Question
Natural uranium is 0.7200% $^{235}\textrm{U}$, 99.27% $^{238}\textrm{U}$, and 0.0055% $^{234}\textrm{U}$. (a) What were the percentages of $^{235}\textrm{U}$, $^{238}\textrm{U}$, and $^{234}\textrm{U}$ in natural uranium when Earth formed $4.5\times 10^{9}$ years ago? (b) What is unreasonable about this result? (c) What assumption is responsible? (d) Where does $^{234}\textrm{U}$ come from if it is not primordial?
1. The calculator can't compute such a large number.
2. This calculation would suggest an amount of $^{234}\textrm{U}$ that exceeds all the ordinary matter in the universe.
3. The assumption that a primordial initial quantity of $^{234}\textrm{U}$ existed, and has since decayed and not been produced, is false.
4. $^{234}\textrm{U}$ is continually produced by radioactive decay.
Solution Video