Some elements do not occur naturally in nature, and are thus classed as artificially created, being man made in high energy accelerators or nuclear reactors. Examples are americium and other elements of atomic number greater than that of plutonium, atomic number 93. All are radioactive with no stable isotopes. Some are available in kilogram quantities, like americium used in fire detectors, whist others are so radioactive with halflives so short that they exist only one atom at a time before they decay again, like element 109, meitnerium.
Some other elements, which do occur naturally, are nevertheless, extremely rare. Examples are, amongst others, polonium, technetium and actinium. All are radioactive with no stable isotopes. Some are produced as daughter products in a radioactive decay series and exist only by way of a secular equilibrium, decaying as fast as they are produced. Others may be produced by the bombardment of other isotopes by high energy cosmic rays from outer space.
See also graphs on abundance (oceanic-, terrestrial-, human-, crustal-, lithospheric-, meteoritic-, solar- etc)
