Caesium

Caesium is the most electropositive stable element and one of only a handful of metals that will melt in your hand — 28.5 °C, just below body temperature — though holding it is a poor idea, since it detonates on contact with water. Its real importance is metrological: since 1967 the second has been defined as exactly 9192631770 oscillations of the radiation from the hyperfine transition in a caesium-133 atom, which means every clock, GPS fix and network timestamp on Earth is ultimately traceable to this element. Bunsen and Kirchhoff found it in 1860 in mineral water using the brand-new technique of flame spectroscopy, and named it after the two blue lines they saw, from Latin caesius, sky-blue. The radioactive fission product caesium-137, with a 30-year half-life, is the isotope that made Chernobyl and Fukushima long-term problems rather than short ones.