The most reactive metallic element is cesium.
Cesium metal is a golden-yellow, active metal with a low melting point, which is extremely easy to be oxidized in air and can react violently with water to produce hydrogen and explode. Cesium has no monomorphic form in nature, and elemental cesium is rarely distributed in the form of salt on land and in the ocean.
Cesium is also an important material in the manufacture of vacuum devices and photocells. The radionuclide Cs-137 is one of the radioactive contaminants that leaked from the Fukushima Daiichi nuclear power plant in Japan.
Cesium is the most metallic of the known elements (including radioactive elements) (note that it is not the metal activity, the most active is lithium).
Extended information:
Caesium (Cs) is a silvery-colored soft metal. Of its isotopes, the naturally occurring cesium 133 is a stable isotope, while the other cesium isotopes are radioactive.
Cesium 137 is one of the byproducts of nuclear bombs, nuclear weapons testing, and nuclear fission in nuclear reactors, and it releases gamma rays. Because cesium 137 has a long half-life of 30 years, if you ingest cesium 137 through eating or breathing, or are irradiated by cesium 137 that settles on the ground, it will have a long-lasting effect on your body.
Cesium 137 is a commonly used source of gamma radiation. The source is used to calibrate radiation therapy equipment used to treat cancer, and can also be used to calibrate radiation monitoring instruments used to monitor radiation levels received by radiation therapy personnel and patients. These medical devices and instruments require periodic calibration to ensure their accuracy.
The effects of cesium 137 on the human body depend on the intensity of the radiation, the duration of exposure and the type of body cells affected. If the Cesium 137 involved is a very high radiation source, it may cause acute radiation sickness such as nausea, fatigue, vomiting and hair loss, etc., and can even cause death if exposed to a direct dose of about 1 sievert.
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