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24.1: Discovery of Radioactivity

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

The student will:

  • describe the roles played by Henri Becquerel and Marie Curie in the discovery of radioactivity.
  • list the most common emissions from naturally radioactive nuclei.


  • alpha particle
  • beta particle
  • gamma rays


No one could have known in the 1800s that the discovery of the science and art form of photography would eventually lead to the splitting of the atom. The basis of photography is the fact that visible light causes certain chemical reactions. If the chemicals are spread thinly on a surface but protected from light by a covering, no reaction occurs. When the covering is removed, however, light acting on the chemicals causes them to darken. At the time of its discovery, photography was a strange and wonderful thing. Even stranger was the discovery by Wilhelm Conrad Röntgen that radiation other than visible light could expose photographic film. He found that film wrapped in dark paper would react when X-rays went through the paper and struck the film.

Becquerel and Radioactivity

When Henri Becquerel heard about Roentgen's discovery, he wondered if his fluorescent minerals would give the same X-rays. Becquerel placed some of his rock crystals on top of a well-covered photographic plate and sat them in the sunlight. The sunlight made the crystals glow with a bright fluorescent light, but when Becquerel developed the film, he was disappointed. He found that only one of his minerals, a uranium salt, had fogged the photographic plate. He decided to try again, leaving them out in the sun for a longer period of time. The weather didn't cooperate, so Becquerel left the crystals and film stored in a drawer for several cloudy days. Before continuing his experiments, Becquerel decided to check one of the photographic plates to make sure the chemicals were still good. To his amazement, he found that the plate had been exposed in spots where it had been near the uranium containing rocks even though some of these rocks had not been exposed to sunlight at all (see the illustration below).

In later experiments, Becquerel confirmed that the radiation from the uranium had no connection with light or fluorescence, but the amount of radiation was directly proportional to the concentration of uranium in the rock. Becqueral had discovered radioactivity.

The Curies and Radium

One of Becquerel's assistants, a young Polish scientist named Maria Sklowdowska (to become Marie Curie after she married Pierre Curie, seen in Figure below), became interested in the phenomenon of radioactivity. With her husband, she decided to find out if chemicals other than uranium were radioactive. The Curies studied pitchblend, the residue of uranium mining, from the mining region of Joachimstahl sent from the Austrian government. From the ton of pitchblend, the Curies separated \begin{align*}0.10 \ \mathrm{g}\end{align*}0.10 g of a previously unknown element, radium, in the form of the compound radium chloride. This radium was many times more radioactive than uranium.

By 1902, the world was aware of a new phenomenon called radioactivity and of new elements which exhibited natural radioactivity. For this work, Becquerel and the Curies shared the 1903 Nobel Prize in Physics. For her subsequent work in radioactivity, Marie Curie won the 1911 Nobel Prize in Chemistry. She was the first female Nobel laureate and the only person ever to receive two Nobel Prizes in two different scientific categories.

Marie Sklodowska Curie before she moved to Paris

Marie Sklodowska Curie before she moved to Paris.

Further experiments provided information about the characteristics of the penetrating emissions from radioactive substances. It was soon discovered that there were three common types of radioactive emissions. Some radiations could pass easily through aluminum foil up to a centimeter thick, while some were stopped by the foil. The three basic types of radiation were named alpha (\begin{align*}\alpha\end{align*}α), beta (\begin{align*}\beta\end{align*}β), and gamma (\begin{align*}\gamma\end{align*}γ) radiation.

Eventually, scientists were able to demonstrate experimentally that the alpha particle is a helium nucleus (a particle containing two protons and two neutrons), the beta particle is a high speed, high energy electron originating from the nucleus, and gamma rays are a very high energy form of electromagnetic radiation (even higher energy than X-rays) of nuclear origin.

Lesson Summary

  • Henri Becquerral, Marie Curie, and Pierre Curie shared the discovery of radioactivity.
  • The most common emissions of radioactive elements were called alpha \begin{align*}( \alpha)\end{align*}(α), beta \begin{align*}( \beta )\end{align*}(β), and gamma \begin{align*}( \gamma )\end{align*}(γ) radiation.

Further Reading / Supplementary Links

To learn more about Marie Curie, visit:

The following webpage provides more information about the discovery of radioactivity.

Review Questions

  1. Match the term with its meaning in the table below.
\begin{align*} \begin{array}{|l|l|} \hline \mathbf{Term} \ \ \ \ \ \ \ \ \ \ &\mathbf{Definition} \\ \hline \text{(1) alpha particle} &\text{(a) high energy electromagnetic radiation} \\ & \\ \text{(2) beta particle} &\text{(b) a high speed electron} \\ & \\ \text{(3) gamma ray} &\text{(c) a helium nucleus} \\ \hline \end{array} \end{align*}

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