History of Atomic Theory
Philosophical ideas on atoms date from ancient times. From 1800-1810, John Dalton’s atomic theory elucidated scientific evidence for atoms. For a century however, debate raged about atoms as real entities or simply as a useful model.
Einstein’s 1905 paper on Brownian motion, using statistical methods, provided conclusive theoretical justification for the existence of atoms. The mammoth task of empirically testing Einstein’s theories fell to one Jean Perrin, a Parisian experimental physicist.
Born in Lille in 1870, Jean Baptiste Perrin began his researches on Cathode and Rontgen rays in the 1890’s. In 1897, awarded a doctorate for earlier work, he began a prestigious readership at the Sorbonne. Serving honourably in The Great War, he returned to his studies and in 1926, was awarded The Nobel Physics Prize. Thirty years at his Paris professorship and then ejected by The Nazis, he died in America in 1942, his remains later reinterred in The Panthéon.
What is Avogadro’s Constant (L)?
Named after Italian scientist Ameado Avogadro, L links the number of molecules in a substance to its molecular mass – one “moles” worth. At a given temperature and pressure, one mole contains 6.0 exp 23 molecules.
For example just 2 grams of hydrogen contains 600,000 billion billion molecules! Knowing L enables scientists to work out the number, mass and size of the individual atoms/molecules in a substance, effectively proving the existence of atoms and the so-called “discontinuity of matter”.
Jean Perrin and The Avogadro Constant
In response to Einstein’s 1905 paper, Perrin considered the consequences of The Kinetic Theory of Gases, developed by scientists such as Maxwell, Boltzmann et al decades earlier. He concluded it was possible to measure the size of gas molecules within a given volume and calculate the number of entities present. This led Perrin to realistic values for L and atomic radii but great uncertainty remained, because of measuring inaccuries and assumptions made about molecular interactions.
Perrin decided to model molecular distributions in liquids, using tiny particles of a soap-like material, gamboge. If quantities of identical, sufficiently small particles were distributed in a liquid, not all would settle out, even if denser than the surrounding liquid and at constant ambient temperature.
Perrin predicted such particles would become distributed at different levels in the liquid. In effect the particles would behave according to the same physical laws which, for example, govern the distribution of atmospheric gas molecules.
Measuring these effects empirically would allow Perrin to:
- Demonstrate the kinetic theories of Boltzmann et al applied to tiny particles in liquids as gamboge particles would be buffeted by the movement of liquid molecules
- Prove experimentally the theories for atoms/molecules proffered by Einstein in 1905 in his thesis on Brownian motion.
- Measure L by a method completely different to that based on the kinetic theory of gases and to establish conclusively that Dalton’s atomic theory was correct.
Perrin showed that microscopic particles in a liquid are, as in a gas, always moving. They are in perpetual movement, under bombardment from molecules of infinitesimally small size, even under conditions of perfect external equilibrium. Later findings decisively proved the veracity of Perrin’s brilliant empirical approach for which he deservedly won the 1926 Nobel Physics Prize.