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In search of the structure of the atom

by ace
In search of the structure of the atom

The philosopher Auguste Comte, from whom we take the motto "order and progress", wrote in 1930: "Every attempt to use mathematical methods in the study of questions of chemistry must be considered profoundly irrational and contrary to the spirit of chemistry. If mathematical analysis is ever to play a prominent role in chemistry – an aberration that is fortunately almost impossible – it will cause a rapid degeneration of this science ”.

This was not Comte's only blunder.

The evolution of science in the last 150 years has shown that the chemistry of substances is a reflection of the structure of its atoms, and that this can only be understood in mathematical terms. In place of aberration, this produced progress that Comte could not even dream of.

Cathode rays were discovered in experiments with vacuum electricity performed in 1869 by J. Plücker and J. W. Hittorf. In 1897, J. J. Thomson showed that they are formed by particles with a negative electric charge, electrons, whose mass is 1,800 times less than that of the hydrogen atom. Thomson won the Nobel Prize in physics in 1906.

If the atom is not indivisible, what is its structure? And if electrons are negatively charged, why are atoms electrically neutral?

Thompson suggested that the electrons would be immersed in a positively charged sea, like fruit candied in an English cake batter. But experiments carried out in 1909 by students H. Geiger and E. Marsden gave results incompatible with this idea. Its advisor, E. Rutherford, then suggested that each atom would have a tiny, positively charged nucleus around which electrons would orbit, like planets around a star.

The problem is that according to electromagnetism, this model would be unstable: electrons would have to emit energy when describing their orbits, and would fall quickly into the nucleus.

In 1913, Danish physicist Niels Bohr made a strange proposal, inspired by the ideas about quantifying energy introduced years earlier by Max Planck. He postulated that electrons could only occupy a certain finite number of orbits around the nucleus, instantly jumping from one orbit to another when the atom absorbs or emits the right amount of energy to do so.

The nucleus would contain positive charges, which were called protons, in number equal to that of electrons. This would be the mysterious atomic number, introduced arbitrarily by D. Mendeleev in 1869 to organize his Periodic Table of the Elements. Bohr's atomic model also led to important discoveries, such as that of the laser.

Bohr won the Nobel Prize in physics in 1922, and was one of the leaders in quantum mechanics. But he never became as famous as his brother Harald. This story is for next week.

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