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Before 2003, Bismuth was thought to be the heaviest stable element, however research at the Institut d'Astrophysique Spatiale in Orsay, France measured the alpha emission half-life of the metal to be (1.9 +/- 0.2) * 10¹⁹ years (meaning it is not a stable element).

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What the above means is that Bismuth is only just unstable, and we would have to go for several trillions more years to see a noticeable disintegration of a sample of bismuth into thallium. It is to be noted, by the way, that Bismuth-209 is the only long-lived, nearly stable isotope of that element. All other isotopes are quite radioactive and decay quite readily.

Nevertheless it's a bit of a surprise, as I was thinking about the quantization of nuclear stability recently. That is to say, the fact that the sudden drop from very nearly stable (Bismuth) to radically unstable and radioactive (Polonium) suggested that nuclear stability was not a gradual drop-off as it would be if there was a kind of non-quantization of stability.

However, the fact that Bismuth is only just unstable makes me wonder if
Lead would be just a smidgen more unstable if it were not for the fact that it exists in a closed-shell ("magic number") configuration for its protons - although it is apparently hypothesized that lead-204 (although it is considered a stable isotope) may be slightly unstable (I would suspect due to a slight neutron deficiency). I can't find any definite experimentation that has been done to confirm this and it is clear that its 10¹⁷ year half-life would impart the same characteristics as Bismuth - i.e. nearly zero radioactivity, and certainly no appreciable disintegration for billions of years or more.

So just some thoughts

[ 09 March 2004: Message edited by: DrConway ]

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Originally posted by Mike Keenan:
I thought it had been established years ago that bismuth was not quite stable.

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My radiochemistry text printed in 1990 says Bismuth-209 has 100% natural abundance and no half-life (meaning it was thought to be a stable isotope).

What was established years ago was that if you find out the binding energy of Bismuth-209, it's a bit lower than what you would expect from theoretical predictions (which means there is a potential for some instability).

So, it was theoretically established that Bismuth might indeed have all radioactive isotopes, but nobody could detect (until now) any signs of decay.

A long half-life means a small probability of decay for any one atom, and concurrently with that, very little radiation given off. Not surprising this one took so long to finally find out.

[ 09 March 2004: Message edited by: DrConway ]