Thomson's Atomic Model: Characteristics, Experiments, Postulates

He Thomson's atomic model was recognized in the world for giving the first light on the configuration of protons and electrons within the structure of the atom. Through this proposal, Thomson suggested that the atoms were uniform and contained positive charge in a homogeneous way, with random deposits of electrons inside each atom.

To describe it, Thomson compared his model with the plum pudding. This simile was later used as an alternative name for the model. However, due to several inconsistencies (theoretical and experimental) about the distribution of electric charges within the atom, the Thomson model was discarded in 1911.

Origin

This atomic model was proposed by the English scientist Joseph John"J.J."Thomson in 1904, with the purpose of explaining the composition of atoms based on the notions of which we had knowledge by then.

Joseph John Thomson

In addition, Thomson was responsible for the discovery of the electron at the end of the 19th century. It is worth noting that Thomson's atomic model was proposed shortly after the discovery of the electron but before knowing the existence of an atomic nucleus.

Therefore, the proposal consisted of a dispersed configuration of all the negative charges within the atomic structure which, in turn, was made up of a uniform mass of positive charge.

characteristics

- The atom has a neutral charge.

- There is a source of positive charge that neutralizes the negative charge of the electrons.

- This positive charge is evenly distributed in the atom.

- In the words of Thomson:"negatively electrified corpuscles"- that is, electrons - are contained within the uniform mass of positive charge.

- The electrons could derive freely inside the atom.

- The electrons had stable orbits, argument based on the Gaussian Law. If the electrons moved through the positive"mass", the internal forces within the electrons were balanced by the positive charge that was automatically generated around the orbit.

- The Thomson model was popularly known in England as a plum pudding model, since the electron distribution proposed by Thomson was similar to the disposition of the plums in that dessert.

Experiments to develop the model

Thomson conducted several tests with cathode ray tubes to test the properties of subatomic particles and lay the groundwork for his model. Cathode ray tubes are glass tubes whose air content has been emptied almost entirely.

These tubes are electrified with a battery that polarizes the tube to have a negative charge end (cathode) and a positively charged end (anode).

They are also sealed on both sides and are subjected to high voltage levels by electrification of two electrodes placed on the cathode of the device. By this configuration the circulation of a particle beam from the cathode to the anode of the tube is induced.

Cathode Rays

Beams of electrons are directed from the cathode to the anode.

There is the origin of the name of this type of tools, since they are called cathode rays due to the point of exit of the particles inside the tube. By painting the anode of the tube with a material such as phosphorus or lead, a reaction is generated at the positive end just when the particle beam hits it.

In his experiments, Thomson determined the deflection of the ray in its path from the cathode to the anode. Subsequently, Thomson tried to validate the properties of these particles: basically the electric charge and the reaction between them.

The English physicist placed two electric plates with opposite charge on the upper and lower ends of the tube. Due to this polarization, the beam was diverted towards the positively charged plate, placed in the upper stop.

In this way, Thomson showed that the cathode ray was made up of negatively charged particles that, due to their opposite charge, were attracted towards the positively charged plate.

Evolution in research

Thomson evolved his assumptions and, after that finding, placed two magnets on both sides of the tube. This incorporation also affected some deviations of the cathode ray.

By analyzing the associated magnetic field, Thomson was able to determine the mass-to-charge ratio of subatomic particles and detected that the mass of each subatomic particle was negligible compared to the atomic mass.

J.J. Thomson created a device that preceded the invention and perfection of what is now known as a mass spectrometer.

This device performs a fairly accurate measurement of the relationship between mass and charge of the ions, which yields extremely useful information to determine the composition of the elements present in nature.

Repeating the experiment

Thomson performed the same experiment on multiple occasions, modifying the metals he used for the placement of the electrodes in the cathode ray tube.

Finally, he determined that the properties of the beam remained constant, regardless of the material used for the electrodes. That is, this factor was not determinant in the execution of the experiment.

Thomson's studies were very useful to explain the molecular structure of some substances, as well as the formation of atomic bonds.

Postulates

Thomson's model put together in a single statement the favorable conclusions of the British scientist John Dalton on the atomic structure, and hinted at the presence of electrons within each atom.

In addition, Thomson also conducted several studies on protons in neon gas, and thus demonstrated the electrical neutrality of atoms. However, the positive charge on the atom was proposed as a uniform mass and not as particles.

Thomson's experiment with cathode rays allowed the enunciation of the following scientific postulates:

- The cathode ray is constituted by subatomic particles of negative charge. Thomson initially defined these particles as"corpuscles".

- The mass of each subatomic particle is just 0.0005 times the mass of a hydrogen atom.

- These subatomic particles are found in all the atoms of all the elements of the Earth.

- The atoms are electrically neutral; that is, the negative charge of the"corpuscles"is equated with the positive charge of the protons.

Controversial model

The atomic model of Thomson was highly controversial within the scientific community, as it was contrary to the Dalton's atomic model .

The latter postulated that atoms were indivisible units, despite the combinations that may be generated during chemical reactions.

Thus, Dalton did not contemplate the existence of subatomic particles - such as electrons - within atoms.

In contrast, Thomson found a novel model that provided an alternative explanation of atomic and subatomic composition, after the discovery of the electron.

The atomic model of Thomson was quickly revealed by the simile with the popular English dessert"plum pudding". The mass of the pudding symbolizes an integral view of the atom and the plums represent each of the electrons that make up the atom.

Limitations

The model suggested by Thomson enjoyed great popularity and acceptance at the time, and served as a starting point to investigate the atomic structure and refine the associated details.

The biggest cause of acceptance of the model was how well it adapted to the observations of Thomson's cathode ray experiments.

However, the model had important opportunities for improvement in order to explain the distribution of electric charges within the atom, both positive and negative charges.

The investigations of Rutherford

Later, in the decade of 1910, the scientific school led by Thomson continued the investigations on the models of atomic structure.

This is how Ernest Rutherford, a former student of Thomson, determined the limitations of Thomson's atomic model, in the company of the British physicist Ernest Marsden and the German physicist Hans Geiger.

The trio of scientists carried out several experiments with alpha particles (α), that is, ionized nuclei of 4He molecules, without the surrounding of electrons.

This type of particles are made up of two protons and two neutrons, which is why the positive charge predominates. Alpha particles are produced in nuclear reactions or by experiments with radioactive decay.

Rutherford designed an arrangement that allowed to evaluate the behavior of the alpha particles when crossing solid substances, such as, for example, gold sheets.

In the analysis of the path it was detected that some particles presented an angle of deviation when penetrating the gold sheets. In other cases a slight bounce was also perceived on the impact element.

After the investigations with alpha particles, Rutherford, Marsden and Geiger contradicted Thomson's atomic model and proposed a new atomic structure instead.

New proposal

The counterproposal of Rutherford and his colleagues was that the atom was made up of a small, high density core, in which positive charges and a ring of electrons were concentrated around it.

The discovery of the atomic nucleus by Rutherford brought with it a new air for the scientific community. However, years later this model was also revoked and replaced by Bohr's atomic model.

References

1. Discovery of the electron and nucleus (s.f.). Retrieved from: khanacademy.org
2. J.J. Thomson Atomic Theory and Biography (s.f.). Retrieved from: thoughtco.com
3. Modern Atomic Theory: Models (2007). Retrieved from: abcte.org
4. Thomson atomic model (1998). Encyclopædia Britannica, Inc. Retrieved from: britannica.com
5. Wikipedia, The Free Encyclopedia (2018). Atomic model of Thomson. Retrieved from: en.wikipedia.org
6. Wikipedia, The free Encyclopedia (2018). Plum pudding model. Retrieved from: en.wikipedia.org