atoms, with three atoms per turn. This gray material resists oxidation by air and is not volatile. Isotopes Naturally occurring tellurium has eight isotopes. Six of those isotopes, 120Te, 122Te, 123Te, 124Te, 125Te, and 126Te, are stable. The other two, 128Te and 130Te, have been found to be slightly radioactive, with extremely long half-lives, including 2.2 × 1024 years for 128Te. This is the longest known half-life among all radionuclides and is about 160 trillion (1012) times the age of the known universe. A further 31 artificial radioisotopes of tellurium are known, with atomic masses ranging from 104 to 142 and with half-lives of 19 days or less. Also, 17 nuclear isomers are known, with half-lives up to 154 days. With the exception of beryllium-8 and beta-delayed alpha emission branches in some lighter nuclides, tellurium (104Te to 109Te) is the lightest element with isotopes known to undergo alpha decay. The atomic mass of tellurium () exceeds that of iodine (), the next element in the periodic table. Occurrence With an abundance in the Earth's crust comparable to that of platinum (about 1 µg/kg), tellurium is one of the rarest stable solid elements. In comparison, even thulium, the rarest of the stable lanthanides have crustal abundances of 500 µg/kg (see Abundance of the chemical elements). This rarity of tellurium in the Earth's crust is not a reflection of its cosmic abundance. Tellurium is more abundant than rubidium in the cosmos, though rubidium is 10,000 times more abundant in the Earth's crust. The rarity of tellurium on Earth is thought to be caused by conditions during preaccretional sorting in the solar nebula, when the stable form of certain elements, in the absence of oxygen and water, was controlled by the reductive power of free hydrogen. Under this scenario, certain elements that form volatile hydrides, such as tellurium, were severely depleted through evaporation of these hydrides. Tellurium and selenium are the heavy elements most depleted by this process. Tellurium is sometimes found in its native (i.e., elemental) form, but is more often found as the tellurides of gold such as calaverite and krennerite (two different polymorphs of AuTe2), petzite, Ag3AuTe2, and sylvanite, AgAuTe4. The town of Telluride, Colorado, was named in hope of a strike of gold telluride (which never materialized, though gold metal ore was found). Gold itself is usually found uncombined, but when found as a chemical compound, it is often