Atom's structure

Thomson atomic model

Thomson atomic model

The Thomson model is a model of the atom proposed in 1904 by Joseph John Thomson. This new nuclear model was an evolution of Dalton's atomic model.

Sir Joseph John Thomson was a British physicist and Nobel Laureate in Physics. He discovered the electron before discovering the atomic nucleus, the first subatomic particle of the atomic structure

This new atomic theory from Thomson attempted to explain two then-known properties of atoms

  • Electrons are negatively charged particles.

  • Atoms do not have a neutral electric charge.

Thomson’s plum pudding model of an atom

Thomson's model has been compared (but not itself) to a dessert: it was also called plum pudding, hence the name of this model.

The plum pudding model has electrons surrounded by a volume of positive charge, much like negatively charged "plums" embedded in a positively charged "pudding.”

On the basis of the Thomson model of an atom

The basis of the Thomson model of an atom are summarized in these 4 points:

  • Thomson discovered electrons through his experiments with cathode ray tubes.

  • In his model, the atom is made up of negatively charged electrons embedded in a positively charged sphere-like in a "raisin pudding."

  • The electrons are evenly distributed throughout the atom.

  • The atom is neutral, so the negative charges on the electrons are offset by the positive charge.

Description of Thomson's atomic model

According to J.J. Thomson's atomic model, the atom consists of electrons placed in a positively charged "soup,” which compensates for the electrically negative charges of the electrons.

According to this model, electrons could spin freely in a drop or cloud of such a positively charged substance. Their orbits were stabilized within the atom. It happens because when an electron moves away from the center of a positively charged cloud, it experiences an increase in the force of attraction toward the center of the cloud.

This attractive force brings you back to the center. The force of attraction to the center of a uniformly charged spherical cloud is directly proportional to the distance from its center.

In Thomson's model, electrons can rotate freely in-ring orbits, which are stabilized by interactions between electrons. The line spectra were explained by the difference in energies when they moved along different trajectories of rings.

Thomson's model became a precursor to the later Bohr atomic model, which depicts the atom as a likeness of the solar system.

Drawbacks of Thomson model of the atom

Thomson's model of the atom was refuted in an experiment on the scattering of alpha particles in gold foil in 1909, which Ernest Rutherford analyzed in 1911. Rutherford's investigation suggested that the atom had a tiny nucleus that contains a large positive charge.

In 1913, Henry Moseley showed experimentally that the nuclear charge in elementary charges is very close to the atomic number.

This work eventually led to creating the Bohr model in the same year, similar to the solar system. According to this model, the nucleus has a positive charge equal to the atomic number and is surrounded by an equal number of electrons in orbital shells.

Thomson's problem

A still-unsolved mathematical physics problem was formulated by considering Thomson’s model - finding the configuration of many charges with the lowest potential energy in a sphere - Thomson's problem.

The objective of Thomson's problem is to determine the minimum electrostatic potential energy configuration of N electrons restricted to the surface of a unit sphere that repels each other with force given by Coulomb's Law.

The physicist J.J. Thomson raised the problem in 1904 after proposing his atomic model based on his knowledge of negatively charged electrons within neutrally charged atoms. Related issues include studying the geometry of the minimum energy configuration and the study of the behavior of large N of the minimum energy.

Publication Date: May 30, 2021
Last Revision: May 31, 2021