![]() The camera records the radiation given off by the isotope. As soon as the exercise ends, the patient lies down. Thallium-201 is injected into the patient's blood-stream as he or she exercises on a treadmill or bicycle. In many cases, the isotope is used as a stress test. Thallium-201 is used by doctors to determine how well a person's heart is working. These particles stick in the atoms and make them radioactive. Radioactive isotopes are produced when very small particles are fired at atoms. A radioactive isotope is one that breaks apart and gives off some form of radiation. Two dozen radioactive isotopes of thallium have also been made. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number written to the right of the element's name is the mass number. Isotopes differ from each other according to their mass number. Isotopes are two or more forms of an element. Two naturally occurring isotopes of thallium exist, thallium-203 and thallium-205. The most common minerals containing thallium are crookesite, lorandite, and hutchinsonite. It is about as common as iodine or tungsten. That puts it in the bottom half among the elements in terms of abundance. Its abundance is estimated to be about 0.7 parts per million. Thallium is quite uncommon in the Earth's crust. As the coating drops off, a new layer forms in its place. When exposed to air, it forms a thin coating of thallium oxide (Tl 2O) that peels off easily. It reacts with acids and with oxygen in the air. Its density is 11.85 grams per cubic centimeter. Thallium has a melting point of 302 ☌ (576 ☏) and a boiling point of 1,457 ☌ (2,655 ☏). It is soft enough to cut with an ordinary knife and will leave a mark on paper if rubbed across it. Thallium is a heavy, bluish-white metal that resembles lead, element 82. Eventually, the decision was made in favor of Crookes. For a short time, there was a difference of opinion as to whether Lamy or Crookes was the "real" discoverer of thallium. Lamy discovered thallium the "old fashioned way," by separating one of its minerals in the laboratory. Interestingly, thallium was discovered at almost the same time by French chemist Claude-Auguste Lamy (1820-78). The discoverer of thallium was British physicist Sir William Crookes (1832-1919). All four elements are named after the color of their spectral lines. Within a period of four years after the work of Bunsen and Kirchhoff, four new elements were discovered: cesium, rubidium, thallium, and indium. The spectroscope is able to detect all the elements present in the mineral. A mineral is made of many elements, each of which gives off its own series of colored (spectral) lines. They could detect the presence of elements without actually seeing them. Scientists were fascinated by the instrument. (See sidebar on Bunsen in the cesium entry in Volume 1.) Forty years later, German chemists Robert Bunsen (1811-99) and Gustav Robert Kirchhoff (1824-87) improved on the instrument and showed how it could be used to study chemical elements. The spectroscope was invented in 1814 by German physicist Joseph von Fraunhofer (1787-1826). One form of thallium is sometimes used to study the flow of blood in the body. For example, thallium sulfate (Tl 2SO 4) has long been used as a rodenticide (rat and mouse poison). Still, some of its compounds have important applications. In Greek, the word thallos means "green twig." The green lines in thallium's spectrum look like green twigs. The brightest lines in the spectrum of thallium are green, which accounts for its name. The spectrum (plural: spectra) of an element consists of a series of colored lines that are different for every element. A spectroscope is a device for analyzing the light produced when an element is heated. Thallium was first discovered by means of a spectroscope. Thallium is also a member of the heavy metals, along with gold, platinum, and lead. The periodic table is a chart that shows how chemical elements are related to one another. Thallium is a member of the aluminum family, Group 13 (IIIA) on the periodic table. Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
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