Amedeo Avogadro Biography

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World of Scientific Discovery on Amedeo Avogadro

Avogadro was born in Turin, Italy, on August 9, 1776, the son of Count Filippo Avogadro and Anna Maria Vercellone. Count Avogadro was a lawyer, civil servant, and senator for the state of Piedmont. Amedeo followed his father into the law and received his doctorate in ecclesiastical law in 1796.

He practiced law for only a few years, however, before his interests shifted to the sciences. He studied mathematics and physics on his own and, in 1806, was appointed demonstrator in physics at the Academy of Turin. In 1809 he was appointed professor of natural philosophy at the College of Vercelli. A decade later he became the first professor of mathematical at Turin although he lost his chair in the revolution of 1822. He was reappointed in 1835 and remained at Turin until he retired in 1850. He died in Turin on July 9, 1856.

Avogadro's name is best known in connection with his hypothesis about the composition of gases. In 1809, the French chemist and physicist Joseph Gay-Lussac had published reports on his research on the combining volumes of gases. Gay-Lussac reported that, under conditions of equal temperature and pressure, gases combine with each other in simple whole number ratios. For example, when hydrogen and oxygen combine to form water, they do so in the proportions of 2 volumes of hydrogen to 1 volume of oxygen, producing two volumes of water vapor in the process.

The conclusion suggested by these results was obvious to Avogadro. Equal volumes of all gases, he said, must contain equal numbers of particles. This statement is known as Avogadro's hypothesis.

The same thought had occurred to John Dalton, originator of the atomic theory. But Dalton had decided to reject this hypothesis, as well as Gay-Lussac's experimental results. Dalton believed that gases were made of atoms that were in contact with each other. Since he thought that atoms of different elements were of different sizes, then equal volumes could not contain equal numbers of particles. He suggested that Gay-Lussac's research was faulty and carried out experiments of his own that produced results more consistent with his own ideas.

It was Dalton, however, who was in error this time. Gay-Lussac and others were able to demonstrate that the particles in a gas are not in contact with each other and actually make up only a very small fraction of the gas itself. Most of the gas is empty space. So the size and shape of atoms in the gas are irrelevant.

Avogadro made one further suggestion to clarify Gay-Lussac's results. Consider the case with water again. Gay-Lussac showed that: 2 volumes of hydrogen + 1 volume of oxygen = 2 volumes of water.

If we assume the simplest case, in which each volume consists of only one particle, the same result can be expressed as: 2 particles of hydrogen (H H) + 1 particle of oxygen (O) = 2 particles of water (H-O H-O). (At the time, water was thought to consist of one atom of hydrogen and one atom of oxygen.)

Obviously, this situation is impossible. One cannot begin with one particle of oxygen and end with two. The solution Avogadro suggested is that the smallest particle of an element can sometimes consist of twoatoms joined together in a molecule, a term he invented for the new particle. Thus, if the smallest particle of oxygen is a molecule of oxygen, we can explain Gay-Lussac's results as follows: 2 particles of hydrodgen (H H) + 1 particle of oxygen (O-O) = 2 particles of water (H-O H-O).

We now know the smallest particle of hydrogen also consists of two atoms together. If we draw the reaction with modern forms of the components and products, Avogadro's solution works as follows: H-H H-H + O-O = H-O-H H-O-H.

Avogadro's hypothesis can also be applied to non-gasses. Equal atomic weights of any solid or liquid have the same number of particles. For example, an atom of carbon has an atomic weight of twelve, while a molecule of water is composed of an atom of oxygen with an atomic weight of sixteen plus two atoms of hydrogen with atomic weights of one each for a total of eighteen. Therefore, twelve grams of carbon will have the same number of particles as eighteen grams of water. This number of particles turns out to be 6.026 x 10²³ and is called Avogadro 's number.

Avogadro's suggestions expanded and improved on Dalton's atomic theory. For a number of reasons, however, his ideas were largely ignored for half a century. Not until Stanislao Cannizarro began to spread Avogadro's ideas in the 1850s did chemists finally understand and adopt them.