Naturally occurring tungsten (74W) consists of five isotopes. Four are considered stable (182W, 183W, 184W, and 186W) and one is slightly radioactive, 180W, with an extremely long half-life of (1.59±0.05)×1018 years. This results in about two alpha decays of 180W per gram of natural tungsten per year, so for most practical purposes, 180W can be considered stable. The other naturally occurring isotopes are also capable of alpha decay and so only observationally stable.