*" ( The remaining natural thorium isotope, 229Th, occurs in minute traces as part of the decay chain of 237Np, the neptunium series, which is much less abundant than the other decay chains in nature and was hence discovered much later : therefore it does not have a historical name . ) " parentheses not really needed here
2.
Due to the relatively short half-life of its starting isotope neptunium-237 ( 2.14 million years ), the fourth chain, the neptunium series with " A " = 4 " n " + 1, is already extinct in nature, except for the final rate-limiting step, decay of bismuth-209 . The ending isotope of this chain is now known to be thallium-205.
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