famous as Avogadro's Law. It was based on the French Chemist Joseph Louis Gay-Lussac's Law of Combining Volumes, 1808 ('Gases under equal conditions of temperature and pressure react with one another in volume ratios of simple whole numbers'), and stated that 'Equal volumes of gases, at the same temperature and pressure, contain the same number of molecules'. This implied that the relative molecular weights of any two gases would be equal to the ratio of the gas densities under the same temperature and pressure.
Avogadro also concluded that simple gases were composed of combinations of two or more atoms, not single atoms as was generally thought then - although he did not employ the term 'atom', he was one of the first to consider that perhaps particles were composed of molecules, and that molecules in turn were composed of the simpler atoms. This resolved the controversy of Gay-Lussac's Law - it could now be understood why the water vapor volume was double that of the oxygen volume used in its formation as had been reported by Gay-Lussac - it was because the oxygen molecule split into two atoms during the formation of water vapor.
However, neither Gay-Lussac's theory nor Avogadro's new hypothesis fitted with the then widely accepted Atomic Theory of Dalton and Berzelius's Dualism Theory. John Dalton had wrongly assumed that a simple compound of two elements could only contain one atom of each and, as he had equated an atom with a particle, he could not see how one oxygen particle could bring forth two water particles. Berzelius, on the other hand, believed that compounds formed due to the cohesion between positive electrical charges and negative electrical charges, and so, of course, a molecule with two similarly-charged atoms (as in the case of oxygen) was just out of the question - they would repel each other after all, wouldn't they?
Dalton and Berzelius were the two leading Chemists of the period, hence, their opinions carried a lot of weight. Avogadro, on the other hand, was a modest, somewhat reclusive personality, who was not associated with the important Scientific Societies in England, France, Germany, Sweden, and moreover was then practically unknown outside Italy. His paper was also not supported by experimental evidence. So his hypothesis - first published under the title "Essai d'une manière de déterminer les masses relatives des molecules élémentaires des corps, et les proportions selon lesquelles elles entrent dans ces combinaisons" in the 14th July Edition of De Lamétherie's 'Journal de Physique, de Chemie, et d'Histoire naturelle' - went unappreciated for a very long time. Nearly fifty years later, at the Karlsruhe Conference of 1860, another Italian Scientist Stanislau Cannizzaro proved the importance of the theory in determining both molar and atomic masses.
Avogadro's hypothesis led to the idea of gram molecular weight and so the number of molecules contained in the gram molecular weight of a substance was named Avogadro's Number in his honor. Avogadro's Number (N) is a fundamental constant that helps Scientists to calculate the exact amounts of pure substances produced in a reaction, and this constant was actually first determined by an Austrian Scientist named Josef Loschmidt (1821-1895) and later other Scientists like James Maxwell, Lord Kelvin, J. J. Perrin, Rutherford, Geiger, R.T. Birge, calculated its value more accurately. The value of N has been determined as 6.023 x 1023 molecules per gram-mole. This is an extraordinarily large number, quite beyond comprehension actually.
To get back to Avogadro however, he remained unacknowledged during his lifetime, but undeterred by the general disregard of his hypothesis, he kept on with his research and later published many other important papers. The most important of these was his four-volumed tome on Physics 'Fisica dei corpi ponderabili' (1837-1841). He became a full member of the Royal Academy of Sciences of Turin in 1819 and also became the first Professor of Mathematical Physics at Turin University in 1820, a position he retained for the rest of his career, except for the period between 1822 and 1834, when, owing to his involvement in the political unrest against the King of Sardinia, he was temporarily discharged. Reinstated in 1834, he remained until his retirement in 1850.
Avogadro died on the 9 July 1856. His family included his wife Felicita and six sons.