Discovery of the electron and nucleus (article) | Khan Academy (2024)

• 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.

• 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 $2$‍ 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.

In the late ${19}^{\text{th}}$‍ 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.

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 $\sim$‍${\displaystyle \frac{1}{2000}}$‍ 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?

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).

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?

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 $\alpha$‍ particles (pronounced alpha particles) at a very thin sheet of pure gold. Alpha particles are helium nuclei ${(}_2^4{\text{He}}^{2+})$‍, 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 $\alpha$‍ 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 $\alpha$‍ particle.

Based on Thomson's plum pudding model, Rutherford predicted that most of the $\alpha$‍ 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 $\alpha$‍ particles.

The results of the experiment, however, were striking. While almost all of the $\alpha$‍ particles passed straight through the gold foil, a few $\alpha$‍ particles (about $1$‍ in $20$‍,$000$‍) were deflected more than ${90}^{\circ }$‍ 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 $15$‍-$\text{inch}$‍ shell at a piece of tissue paper and it came back and hit you."

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 $\alpha$‍ particles were deflected drastically, presumably due to the rare collision with a gold nucleus.

• Since most of the $\alpha$‍ particles passed straight through the gold foil, the atom must be made up of mostly empty space!

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 $\alpha$‍ 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.

• 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.