Isotopes are versions of a chemical element that vary based on the number of neutrons in their nuclei. A natural isotope is an atom that occurs in nature, while an artificial isotope only occurs in laboratory conditions. Isotopes of both types may be either stable or radioactive.
Most natural isotopes are stable, which means that they do not release radiation. The few radioactive isotopes that appear in nature belong to three groups: those with very long radioactive half-lives, products of long-lived isotopes' decay and those that are actively synthesized by natural processes. Uranium-238 is an example of the first type, thorium-234 is an example of the second type and carbon-14 is an example of the third type. The number following an element's name indicates the number of neucleons in the isotope.
Some radioactive elements have such short half-lives that they are never found in nature. An element's half-life is the amount of time it takes for half of the element to decay. All isotopes of these synthetic elements are produced in laboratories, and due to their very short half-lives have no commercial purposes outside of research. Every element from atomic number 99 onward is artificial, and most naturally occurring elements with no stable isotopes are so rare that they are also synthesized.