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Discovery of the electron and nucleus

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 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 matterminustiny, 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 19, start superscript, start text, t, h, end text, end superscript 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 discoveryminusthe 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 \simstart fraction, 1, divided by, 2000, end fraction 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?

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?

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 alpha particles (pronounced alpha particles) at a very thin sheet of pure gold. Alpha particles are helium nuclei left parenthesis, start subscript, 2, end subscript, start superscript, 4, end superscript, start text, H, e, end text, start superscript, 2, plus, end superscript, right parenthesis, 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.
The apparatus used in Rutherford's gold foil experiment.
In Rutherford's gold foil experiment, a beam of alpha particles that was shot at a thin sheet of gold foil. Most of the alpha particles passed straight through the gold foil, but a small number were deflected slightly, and an even smaller fraction were deflected more than 90, degrees from their path. Image from Openstax, CC BY 4.0.
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, degrees 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-start text, i, n, c, h, end text 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 alpha particles from their paths (see left image). However, what Rutherford actually observed did not match his prediction (see image on right)minusa 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 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!
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 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.

Want to join the conversation?

  • blobby green style avatar for user rajaharis294
    How can we calculate e/m value of elactron?
    (6 votes)
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  • blobby green style avatar for user afeefdk.07
    comparing thomsons exp. and rutherford's exp. why does the rays deflect in rutherford's model due to presence of nucleus but travel straight in thomson's exp.
    (5 votes)
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  • primosaur seed style avatar for user sri shakthi
    is anode rays consist of proton ?
    (1 vote)
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    • leafers tree style avatar for user raghuwanshiparth
      You are right, both kinds of rays are emitted simultaneously. the cathode rays consist of electrons, while the anode/canal rays are the positively charged gaseous ions.
      The experiment for canal rays was carried out in modified cathode ray tube, by E. Goldstein.

      :: some properties of anode rays:
      1. The positively charged particles depend upon the nature of gas present in the cathode ray tube. Those are simply the positively charged gaseous ions.
      2. The charge to mass ratio of particles depends on the gas from which it originates.
      3. Some positively charged particles carry a multiple of a fundamental unit of electric charge.
      4. The behavior of these particles in a magnetic or electric field is opposite to that of electrons or cathode rays.

      THE SMALLEST AND LIGHTEST POSITIVE ION WAS OBTAINED FROM HYDROGEN AND WAS CALLED PROTON.

      Image of a canal ray experiment:-

      https://www.google.co.in/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwi41_ra0tTVAhXCOY8KHZJNC7kQjRwIBw&url=http%3A%2F%2Fchem-guide.blogspot.com%2F2010%2F04%2Fdiscovery-of-proton.html&psig=AFQjCNEUcp3n61_aLrSJ5u4dzti4Ozepvg&ust=1502728671172162

      Hope it's helpful.
      (9 votes)
  • mr pink red style avatar for user Thiziri Mnlight
    In th Rutherford experiment, why did he use alpha particles ( He2+) ? wouldn't it have been better if he had used ( H+) particles ( which we now know are protons ) because these are smaller ?
    (4 votes)
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    • old spice man green style avatar for user Matt B
      Rutherford was interested in radio-chemistry (radioactivity) and what it might reveal about the atom. is focus was on alpha particles (from alpha rays, type of radioactivity), which he then discovered consisted of two protons and two neutrons: the equivalent of a helium nucleus (He2+)
      (3 votes)
  • leafers seed style avatar for user Ali shair Awan
    How Rutherford told that atom is like a solar system?on which base ?
    (2 votes)
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    • duskpin ultimate style avatar for user Davide Ghazal
      Because he used classical physics (although he quantized the angular momentum) to conceive his model....and classical physics is based on intuitive thinking and empirical observations....thinking of subatomic particles as if they were planets is therefore, in line with the way we perceive reality....(a wrong way according to quantum mechanics).....
      (6 votes)
  • scuttlebug green style avatar for user Tzviofen ✡
    2 questions:
    1. How did Rutherford know that the atom was net neutral?
    2. Why did he decide that the nucleus was in the -middle- of the atom?
    (4 votes)
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  • leaf green style avatar for user dyoffis
    why did we have to name positive ions cations and not anions, since most of the naming in science seems to be such that the names are easy and a lot telling, why didnt we call positively charged ions anions since a positively charged electrode is called an anode, which leads to a whole bunch of confusion for me atleast.
    (2 votes)
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    • leaf red style avatar for user Richard
      The negatively charged electrode is the anode, this is where the electrons in a current originate. The positively charged electrode is the cathode and is where the electrons finish their journey at. Electrons are negatively charged and are pushed away from the negative charge of the anode towards the positive charge of the cathode which they find an attraction to.

      As for the anions and cations part, the names originate from William Whewell who worked with famous scientist Michael Faraday and his work in electricity. Whewell actually coined a lot of famous words including anode, cathode, ion, and probably most famously; scientist. Whewell suggested using anions for the ions that were attracted to the negatively charged anode and cations for the ions that were attracted to the positively charged cathode.

      Hope that helps.
      (5 votes)
  • marcimus pink style avatar for user Sagnik Sarkar
    Why only alpha-particles were fired? Why not beta or why not gamma-particles?
    (3 votes)
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    • female robot amelia style avatar for user Nikitha A
      alpha-particles are nuclie of Helium atoms and thus +ve charged and thus when it hit the nucleus of the gold atoms the alpha-particles reflected back.

      beta-particles are -ve charged and gamma-particles are neutral charged. To carry out his experiment he needed rays with +ve charge thus he chose alpha-particles, also they have the least penetrating power among all the 3 rays so that they hit the fluorescent screen and not prenetrate thought it. I think this is the reason why but i am not entirely sure.
      (1 vote)
  • piceratops tree style avatar for user Lakshmi.manda07
    How did Rutherford make the alpha particles of helium and how did he concentrate those particles so as to make them travel out of a pinhole of the lead box to bombard the gold foil?
    (2 votes)
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  • starky ultimate style avatar for user Akash Jaiswal
    How Rutherford was able to made that much of thin foil which is few atoms thin?
    (0 votes)
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