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Which Alkali Metals Are Radioactive?

The elements in the first column of the periodic table are often called the alkali metals. All of them are very reactive and hence seldom if ever found in pure form in nature. As you go down the column, they tend to become more reactive, so that lithium is the least reactive of the group, while francium is the most reactive. Several isotopes of the alkali metals are unstable and hence highly radioactive.

Francium
- Francium is a strange beast seldom encountered except in trace quantities. According to Jefferson Lab, it's estimated the Earth's crust contains a mere ounce of francium in total at any one time. Francium is formed by the radioactive decay of heavier elements, but it's so unstable it breaks down quickly, releasing radiation in the form of alpha or beta particles. Scientists can produce francium through neutron bombardment of a couple other elements, but at present it has no important uses.
Cesium Isotopes
- Cesium metal is much more common in nature. The most common isotope or variant of cesium, cesium-133, is not radioactive, and it makes up about 1.9 milligrams per kilogram of mass in Earth's crust. There are, however, some 11 radioactive isotopes of cesium. Three of these have half-lives greater than two years; the remaining eight isotopes have half-lives of less than two weeks. Some of these radioactive isotopes are among the products of nuclear fission and radioactive decay in nuclear reactors, so they are a possible concern in the event of an accidental radiation release or a nuclear blast.
Rubidium Isotopes
- There are 24 isotopes of rubidium, but only two of these are actually found in nature. Of the two found in nature, however, one is radioactive. About 28 percent of the rubidium found on Earth is rubidium-87, which has a half-life of 4.9 x 10^10 years and releases beta particles as it decays. The other non-natural isotopes of rubidium are all unstable and radioactive; just as with cesium, they can be produced through neutron bombardment or decay of heavier elements.
Potassium Isotopes
- Virtually all the potassium you'll find in nature is potassium-39 or potassium-41, both of which are stable. A fraction of a percent of the element here on Earth is potassium-40, which is radioactive and has a half-life of 1.265 x 10^9 years. It can decay by capturing an electron, which melds with a proton to form a neutron and a neutrino; alternatively, it can decay by converting a neutron into a proton, an electron and a neutrino and releasing an electron (a beta particle) in the process. Various other radioactive isotopes of potassium are known but do not exist in nature.
Lithium &Sodium
- All lithium and sodium found in nature is stable and thus nonradioactive. Nuclear physicists have discovered various radioactive isotopes of sodium, the longest of which has a half-life of ~2.6 years. The longest-lived isotope of lithium has a half-life measured in milliseconds. None of these radioactive lithium or sodium isotopes occur in nature.

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