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Henri Becquerel Atomic Theory

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  • 0:04 Radioactivity and…
  • 1:50 Becquerel's Atomic Theory
  • 3:59 Lesson Summary
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Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

Henri Becquerel made an important contribution to our understanding of atomic theory when he discovered the existence of radioactivity. In this lesson, learn about him and his amazing discovery.

Radioactivity and Henri Becquerel

You might know someone who has had radiation therapy for cancer or another disease. This is a very common treatment for a variety of medical conditions, and it harnesses the power of radioactivity. Some atoms are inherently unstable and their nuclei break down very easily. When this happens, a tiny, high energy particle is released. The process in which atomic nuclei break down and release these high energy particles is called radioactivity. The particles that they release are called radiation.

In the past century, understanding and harnessing the power of radioactivity has led to many new technologies. Although radioactivity can be used in many beneficial ways, it can also be dangerous. Exposure to radioactivity can not only treat cancer, but can actually cause cancer as well.

Although we now know a lot about how radioactivity works, it wasn't always this way. Back in 1885, no one knew that some atoms could be radioactive. The first person to conclude that atoms could be radioactive was a French scientist named Henri Becquerel.

Becquerel was born in 1852 in Paris, France. Both his father and grandfather were well known physicists and he followed them into the family business, studying physics at the Ecole Polytechnique. In 1892, Becquerel became the third member of his family to serve as chair of the physics department at the Museum of Natural History in Paris. Becquerel spent many years studying light, until a surprisingly cloudy day led to a momentous discovery.

Becquerel's Atomic Theory

Inspired by the recent discovery of X-rays, Becquerel thought that phosphorescent elements like uranium might emit some kind of penetrating radiation similar to X-rays when exposed to light. To test this idea, he placed uranium salts on top of a photographic plate that had been wrapped in black paper to protect it from the sunlight. After several hours of exposure to the sun, he noticed that the image of the uranium could be seen on the photographic plate, so he thought that the sun was causing the uranium to emit some kind of invisible radiation.

Then, one day in 1896, he prepared his experiment as usual, but by the time he finished getting everything ready, it was too cloudy. He put the whole set-up into a drawer and left it for several days because it remained overcast and rainy. After a few days, he decided to just go ahead and develop the photographic plate. He didn't expect to see much because the uranium had never been exposed to the sunlight, but instead, the image was much stronger than it had been before. Why did that happen? Becquerel realized that the radiation came from the uranium itself and it didn't depend on the sun at all.

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