|Name, Symbol, Number||radium, Ra, 88|
|Chemical series||alkaline earth metals|
|Group, Period, Block||2, 7, s|
|Appearance||silvery white metallic|
|Atomic mass||(226) g/mol|
|Electron configuration||[Rn] 7s2|
|Electrons per shell||2, 8, 18, 32, 18, 8, 2|
|Density (near r.t.)||5.5 g/cm³|
|Melting point||973 K
(700 °C, 1292 °F)
|Boiling point||2010 K
(1737 °C, 3159 °F)
|Heat of fusion||8.5 kJ/mol|
|Heat of vaporization||113 kJ/mol|
|Crystal structure||cubic body centered|
(strongly basic oxide)
|Electronegativity||0.9 (Pauling scale)|
|Ionization energies||1st: 509.3 kJ/mol|
|2nd: 979.0 kJ/mol|
|Atomic radius||215 pm|
|Electrical resistivity||(20 °C) 1 µΩ·m|
|Thermal conductivity||(300 K) 18.6 W/(m·K)|
|CAS registry number||7440-14-4|
Radium (chemical symbol Ra, atomic number 88) is an extremely radioactive element that is classified as an an alkaline earth metal. When freshly prepared, the pure metal is brilliant white, but it blackens when exposed to air. It is found in trace amounts in uranium ores. Its most stable isotope, Ra-226, has a half-life of 1,602 years and decays into radon gas, which is also radioactive.
The applications of radium are mainly based on its radioactivity. For instance, it is used in controlled doses for radiation therapy for certain types of cancer, and its mixture with beryllium is used as a neutron source in certain physics experiments. It was once used in luminescent paints on watch dials, and in the early twentieth century it was added to products like toothpaste, hair creams, and certain foodstuffs, based on the belief that it had curative properties. These latter uses were discontinued when the adverse effects of radium were discovered.
Radium needs to be handled and stored with extreme care. Exposure to radiation from this element can lead to sores on the skin and other health problems. If inhaled or ingested, radium can replace calcium in bone tissue and cause bone cancer.
Radium is a decay product of uranium and is therefore found in all uranium-bearing ores. It was originally acquired from pitchblende ore from Joachimsthal, Bohemia. (Seven metric tons of pitchblende yielded one gram of radium.) Some of this element can be obtained from the carnotite sands of Colorado, but there are richer ores in the Democratic Republic of the Congo and the Great Lakes area of Canada. It can also be extracted from uranium processing waste. Large uranium deposits are located in Ontario (Canada), New Mexico and Utah (United States), Australia, and other parts of the world.
Radium (from the Latin word radius, meaning "ray") was discovered by Maria Skłodowska-Curie and her husband Pierre in 1898. The Curies were studying pitchblende, a variety of the uranium ore uraninite (mainly uranium dioxide, UO2) obtained from North Bohemia (area around Jáchymov). When they removed uranium from the ore, they found that the remaining material was still radioactive. They then separated out a radioactive mixture, consisting mostly of barium, which gave a brilliant red flame color and spectral lines that had never been documented before.
In 1902, Marie Curie and Andre Debierne isolated radium in its pure metallic form. Their method involved electrolysis of a solution of pure radium chloride, using a mercury cathode, and distillation of the product in an atmosphere of hydrogen gas.
Historically, the radioactive decay products of radium were labeled Radium A, B, C, and so forth (see Radioactivity below). These are now recognized as isotopes of other elements. On February 4, 1936, radium E became the first radioactive element to be made synthetically.
During the 1930s, it was found that workers exposed to radium when handling luminescent paints suffered from serious health problems, including sores, anemia, and bone cancer. This use of radium was stopped soon afterward. The reason for this problem is that the body treats radium as though it were calcium. Thus, radium becomes deposited in the bones, where radioactivity degrades the marrow and damages bone cells. Marie Curie's premature death has been attributed to her extensive work with radium.
Radium is the heaviest of the alkaline earth metals. It lies directly below barium in group 2 (former group 2A) of the periodic table, and its chemical properties therefore most closely resemble those of barium. In addition, it is placed in period 7, between francium and actinium.
Radium is intensely radioactive, emitting three types of radiation: alpha particles, beta particles, and gamma rays. When mixed with beryllium, radium produces neutrons. Another remarkable property of radium preparations is that they keep themselves warmer than their surroundings.
Radium has 25 known isotopes, four of which—Ra-223, Ra-224, Ra-226, and Ra-228—are are found in nature and are generated by the decay of uranium or thorium. The common isotope is Ra-226, a product of U-238 decay. It is the longest-lived isotope of radium, with a half-life of 1,602 years. The next longest-lived isotope is Ra-228, a product of Th-232 breakdown, with a half-life of 6.7 years.
Radium is over one million times more radioactive than the same mass of uranium. It loses about one percent of its activity in 25 years, being transformed into elements of lower atomic weight. The final product of disintegration is lead.
The decay of radium occurs in stages. The successive main products were called radium emanation (or exradio), radium A, radium B, radium C, and so forth. These products have been studied and are now known to be isotopes of other elements, as follows.
The SI unit of radioactivity is the becquerel (Bq), corresponding to one disintegration per second. The curie, a non-SI unit, is defined as the amount of radioactivity that has the same disintegration rate as 1 gram of Ra-226 (3.7 x 1010 disintegrations per second, or 37 GBq).
Given that radium has a geologically short half-life and intense radioactivity, its naturally occurring compounds are quite rare, found almost exclusively in uranium ores. When the compounds are heated in a flame, the flame color turns crimson carmine (a rich red or crimson color, with a shade of purple), and they produce characteristic spectra.
Compounds of radium include its oxide (RaO), fluoride (RaF2), chloride (RaCl2), bromide (RaBr2), and iodide (RaI2). Of these, radium chloride was the first to be prepared in a pure state, and was the basis of Marie Curie's original separation of radium from barium.
At the turn of the twentieth century, radium was a popular additive in products like toothpaste, hair creams, and even food items, based on its assumed curative powers. Such products soon fell out of vogue and were prohibited by authorities in many countries, after it was discovered they could have serious adverse health effects.
Until the 1950s, radium was used in self-luminous paints for watches, clocks, and instrument dials. Unfortunately, more than 100 former watch dial painters who used their lips to hold the paintbrush died from the radiation. Subsequently, this use was also discontinued. Nonetheless, objects with this paint may still be dangerous and must be handled properly. Currently, tritium (which also carries some risks) is used instead of radium, as it is considered safer than radium.
More recently, radium is being replaced by other radioisotopes—such as cobalt-60 and cesium-137—when there is a need for radioactive sources that are safer to handle or those that emit more powerful radiation.
Radium is highly radioactive and its decay product, radon gas, is also radioactive. The energy emitted by the radioactive decay of radium ionizes gases, affects photographic plates, causes sores on the skin, and produces many other detrimental effects. As radium is chemically similar to calcium, it can potentially replace calcium in bone tissue, causing great harm. Inhalation, injection, ingestion, or body exposure to radium can cause cancer and other body disorders. Stored radium should be properly ventilated, to prevent the accumulation of radon.
All links retrieved November 28, 2007.
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