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Hydrogen (H) has three naturally occurring isotopes, sometimes denoted 1H, 2H, and 3H. Other, highly unstable nuclei (4H to 7H) have been synthesized in the laboratory but not observed in nature. Hydrogen is the only element that has different names for its isotopes in common use today. The 2H (or hydrogen-2) isotope is usually called deuterium, while the 3H (or hydrogen-3) isotope is usually called tritium. The ordinary isotope of hydrogen, with no neutrons, is sometimes called "protium". Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is never found in nature as a free element. Its only stable isotope, 89Y, is also its only naturally occurring isotope.