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Rubidium
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<p>Rubidium is a <a href="/facts/Chemical_element/0jducgWD">chemical element</a>; it has <a href="/facts/Symbol_(chemistry)/2Fe1VAL0">symbol</a> Rb and <a href="/facts/Atomic_number/bgAr581S">atomic number</a> 37. It is a very soft, whitish-grey solid in the <a href="/facts/Alkali_metal/IkWbCLk2">alkali metal</a> group, similar to <a href="/facts/Potassium/FWPwS7Rj">potassium</a> and <a href="/facts/Caesium/dXKN1S5k">caesium</a>. Rubidium is the first <a href="/facts/Alkali_metal/IkWbCLk2">alkali metal</a> in the group to have a density higher than <a href="/facts/Properties_of_water/FFsl65l1">water</a>. On Earth, natural rubidium comprises two <a href="/facts/Isotope/KD9B1y6o">isotopes</a>: 72% is a stable isotope 85Rb, and 28% is slightly <a href="/facts/Radioactive/Ya6FVjt4">radioactive</a> 87Rb, with a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 48.8 billion years – more than three times as long as the estimated <a href="/facts/Age_of_the_universe/BaByputq">age of the universe</a>. </p><p>German chemists <a href="/facts/Robert_Bunsen/HYqSh9tH">Robert Bunsen</a> and <a href="/facts/Gustav_Kirchhoff/Ygfzlywy">Gustav Kirchhoff</a> discovered rubidium in 1861 by the newly developed technique, <a href="/facts/Atomic_emission_spectroscopy/5s0wg1sX">flame spectroscopy</a>. The name comes from the <a href="/facts/Latin/pM3Kge85">Latin</a> word <i>rubidus</i>, meaning deep red, the color of its emission spectrum. Rubidium's compounds have various chemical and electronic applications. Rubidium metal is easily vaporized and has a convenient spectral absorption range, making it a frequent target for <a href="/facts/Laser/jk6PTvvT">laser</a> manipulation of <a href="/facts/Atom/FoQ4kGN5">atoms</a>. Rubidium is not a known nutrient for any <a href="/facts/Organism/j8JkL09u">living organisms</a>. However, rubidium <a href="/facts/Ion/3bePpDna">ions</a> have similar properties and the same charge as potassium ions, and are actively taken up and treated by <a href="/facts/Animal_cell/SkwyPLMA">animal cells</a> in similar ways. </p>