1811 – Italy

‘Equal volumes of all gases at the same temperature and pressure contain the same number of molecules’

In 1811, when Avogadro proposed his HYPOTHESIS, very little was known about atoms and molecules. Avogadro claimed that the same volume of any gas under identical conditions would always contain the same number of fundamental particles, or molecules. A litre of hydrogen would contain exactly the same number of molecules as a litre of oxygen or a litre of carbon dioxide.

Drawing of AVOGADRO ©

In 1814 ANDRE AMPERE was credited with discovering that if a gas consisted of a single element, its atoms could clump in pairs. The molecules of oxygen consisted of pairs of oxygen atoms, and the molecules of chlorine, pairs of chlorine atoms.
Diatomic gases possess a total of six degrees of simple freedom per molecule that are related to atomic motion.

This provides a way of comparing the weights of different molecules. It was only necessary to weigh equal volumes of different gases and compare them. This would be exactly the same as comparing the weights of the individual molecules of each gas.

Avogadro realised that GAY-LUSSAC‘s law provided a way of proving that an atom and a molecule are not the same. He suggested that the particles (molecules) of which nitrogen gas is composed consist of two atoms, thus the molecule of nitrogen is N2. When one volume (one molecule) of nitrogen combines with three volumes (three molecules) of hydrogen, two volumes (two molecules) of ammonia, NH3, are produced.

N2 + 3H2 ↔ 2NH3

However, the idea of a molecule consisting of two or more atoms bound together was not understood at that time.

Avogadro’s law was forgotten until 1860 when the Italian chemist STANISLAO CANNIZZARO (1826-1910) explained the necessity of distinguishing between atoms and molecules.

Avogadro’s constant
From Avogadro’s law it can be deduced that the same number of molecules of all gases at the same temperature and pressure should have the same volume. This number has been determined experimentally: it’s value is 6.022 1367(36) × 1023AVOGADRO’S NUMBER


That at the same temperature and pressure, equal volumes of all gases have the same number of molecules allows a simple calculation for the combining ratios of all gases – by measuring their percentages by volume in any compound. This in turn facilitates simple calculation of the relative atomic masses of the elements of which it is composed.

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EMPEDOCLES of AGRIGENTUM (Sicily) (c.494 – c.434 BCE)

‘The four roots of all things are: AIR, WATER, FIRE and EARTH’

Two forces exist – LOVE and STRIFE.

The view of Empedocles developed the monists’ ideas that all substances are derived from a single source, into the concept of objects consisting of different compositions of these four basic elements.
The materials of the natural world being wrought from different blends of the four elemental principles, brought about through the eternal conflict between Love and Strife; their waxing and waning applied to cause mixing when Love is dominant, or separation by Strife.
Empedocles argued that this was the cause of transformation not just of the elements but also of the lives of people and cultures.

Popular mythology described how Aphrodite fashioned the human eye out of the four elements, held together by Love. She kindled the fire of the eye at the hearth fire of the universe, so that it would act like a lantern, transmitting the fire of the eye out into the world and making sight possible.
Empedocles realised that there must be more to sight than this, and that the darkness of night is caused by the body of the Earth getting in the way of light from the sun.

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