The shortest-lived isotope of lithium is 4Li, which decays through proton emission and has a half-life of 7.6 × 10 −23 s. All of the remaining radioactive isotopes have half-lives that are shorter than 8.6 ms. Seven radioisotopes have been characterized, the most stable being 8Li with a half-life of 838 ms and 9Li with a half-life of 178 ms. As a result of this, though very light in atomic weight, lithium is less common in the Solar System than 25 of the first 32 chemical elements. The two lithium nuclei have lower binding energies per nucleon than any other stable nuclides other than hydrogen-1, deuterium and helium-3. Both natural isotopes have anomalously low nuclear binding energy per nucleon (compared to the neighboring elements on the periodic table, helium and beryllium) lithium is the only low numbered element that can produce net energy through nuclear fission. Naturally occurring lithium is composed of two stable isotopes, 6Li and 7Li, the latter being the more abundant (95.15% natural abundance). Lithium can float on the lightest hydrocarbon oils and is one of only three metals that can float on water, the other two being sodium and potassium. Apart from helium and hydrogen, as a solid it is less dense than any other element as a liquid, being only two-thirds as dense as liquid nitrogen (0.808 g/cm 3). It is the least dense of all elements that are solids at room temperature the next lightest solid element (potassium, at 0.862 g/cm 3) is more than 60% denser. Lithium has a very low density (0.534 g/cm 3), comparable with pine wood. Its melting point of 180.50 ☌ (453.65 K 356.90 ☏) and its boiling point of 1,342 ☌ (1,615 K 2,448 ☏) are each the highest of all the alkali metals while its density of 0.534 g/cm 3 is the lowest. When cut, it possesses a silvery-white color that quickly changes to gray as it oxidizes to lithium oxide. Lithium metal is soft enough to be cut with a knife. Molten lithium is significantly more reactive than its solid form. Lithium's low reactivity is due to the proximity of its valence electron to its nucleus (the remaining two electrons are in the 1s orbital, much lower in energy, and do not participate in chemical bonds). Because of this, lithium is a good conductor of heat and electricity as well as a highly reactive element, though it is the least reactive of the alkali metals. Like the other alkali metals (which are sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr)), lithium has a single valence electron that is easily given up to form a cation. The alkali metals are also called the lithium family, after its leading element. Lithium ingots with a thin layer of black nitride tarnish Lithium salts have proven to be useful as a mood stabilizer and antidepressant in the treatment of mental illness such as bipolar disorder. Lithium is present in biological systems in trace amounts its functions are uncertain. These uses consume more than three-quarters of lithium production. Lithium and its compounds have several industrial applications, including heat-resistant glass and ceramics, lithium grease lubricants, flux additives for iron, steel and aluminium production, lithium metal batteries, and lithium-ion batteries. The transmutation of lithium atoms to helium in 1932 was the first fully man-made nuclear reaction, and lithium deuteride serves as a fusion fuel in staged thermonuclear weapons. For related reasons, lithium has important uses in nuclear physics. Because of its relative nuclear instability, lithium is less common in the solar system than 25 of the first 32 chemical elements even though its nuclei are very light: it is an exception to the trend that heavier nuclei are less common. The nucleus of the lithium atom verges on instability, since the two stable lithium isotopes found in nature have among the lowest binding energies per nucleon of all stable nuclides. Lithium metal is isolated electrolytically from a mixture of lithium chloride and potassium chloride. Due to its solubility as an ion, it is present in ocean water and is commonly obtained from brines. It never occurs freely in nature, but only in (usually ionic) compounds, such as pegmatitic minerals, which were once the main source of lithium. When cut, it exhibits a metallic luster, but moist air corrodes it quickly to a dull silvery gray, then black tarnish. Like all alkali metals, lithium is highly reactive and flammable, and must be stored in vacuum, inert atmosphere, or inert liquid such as purified kerosene or mineral oil. Under standard conditions, it is the least dense metal and the least dense solid element. It is a soft, silvery-white alkali metal. Lithium (from Greek: λίθος, romanized: lithos, lit.'stone') is a chemical element with the symbol Li and atomic number 3.
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