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Isotopes of thorium
Although thorium (Th), with atomic number 90, has 6 naturally occurring isotopes, none of these isotopes are stable; however, one isotope, 232Th, is ''relatively'' stable, with a half-life of 14.05 billion years, considerably longer than the age of the earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium. As such, thorium is considered to be mononuclidic. It has a characteristic terrestrial isotopic composition and thus an atomic mass can be given.
Standard atomic mass: 232.03806(2) u.
Thirty radioisotopes have been characterized, with the most stable (after 232Th) being 230Th with a half-life of 75,380 years, 229Th with a half-life of 7,340 years, and 228Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, 229Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 7.6 ± 0.5 eV.
The known isotopes of thorium range in mass number from 209 to 238.
== Some notable isotopes ==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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