Thomson's cathode ray experiment and Rutherford's gold foil experiment.

Key points

  • J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons.
  • Thomson's plum pudding model of the atom had negatively-charged electrons embedded within a positively-charged "soup."
  • Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus.
  • Based on these results, Rutherford proposed the nuclear model of the atom.

Introduction: Building on Dalton's atomic theory

In a previous article on Dalton's atomic theory, we discussed the following postulates:
  • All matter is made of indivisible particles called atoms, which cannot be created or destroyed.
  • Atoms of the same element have identical mass and physical properties.
  • Compounds are combinations of atoms of 22 or more elements.
  • All chemical reactions involve the rearrangement of atoms.
Dalton's ideas proved foundational to modern atomic theory. However, one of his underlying assumptions was later shown to be incorrect. Dalton thought that atoms were the smallest units of matter-tiny, hard spheres that could not be broken down any further. This assumption persisted until experiments in physics showed that the atom was composed of even smaller particles. In this article, we will discuss some of the key experiments that led to the discovery of the electron and the nucleus.

J.J. Thomson and the discovery of the electron

In the late 19th19^{\text{th}} century, physicist J.J. Thomson began experimenting with cathode ray tubes. Cathode ray tubes are sealed glass tubes from which most of the air has been evacuated. A high voltage is applied across two electrodes at one end of the tube, which causes a beam of particles to flow from the cathode (the negatively-charged electrode) to the anode (the positively-charged electrode). The tubes are called cathode ray tubes because the particle beam or "cathode ray" originates at the cathode. The ray can be detected by painting a material known as phosphors onto the far end of the tube beyond the anode. The phosphors spark, or emit light, when impacted by the cathode ray.
A diagram of a cathode ray tube.
A diagram of J.J. Thomson's cathode ray tube. The ray originates at the cathode and passes through a slit in the anode. The cathode ray is deflected away from the negatively-charged electric plate, and towards the positively-charged electric plate. The amount by which the ray was deflected by a magnetic field helped Thomson determine the mass-to-charge ratio of the particles. Image from Openstax, CC BY 4.0.
To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles.
Thomson also placed two magnets on either side of the tube, and observed that this magnetic field also deflected the cathode ray. The results of these experiments helped Thomson determine the mass-to-charge ratio of the cathode ray particles, which led to a fascinating discovery-the mass of each particle was much, much smaller than that of any known atom. Thomson repeated his experiments using different metals as electrode materials, and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. From this evidence, Thomson made the following conclusions:
  • The cathode ray is composed of negatively-charged particles.
  • The particles must exist as part of the atom, since the mass of each particle is only \sim12000\dfrac{1}{2000} the mass of a hydrogen atom.
  • These subatomic particles can be found within atoms of all elements.
While controversial at first, Thomson's discoveries were gradually accepted by scientists. Eventually, his cathode ray particles were given a more familiar name: electrons. The discovery of the electron disproved the part of Dalton's atomic theory that assumed atoms were indivisible. In order to account for the existence of the electrons, an entirely new atomic model was needed.
Concept check: Why did Thomson conclude that electrons could be found in atoms of all elements?
As part of his experiments with cathode ray tubes, Thomson tried changing the cathode material, which was the source of the particles. Since the same particles were emitted even when the cathode materials were changed to different metals, Thomson concluded that the particle was a fundamental part of all atoms.

The plum pudding model

Thomson knew that atoms had an overall neutral charge. Therefore, he reasoned that there must be a source of positive charge within the atom to counterbalance the negative charge on the electrons. This led Thomson to propose that atoms could be described as negative particles floating within a soup of diffuse positive charge. This model is often called the plum pudding model of the atom, due to the fact that its description is very similar to plum pudding, a popular English dessert (see image below).
The plum pudding model of the atom on the right, and a picture of plum pudding dessert on the left.
The plum pudding model depicts the electrons as negatively-charged particles embedded in a sea of positive charge. The structure of Thomson's atom is analogous to plum pudding, an English dessert (left). Image from Openstax, CC BY 4.0.
Given what we know now about the actual structure of atoms, this model might sound a little far-fetched. Luckily, scientists continued to investigate the structure of the atom, including testing the validity of Thomson's plum pudding model.
Concept check: Thomson proposed an atomic model with distinct negative charges floating within a "sea" of positive charge. Can you think of another model of the atom that would explain Thomson's experimental results?
There are many models of the atom that might explain Thomson's results! Other models proposed at the time included Hantaro Nagaoka's "planetary model," which had the electrons revolving around a positively charged "planet" like the rings around Saturn.

Ernest Rutherford and the gold foil experiment

The next groundbreaking experiment in the history of the atom was performed by Ernest Rutherford, a physicist from New Zealand who spent most of his career in England and Canada. In his famous gold foil experiment, Rutherford fired a thin beam of α\alpha particles (pronounced alpha particles) at a very thin sheet of pure gold. Alpha particles are helium nuclei (24He2+)(_2^4\text{He}^{2+}), and they are given off in various radioactive decay processes. In this case, Rutherford placed a sample of radium (a radioactive metal) inside a lead box with a small pinhole in it. Most of the radiation was absorbed by the lead, but a thin beam of α\alpha particles escaped out of the pinhole in the direction of the gold foil. The gold foil was surrounded by a detector screen that would flash when hit with an α\alpha particle.
Believe it or not, the use of gold was not simply a result of Rutherford's extravagant taste. Gold is incredibly malleable, which means it can be hammered into extremely thin sheets. In fact, the thinnest gold sheets can have widths as small as 0.00004 cm0.00004\text{ cm}, which is only only a few hundred atoms thick! A foil this thin was necessary for Rutherford to carry out his experiment successfully. If the foil were any thicker, the α\alpha particles might not have been able to penetrate it.
The apparatus used in Rutherford's gold foil experiment.
In Rutherford's gold foil experiment, a beam of α\alpha particles that was shot at a thin sheet of gold foil. Most of the α\alpha particles passed straight through the gold foil, but a small number were deflected slightly, and an even smaller fraction were deflected more than 9090^{\circ} from their path. Image from Openstax, CC BY 4.0.
Based on Thomson's plum pudding model, Rutherford predicted that most of the α\alpha particles would pass straight through the gold foil. This is because the positive charge in the plum pudding model was assumed to be spread out throughout the entire volume of the atom. Therefore, the electric field from the positively charged "soup" would be too weak to significantly affect the path of the relatively massive and fast-moving α\alpha particles.
The results of the experiment, however, were striking. While almost all of the α\alpha particles passed straight through the gold foil, a few α\alpha particles (about 11 in 2020,000000) were deflected more than 9090^{\circ} from their path! Rutherford himself described the results with the following analogy: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 1515-inch\text{inch} shell at a piece of tissue paper and it came back and hit you."
The expected results of Rutherford's gold foil experiment according to the Thomson model (left), and the actual results of his experiment (right).
Based on the plum pudding model of the atom, it was assumed that there was nothing dense or heavy enough inside the gold atoms to deflect the massive α\alpha particles from their paths (see left image). However, what Rutherford actually observed did not match his prediction (see image on right)-a new atomic model was needed!

The nuclear model of the atom

Based on his experimental results, Rutherford made the following conclusions about the structure of the atom:
  • The positive charge must be localized over a very tiny volume of the atom, which also contains most of the atom's mass. This explained how a very small fraction of the α\alpha particles were deflected drastically, presumably due to the rare collision with a gold nucleus.
  • Since most of the α\alpha particles passed straight through the gold foil, the atom must be made up of mostly empty space!
Picture of red electrons orbiting a small black sphere representing the nucleus.
The nuclear model of the atom. Image of Rutherford atom from Wikimedia Commons, CC-BY-SA-3.0.
This led Rutherford to propose the nuclear model, in which an atom consists of a very small, positively charged nucleus surrounded by the negatively charged electrons. Based on the number of α\alpha particles deflected in his experiment, Rutherford calculated that the nucleus took up a tiny fraction of the volume of the atom.
The nuclear model explained Rutherford's experimental results, but it also raised further questions. For example, what were the electrons doing in the atom? How did the electrons keep themselves from collapsing into the nucleus, since opposite charges attract? Luckily, science was ready for the challenge! Physicists such as Niels Bohr continued to design experiments to test the nuclear model of the atom, which eventually evolved into the modern quantum mechanical model.

Summary

  • J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons.
  • Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup."
  • Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus.
  • Based on these results, Rutherford proposed the nuclear model of the atom.

Attributions

This article was adapted from the following articles:
  1. "Atomic Theory" from UC Davis ChemWiki, CC BY-NC-SA 3.0 US.
The modified article is licensed under a CC-BY-NC-SA 4.0 license.

Additional References

Zumdahl, S.S., and Zumdahl S.A. (2003). Atomic Structure and Periodicity. In Chemistry (6th ed., pp. 290-94), Boston, MA: Houghton Mifflin Company.
Kotz, J. C., Treichel, P. M., Townsend, J. R., and Treichel, D. A. (2015). Key Experiments: How Do We Know the Nature of the Atom and Its Components? In Chemistry and Chemical Reactivity, Instructor's Edition (9th ed., pp. 54-55). Stamford, CT: Cengage Learning.
Loading