All of the elements produced by artificial means are radioactive to a significant degree. These include technetium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, lawrencium, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, darmstadtium, unununium, ununbium, ununtrium, ununquadium and ununpentium. Also radioactive are actinium and the naturally occurring actinons: thorium, protactinium, and uranium. The tritium isotope of hydrogen, H3, is radioactive, as are the heavy elements polonium, astatine, radon, francium, radium, and actinium. Among the alkali metals, the rare (about 0.01 percent) naturally occurring 40K isotope accounts for potassium's radioactivity; that of rubidium comes from the beta-emitting 87Rb isotope (28 percent). Of the transition metals, only two exhibit natural radioactivity: the weakly radioactive vanadium50 (0.24 percent), and the radioactive rhenium187 (62.93 percent). Among the rare earth metals, lanthanum, neodymium, promethium, gadolinium, and lutetium have naturally occurring radioactive isotopes: the very long-lived radioactive lanthanum138 (0.089 percent); the weakly radioactive neodymium144, the lightest natural nuclide that decays by alpha emission; and radioactive lutetium176 (2.59 percent). At least one of gadolinium's seven stable isotopes is radioactive. All of promethium's isotopes are radioactive and short-lived.