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Natural Science - Year II

Unit 53: arie and Pierre Curie and Radioactivity

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History Weblecture for Unit 53

This Unit's Homework Page History Lecture Science Lecture Lab Parents' Notes

History Lecture for Unit 53: The Breakdown of the Atom

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The Breakdown of the Atom

At the beginning of the last decade of the nineteenth century, the president of one of America's prestigious colleges advised students of science to choose some other field than physics, claiming that nothing was left to do except measure a few constants to better accuracy. The periodic table proclaimed the existence and characteristics of a number of different but comfortably solid atomic elements, the laws of chemical combination described by Dalton and the chemists showed how these would combine into molecules. The effects of electrical and magnetic on each other was well known. The laws of genetic inheritance were being worked out according to the mathematics of statistical probability. Except for Boltzmann's little theory about the statistical nature of thermodynamics, which introduced an element of chance into what should have been a solidly deterministic view of the world, nature appeared to be well-ordered and well under the control of physical theory.

Thirty years later, all that had changed. The physical scientists of the twentieth century stood faced with atoms which decayed, and were mostly space, space and time which contracted or expanded at will, energy and matter which could change places and form with one another, and a universe vastly larger than anything anyone had ever imagined before.

Roëntgen and X-Rays

Maxwell's realization that electromagnetic radiation was the source of visible light spurred many physicists to study non-visible electromagnetic radiation -- that is, light with wavelengths longer or shorter than can be detected by the human eye. Even before Maxwell's identification of EM waves with light, the astronomer William Herschel had discovered that when he used a prism to split sunlight, he could detect heat below the visible red area of the spectrum. This infrared light could not be seen, but could be detected by a change in temperature where it struck matter. Chemists experimenting with light-driven reactions found that reactions using ultraviolet light (beyond the violet end of the spectrum) occurred more rapidly than reactions using light in the visible or infrared spectrum.

Spectrum Beyond Visible Light

Obviously, ultraviolet light carried more energy than visible light. After much experimentation, physicists figured out how to make cathode ray tubes (CRTs--the forerunner of the television and computer screen), as a reliable way of generating high energy radiation. Electric current is passed through a tube with gas at a very low pressure. The fast-moving electrons in the current collide with the gas particles, causing a characteristic glow.

In 1885, while he was working with such a "Crookes" tube, Wilhelm Roëntgen discovered that the radiation from the tube caused a glow on a piece of fluorescent paper at some distance from the tube. He wrapped the tube in black paper, but it didn't stop the glow. Apparently his "X-rays" could penetrate solid matter of low density. Roëntgen immediately recognized the medical possibilities for his rays, and used them to make a "penetrating photograph" of a human body, showing its skeletal structure.

Read Roëntgen's own announcement of his new kind of ray.

  • What materials did Roëntgen use to try to stop the X-rays? Which could X-rays pass through easily? Which reduced the intensity of fluorescence? Which block X-rays the best?
  • What does Roëntgen mean by "caloric effects"?
  • Was Roëntgen able to get X-rays to refract (bend) by passing them through a prism?

Becquerel and Uranium Rays

A year after Roëntgen's discovery, the French scientist Henri Becquerel was studying X-rays and discovered yet another kind of ray that could expose photographic plates even when these were covered in paper opaque to visible light. These rays did not come from a light source like a Crookes's tube. Instead, they came from matter itself.

Becquerel had left a sample of uranium crystals on top of a set of wrapped photographic plates. When he opened the package to expose the plates to X-rays for his experiments, he found they were already "fogged" in a pattern that matched the sample package of uranium.

Read about Becquerel's experiments and look at his plates (this is actually a set of instructions for teaching about Becquerel's discovery -- you can skip those and concentrate on the historical information and extracts from Becquerel's diaries).

  • How did Becquerel show that the uranium was not "leaking" chemicals onto the plates?
  • How did Becquerel show that the uranium crystals did not need to be "charged" with sunlight before exposing the plates?
  • What materials did Becquerel test to see which were transparent to the new ray? Which were most transparent? Which were most opaque?

Thomson and Beta Rays

Many people began to work with Becquerel's new radiation, trying to see how it responded to different conditions. One of the first discoveries was that, unlike normal light and X-rays, Becquerel's radiation would bend when it passed through a magnetic field....some of the light bending one way, the rest in the opposite direction. These two kinds of rays were named α-rays (alpha rays) and β-rays (beta rays). Paul Villard, a French physicist working with radioactive materials, found a third kind of ray, γ-rays (gamma rays) which also penetrated metal sheets, exposed wrapped photograph plates, but which did not bend in a magnetic field. These were later identified as a form of electromagnetic radiation, but α-rays and β-rays were not light, even though they shared some characteristics of light.

Working on β-rays, the English physicist Joseph John Thomson (often "J.J." in his publications) was able to prove that these rays acted like negatively-charged particles passing through a magnetic field. Although he could not measure the charge and mass separately, he was able to vary the magnetic field, measure the amount of deflection (bending) for the rays, and using Maxwell's equations, show that the charge-to-mass ratio for the particles in the ray was a fixed amount that indicated either the charge on the particle was very large, or that the particle itself was very small, about 1/2000th of the mass of the hydrogen atom.

Take a look at the discovery of the electron at the Center for the History of Physics. Follow the links to read about Thompson's life, his mysterious rays, and his 1897 experiments.

  • Why did Thomson think he had found a particle "more fundamental" than the atom?
  • How did Thomson describe the atom? Where was the positive charge (required because most atoms are neutrally charged)?

The Curies and Radiation

Read about Marie Curie at the same Center for History of Physics. Note: this article has several sections. Pay particular attention to the Research Breakthroughs pages.

  • Does the amount of radiation depend on whether the radioactive material is in a compound?
  • What evidence did Marie have that at least a third radioactive element that was neither uranium or thorium, existed in her pitchblende sample?
  • What problems did the Curies have in getting other scientists to accept their discovery of polonium and radium?

Number 10, The School of Chemistry and Physics, looking very much as it did in the late 1890s, when the Curies worked here in their lab.


A plaque marks the building and commemorates the discovery of radium and radiation.

Curies Tomb

The Curies' tombs in the Pantheon, Paris.
Prioir to November, 2021, Marie Curie was the only woman buried here.

Study/Discussion Questions:

Further Study/On Your Own