notable by its absence as it nigh instantaneously decays into two alpha particles whenever it is formed. The triple alpha process in stars can only occur at energies high enough for beryllium-8 to encounter a third alpha particle before it decays. This is why most main sequence stars spend billions of years hydrogen burning but never or only briefly during their red giant phase initiate helium burning. Strontium-90 is a common fission product of the fission of uranium and has been produced in appreciable quantities by humanmade nuclear reactions as well as a tiny secular equilibrium concentration in uranium due to spontaneous fission. Radioisotopes of alkaline earth metals are usually "bone seekers" as they behave chemically similar to calcium and may do significant harm to bone marrow (a rapidly dividing tissue) when they accumulate there. This property is also made use of in radiotherapy of certain bone cancers as the chemical properties allow the radionuclide to target the cancerous growth in the bone while leaving the rest of the body unharmed. Compared to their neighbors in the periodic table, alkaline earth metals tend to have more stable isotopes, as they possess an even number of protons and for any given even mass number, isobars with both an even number of protons and an even number of neutrons tend to be more stable due to nucleon pairing effects. History Etymology The alkaline earth metals are named after their oxides, the alkaline earths, whose old-fashioned names were beryllia, magnesia, lime, strontia, and baria. These oxides are basic (alkaline) when combined with water. "Earth" was a term applied by early chemists to nonmetallic substances that are insoluble in water and resistant to heating—properties shared by these oxides. The realization that these earths were not elements but compounds is attributed to the chemist Antoine Lavoisier. In his Traité Élémentaire de Chimie (Elements of Chemistry) of 1789 he called them salt-forming earth elements. Later, he suggested that the alkaline earths might be metal oxides, but admitted that this was mere conjecture. In 1808, acting on Lavoisier's idea, Humphry Davy became the first to obtain samples of the metals by electrolysis of their molten earths, thus supporting Lavoisier's hypothesis and causing the group to be named the alkaline earth metals. Discovery The calcium compounds calcite and lime have been known and used since prehistoric times. The same is true for the beryllium compounds beryl and emerald.