20/05/2020

In Folha de S.Paulo, the search for the structure of the atom.

O filósofo Auguste Comte – Wikimedia Commons

Reproduction of Marcelo Viana's column in Folha de S.Paulo.

The philosopher Auguste Comte, from whom we take the motto "order and progress," wrote in 1930: "Any attempt to use mathematical methods in the study of chemical questions must be considered profoundly irrational and contrary to the spirit of chemistry. If mathematical analysis should ever come to have 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 over the last 150 years has shown that the chemistry of substances reflects the structure of their atoms, and that this can only be understood in mathematical terms. Instead of being an aberration, this has produced progress that Comte could never have dreamed of.

Cathode rays were discovered in experiments with electricity in a vacuum conducted in 1869 by J. Plücker and J.W. Hittorf. In 1897, J.J. Thomson showed that they are formed by negatively charged particles, electrons, whose mass is 1,800 times smaller 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 have a negative charge, why are atoms electrically neutral?

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

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

In 1913, the Danish physicist Niels Bohr made a strange proposal, inspired by ideas about the quantization of energy introduced years earlier by Max Planck. He postulated that electrons could only occupy a certain finite number of orbits around the nucleus, jumping instantaneously 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 equal number to the electrons. This would be the mysterious atomic number, arbitrarily introduced by D. Mendeleev in 1869 to organize his Periodic Table of Elements. Bohr's atomic model also led to important discoveries, such as that of the laser.

To read the full text, visit the newspaper's website.