June 2006


Radium chloride, RaCl2, played an important role in separating the element radium from uranium ore by history's most famous scientist couple, Marie Sklodowska Curie (also known as Madame Curie) and her husband, Pierre Curie. By discovering two new radioactive chemical elements, their work greatly increased our understanding of radioactivity, a fundamental property of certain elements.

Marie and Pierre Curie in their
laboratory Courtesy: University of
Frankfurt From a NASA website.

Radium, atomic number 88, is a silver white metal that is naturally radioactive. Radioactivity is a very important source of Earth's internal heat; it is likely the "engine" that drives the "drift" of the continents, volcanic eruptions and earthquakes. Geologists use the natural radioactivity of some minerals to help them estimate the ages of rocks. Similarly, archaeologists use radioactivity to estimate the age of artifacts of past human life.

A New Field of Research

Radioactivity in uranium compounds had just been discovered in 1896 by the French physicist Henri Becquerel when Marie set out to learn more about it. She liked the idea of investigating an entirely new phenomenon, about which almost nothing had been written. There were very few women scientists in the late 1800's, and Marie would become world famous for her discoveries. In addition to being a pioneering woman scientist, she was the first scientist ever to win two Nobel Prizes for her research.

Probing Pitchblende

Henri Becquerel had written that certain compounds of the element uranium release a steady stream of energy. Using a uranium ore known as pitchblende, Marie separated out a uranium compound and measured the strength of uranium's radioactivity, using a device invented by her husband and brother-in-law. She was surprised to find that after removing uranium from pitchblende, the residue was more radioactive than before. After checking and re-checking this result, she reasoned that the ore must contain at least one other strongly radioactive element, yet unknown to science.

Hard Work in a Makeshift Laboratory

The Curie's laboratory on the grounds of
the School of Industrial Physics and
Chemistry in Paris Courtesy of the 
Radiological History & Heritage Charitable Trust

In 1898, the Curies announced they had evidence for the existence of two new, naturally-occurring radioactive elements, polonium and radium, found in pitchblende. The next step was to produce samples of the elements. Pierre abandoned his study of crystals to help Marie isolate the new radioactive substances.

For the next eight years the couple toiled together over several tons of uranium ore waste that had been carted to Paris from an Austrian mine. The Curies knew they needed a large amount of pitchblende because the unknown radioactive elements were present in the ore at very low levels.

To work with a large quantity of ore, the scientists needed a spacious laboratory, but all that was available was a dilapidated wooden shed on the grounds of The School of Industrial Physics and Chemistry in Paris. A visiting chemist declared it looked more like "a cross between a stable and a potato shed" than a laboratory. The couple worked very hard, with Marie processing the ore to separate out the portions holding the new elements and Pierre measuring the radioactivity of the product after each step. By 1902, Marie had succeeded in chemically separating out one decigram (one-tenth of a gram = 0.00022 pound) of RaCl2. Working with another researcher after Pierre's accidental death in 1906, Marie produced pure radium in 1908.

Naming Privileges
Marie coined the term "radioactivity" from the Latin word for ray, radius, to denote the energy "rays" that came from elements like uranium. She also named radium and polonium. Radium was named for its radioactive properties, but polonium was named for the scientist's homeland, Poland, which had been erased temporarily from the map of Europe by oppressor countries.

Using Chlorine to Produce Pure Radium

Producing RaCl2 from ore was an important intermediate step in producing pure radium. The Curies noted that radium behaved chemically very similarly to the element barium with which it was mixed in the ore, and that they would have to devise a method to separate radium from barium in processing pitchblende.

They took advantage of the fact that RaCl2 apparently was less soluble, or "dissolvable," than barium chloride, BaCl2, in water or alcohol. The couple developed a procedure to repeatedly precipitate a solid in a series of chemical treatments. Each time they precipitated the solid, it was found to be more radioactive than the last time, a sign that the solid was becoming more enriched in RaCl2 compared to BaCl2.

High Risk Research

The Curies made tremendous contributions to our understanding of radioactivity, but their work took a toll on their health. Concentrating small quantities of highly radioactive radium exposed them to dangerously high levels of radiation.

Today, scientists have a good understanding of the health effects of radiation, and it is possible to use radioactive materials safely, to society's benefit. Most of the radioactive materials used today are produced in nuclear reactors or particle accelerators. The companies that produce these materials are licensed and inspected by government agencies.

The symbol above is the tri-foil, the universal sign for radiation. It is posted where radioactive materials are handled, or where radiation-producing equipment is used. The tri-foil is used as a warning to protect people from being inappropriately exposed to radioactivity.

Modern Benefits and Uses of Radioactive Materials

Radioactive materials are used productively today in medicine, scientific research, industrial production processes, mining, energy production, pollution clean-up and even criminal investigations. In the field of healthcare alone, radioactive materials have improved doctors' ability to diagnose and treat diseases. Radioactive elements injected into the body allow doctors, using special cameras, to map blood flow to the heart, scan bones for cancer or kidneys for proper functioning.

Radioactive iodine is used successfully to treat cancer. And radioactive materials are used in medical research to hunt for cures for serious diseases such as cancer, AIDS and Alzheimer's disease.

Follow-up Activities:

  1. Look it up: List the eight naturally-occurring radioactive elements in order of their atomic weights.

  2. Look it up: Explain how these radioactive elements are used to benefit society: technetium-99, phosphorus-32, strontium-85.

  3. Calculate: One ton of pitchblende contains approximately 0.14 grams of radium. How much pitchblende would have to be processed to obtain one gram of radium?

For a list of previous "Chlorine Compound of the Month" features, click here.


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