Atomic Models- Thomson and Rutherford

The Thomson and Rutherford models were one of the atomic models accepted throughout the years until we arrived at the quantum model of the atom. 

Thomson proposed that atoms could be described as negative particles floating within a soup of diffused positive charges. This model is often called the plum pudding model of the atom since its description is very similar to plum pudding, a famous English dessert. Rutherford proposed the nuclear model, in which an atom consists of a tiny, positively charged nucleus surrounded by negatively charged electrons. Rutherford calculated that the nucleus took up a tiny fraction of the atom’s volume based on the number of alpha particles deflected in his experiment. 

Thomson and Rutherford models

One of several historical, scientific models of the atom is the plum pudding model. In a paper published in 1867, Lord Kelvin proposed a vortex atom, and J.J. Thomson continued his work on that hypothesis. 

J.J. Thomson abandoned his 1890 “nebular atom” hypothesis, which proposed that the vortex arrangement and periodic regularity found in chemical elements shared similarities and was based on the vortex theory of the atom. 

Thomson, an astute and practical scientist, based his atomic model on available experimental evidence at the time. He followed Lord Kelvin’s lead, as Kelvin had proposed a positive sphere atom a year before.

J.J. Thomson’s approach to scientific discovery, which proposed ideas to guide future experiments, is reflected in his proposal for a positive volume charge based on Kelvin’s model.

J.J. Thomson proposed the model in 1904, shortly after discovering the electron but before discovering the atomic nucleus, to explain two known properties of atoms at the time: electrons are negatively charged particles and atoms have no net electric charge. 

A positive charge volume surrounds electrons in the plum pudding model, similar to negatively charged “plums” embedded in a positively charged “pudding.” 

At this point, differences are seen between Rutherford and Thomson’s model. Ernest Rutherford, his student, used the plum pudding model to devise experiments to investigate atom composition further. 

Furthermore, Thomson’s model improved on previous solar system models proposed by Joseph Larmor and the Saturnian ring model for atomic electrons proposed by Nagaoka in 1904 (after James Clerk Maxwell’s model of Saturn’s rings), as these could not withstand classical mechanics as electrons spiralled into the nucleus in the solar system models, so they were dropped in favour of Thomson’s model.

Rutherford disproved Thomson’s model with his well-known gold foil experiment in 1911, demonstrating that the atom has a tiny and heavy nucleus. In addition, Rutherford devised an experiment in which alpha particles emitted by a radioactive element served as probes into the world of unseen atomic structure. 

Thomson was correct in predicting that the beam would pass straight through the gold foil. Although most of the rays were not deflected as they passed through the foil, a few were.

To explain the unexpected experimental results, Rutherford presented his physical model for subatomic structure. 

It consists of a centrally charged atom (the modern atomic nucleus, though Rutherford did not use the term “nucleus” in his paper) surrounded by a cloud of (presumably) orbiting electrons. However, Rutherford only committed to a small central region of highly positive or negative values in this May 1911 paper.

According to the Rutherford paper, the central charge of an atom should be “proportional” to its atomic mass in hydrogen mass units (roughly half in Rutherford’s model).

This gold mass number is 197 (not precisely known at the time) and Rutherford estimated it to be 196 u. On the other hand, Rutherford did not attempt to establish a direct relationship between central charge and atomic number because gold’s “atomic number” (at the time, simply its position in the periodic table) was 79. The charge was calculated to be around +100 units (he had suggested 98 units of positive control to make half of 196). 

As a result, Rutherford never proposed that the two numbers (79 on the periodic table and 98 or 100 on the nuclear charge) could be identical.

Difference between Thomson and Rutherford atomic models 

Thomson and Rutherford’s model was the first to explain the structure of an atom. J.J. Thomson proposed a model to explain the design of the atom after discovering the electron. Later, Rutherford discovered the nucleus and proposed a new model incorporating electrons and the nucleus. The main difference between the Thomson and Rutherford atomic models is that the Thomson model contains no information about the nucleus, whereas the Rutherford model does.

Conclusion

The main difference between the Thomson and Rutherford atomic models is that the Thomson model contains no information about the nucleus of an atom, whereas the Rutherford model does. J.J. Thomson was the first to discover the electron, a subatomic particle, in 1904. The “plum pudding model of the atom” was the name given to his proposed model. Ernest Rutherford proposed a new model for the atom in 1911, following his discovery of the atomic nucleus in 1909.