Bismuth-209

Until recently, bismuth-209 was considered to be a stable isotope, in fact, it was thought to be the heaviest stable isotope, but scientists weren't very happy about the "stable" designation. Measurements of atomic mass and nuclear decay schemes since the 1940's indicated that bismuth-209 should be just a bit too heavy to be truly stable. Then, during the night of March 14-15 2002, a team of scientists from the Institut d’Astrophysique Spatiale d’Orsay (IAS - CNRS, Université Paris XI) stumbled across the alpha decay products of bismuth-209. 

The half-life for the decay of bismuth-209 to  thallium-205 has been reported as 1.9 x 10¹⁹ years. 
In comparison, the estimated age of the universe, starting  from the time of the Big Bang, is just  13.75 x 10⁹ years. Bismuth-209 is therefore a primordial isotope, one that has existed in its current form since before the Earth was formed. 288 primordial isotopes are known, including the 255 stable isotopes plus 33 unstable isotopes with exceptionally long half-lives like bismuth-209.

Reference

http://www.cnrs.fr/cw/en/pres/compress/bismuth.htm 

Further Reading 

Isotopes
Properties & Uses of Radiation
Nuclear Decay
Half-life Calculations

Suggested Study Questions

1. For an atom of bismuth-209, give the
• symbol
• atomic number
• mass number
• number of protons in the nucleus
• number of neutrons in the nucleus
2. For bismuth-209,
• Calculate the ratio of neutrons to protons
• If the nucleus of a heavy metal atom is considered to be stable of the neutron to proton ratio is 1.5:1, would bismuth-209 be predicted to be stable or unstable? Explain your answer.
3. For an atom of thallium-205, give the
• symbol
• atomic number
• mass number
• number of protons in the nucleus
• number of neutrons in the nucleus
4. Give the symbol for an alpha particle.
5. Write an equation for the nuclear decay of bismuth-209 to thalium-205.
6. Current predictions are that the Earth will be demolished during the death of the Sun in about 7 x 10⁹ years. Assuming you had 1 tonne of bismuth-209 today,
   • How much bismuth-209 would remain in 7 x 10⁹ years?
   • How much thallium-205 would have been produced as a result of bismuth-209 decay?
7. Bismuth-209 is produced on earth when lead-209 undergoes beta decay. Write an equation to represent this nuclear reaction.
8. The half-life of lead-209 is about 3.25 hours. If you had isolated 60 grams of lead-209, how long would it take for you to have less than about 1 gram left?