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Perrin’s atomic model: characteristics, postulates

With this, the electrical nature of cathode rays was demonstrated and shed light on the electrical nature of the atom, understanding it as the smallest and indivisible unit of matter . In 1901 Jean Baptiste Perrin suggested that the attraction of negative charges surrounding the center (positive charge) is counteracted by the force of inertia.

Atomic model of Perrin
Jean Baptiste Perrin

This model was later supplemented and perfected by Ernest Rutherford , who asserted that all the positive charge of the atom was located in the center of the atom, and that the electrons orbited around it.

Characteristics of the Perrin atomic model

The most prominent features of Perrin’s atomic model are as follows:

– The atom is made up of a large positive particle in its center, in which most of the atomic mass is concentrated.

– Several negative charges orbit around this concentrated positive charge that compensate for the total electrical charge.

Atomic model of Perrin
Illustration of Perrin’s atomic model

Perrin’s proposal compares the atomic structure to a solar system, where the concentrated positive charge would fulfill the role of the Sun and the surrounding electrons would fulfill the role of the planets.

Perrin was the pioneer in suggesting the discontinuous structure of the atom in 1895. However, he never insisted on designing an experiment that would help to verify this conception.

Experiment

As part of his doctoral training, Perrin served as Physics Assistant at the Ecole Normale Supérieure in Paris, between 1894 and 1897.

Cathode rays

The cathode ray experiment arose from research with Crookes tubes, a structure invented by the English chemist William Crookes in the 1870s.

The Crookes tube is made up of a glass tube that contains only gases inside. This configuration has a metal piece at each end, and each piece is connected to an external voltage source.

When the tube is energized, the air inside it ionizes and, consequently, it becomes a conductor of electricity and closes the open circuit between the electrodes at the ends.

Inside the tube the gases take on a fluorescent appearance, but until the late 1890s scientists were not clear about the cause of this phenomenon.

By then it was unknown if the fluorescence was due to the circulation of elementary particles within the tube, or if the rays took the shape of the waves that carried them.

Perrin’s investigations

In 1895 Perrin replicated the cathode ray experiments by connecting a discharge tube to a larger empty container.

In addition, Perrin placed an impermeable wall for ordinary molecules and replicated Crookes’ configuration by placing a Faraday Cage, contained within a protective chamber.

If the rays passed through the impermeable wall for ordinary molecules inside the Faraday cage, it would automatically be shown that the cathode rays were composed of fundamental electrically charged particles.

Verification method

To corroborate this, Perrin connected an electrometer near the impermeable wall to measure the electrical charges that would be produced when cathode rays struck there.

When conducting the experiment, it was evidenced that the collision of the cathode rays against the impermeable wall induced a small measurement of negative charge in the electrometer.

Subsequently, Perrin deflected the cathode ray flux by forcing the system through the induction of an electric field, and forced the cathode rays to impact against the electrometer. When that happened, the meter registered a considerably higher electrical charge compared to the previous record.

Thanks to Perrin’s experiments, it was shown that cathode rays were made up of particles with negative charges.

Later, in the early 20th century, JJ Thomson formally discovered the existence of electrons and their charge-mass relationship, based on Perrin’s research.

Postulates

In 1904 the British scientist JJ Thomson enunciated his proposal for an atomic model, also known as the plum pudding model.

In this model, the positive charge was understood as a homogeneous mass and the negative charges would be randomly dispersed on said positive mass.

In the analogy, the positive charge would be the mass of the pudding, and the negative charges would be represented by the plums. This model was refuted by Perrin in 1907. In his proposal, Perrin indicates the following:

  • The positive charge is not spread throughout the entire atomic structure. Rather, it is concentrated in the center of the atom.
  • Negative charges are not scattered throughout the atom. Instead, they are arranged in an orderly fashion around the positive charge, toward the outer edge of the atom.

Limitations

Perrin’s atomic model has two major restrictions, which were subsequently overcome thanks to the contributions of Bohr (1913) and quantum physics.

The most significant limitations of this proposal are:

  • There is no explanation for why the positive charge remains concentrated in the center of the atom.
  • The stability of the orbits of negative charges around the center of the atom is not understood.
  • According to Maxwell’s electromagnetic laws, negative charges would describe spiral orbits around positive charges, until they collided with them.

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