There are seven naturally occurring isotopes of Thorium but none are stable. 232Th is one of the two nuclides beyond bismuth (the other being 238U) that have half-lives measured in billions of years; its half-life is 14.05 billion years, about three times the age of the Earth, and slightly longer than the age of the universe. Four-fifths of the thorium present at Earth's formation has survived to the present. 232Th is the only isotope of thorium occurring in quantity in nature. Its stability is attributed to its closed nuclear subshell with 142 neutrons. Thorium has a characteristic terrestrial isotopic composition, with atomic weight . It is one of only four radioactive elements (along with bismuth, protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.
There are seven naturally occurring isotopes of Thorium but none are stable. 232Th is one of the two nuclides beyond bismuth (the other being 238U) that have half-lives measured in billions of years; its half-life is 14.05 billion years, about three times the age of the Earth, and slightly longer than the age of the universe. Four-fifths of the thorium present at Earth's formation has survived to the present. 232Th is the only isotope of thorium occurring in quantity in nature. Its stability is attributed to its closed nuclear subshell with 142 neutrons Thorium has a characteristic terrestrial isotopic composition with atomic weight . It is one of only four radioactive elements (along with bismuth, protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.