An element, aka chemical element, is a pure chemical substance consisting of a single type of atom distinguished by its atomic number, which is the number of protons in its atomic nucleus. Elements are divided into metals, metalloids, and nonmetals.
As of 2011, 118 elements have been identified, the latest being ununseptium in 2010. Of these, only the first 98 are known to occur naturally on Earth; 80 elements are stable, while the others are radioactive, decaying into lighter elements over various timescales from fractions of a second to billions of years. The 18 radioactive elements that occur naturally are either very long-lived primordial isotopes (such as uranium and thorium) or radioactive decay daughters or nuclear reaction products formed from these elements combining with naturally occurring neutrons. Those elements that do not occur naturally on Earth have been produced artificially as the synthetic products of nuclear reactions.