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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 6: The Discovery of Radioactivity

Roentgen Discovers X-rays

Figure 12-10. Roentgen Discovers X-rays

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Roentgen Discovers X-rays

Figure 12-10. Roentgen Discovers X-rays

One of Roentgen's first X-ray images. It shows his wife's hand.

In 1895, German physicist Wilhelm Roentgen (1845–1923) was doing research with a cathode ray tube when he made an unexpected discovery. A fluorescent screen near the tube began to glow. Roentgen soon deduced that the screen was being hit with some kind of rays or particles emitted by the cathode ray tube. The radiation from the tube was not the same as cathode rays; cathode rays were easily blocked by cardboard. The new radiation had much greater penetrating power and passed through many objects unchanged. Roentgen held his hand in front of the fluorescent screen and saw the shadow of his bones. He named the mysterious new rays "X-rays." (Figure 12-10)

Inspired by Roentgen's work, French physicist Henri Becquerel (1852–1908) investigated other fluorescent substances looking for other sources of X-rays.

Becquerel's Photographic Plate Exposed to Uranium

Figure 12-11. Becquerel's Photographic Plate Exposed to Uranium

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Becquerel's Photographic Plate Exposed to Uranium

Figure 12-11. Becquerel's Photographic Plate Exposed to Uranium

A photographic plate created by Becquerel. The darkened areas were exposed to radiation from a uranium compound.

One of the substances was a compound uranyl potassium sulfate. Becquerel sprinkled the uranium compound on a photographic plate encased in paper and put it in the sunshine. He hoped the sun's rays would cause the uranium to emit X-rays, which would produce an image on the plate. Becquerel did indeed find the expected image, and he planned to repeat the experiment the following day. The following two days were cloudy, however, so he put the plate in a drawer and waited for the weather to clear. A few days later, on a whim, Becquerel decided to develop the plate despite the fact that it had never been exposed to sunlight. To his surprise, the plate had the same kind of dark, fuzzy images where the uranium salt covered it, and the conclusion was inescapable: Some kind of radiation was coming from the uranium compound itself, which had nothing to do with sunlight or X-rays. (Figure 12-11)

Becquerel also discovered that the new radiation could ionize air. The ions made the air conductive, and by measuring the conductivity one could determine how radioactive a substance was. Using a sensitive electrometer developed by her husband, Polish scientist Marie Curie (1867–1934) investigated the radioactivity of uranium. (Figure 12-12)

Marie Curie

Figure 12-12. Marie Curie

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Marie Curie

Figure 12-12. Marie Curie

Marie Curie's first discovery was that radiation depended only on the quantity of uranium present and not on the amount of other substances with which it may be interacting.

Curie discovered that the amount of radiation given off by uranium was constant over a wide range of conditions. The radiation did not depend on the uranium's temperature, whether it was in a compound or pure, or in a solution or solid. She concluded that the radiation was not produced by some kind of chemical interaction with other substances or as a result of fluorescence; the radiation came from the uranium atoms themselves. Because the production of radiation does not increase with temperature, it differs from most chemical reactions in an important way; it does not have activation energy. However, there is also an important similarity. The production of radiation follows the pattern of first-order kinetics. In other words, the amount of radiation given off depends only on the amount of radioactive material, not on the amount of any other substance that might be interacting with it.

For this pioneering work in radioactivity, Marie Curie, her husband Pierre (1859–1906), and Henri Becquerel were awarded the Nobel Prize in Physics in 1903. The Curies went on to discover two new radioactive elements, polonium and radium, and for this Marie Curie won the Nobel Prize in chemistry in 1911. She is not only the first woman to win a Nobel Prize, but also the first person to win two Nobel Prizes. The element curium (Cm) was named in her honor.

Glossary

X-rays

The second-highest energy form of electromagnetic radiation, exceeded by gamma rays.

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