|Animation of the decay chains of a uranium-238 nucleus. U-238 is a radioactive element with 92 protons (red), indicated to the lower left of its chemical symbol, and 146 neutrons (yellow), giving it a total atomic mass of 238 (upper left of symbol). It is unstable, and decays by emission of an alpha particle, which consists of two protons and two neutrons. This produces thorium-234. Th-234 undergoes beta decay, in which a neutron converts into a proton by emitting an electron (blue) and an electron antineutrino (orange), forming a protactinium-234 nucleus. These processes continue, from Pa-234 to U-234 to Th-230, radium-226, radon-222 and then to Polonium-218. Po-218 can decay by both beta (left) and alpha (right) decay, forming astatine-218 and lead-214 respectively. These both decay to bismuth-214, which can again decay both ways, forming thallium-210 by alpha decay (left) and Po-214 by beta decay (right). These both decay to Pb-210, which decays to mercury-206 by alpha decay (left) or Bi-210 by beta decay (right). These both decay to Tl-206, which decays to Pb-206. This isotope of lead is stable, and is the end of the decay chain. Naturally occurring uranium contains a combination of all of these nuclei. They have different half lives, from the 4.5 billion years of U-238 itself to as little as 164 microseconds for Po-214.