Answer
$^{31}_{15}P$.
Work Step by Step
$$\frac{1}{2}=\frac{r}{r_U}$$
Use equation 30–1 to find the atomic mass number, A. For uranium, we assume we have uranium-238. It is the most common isotope.
$$\frac{1}{2}=\frac{(1.2\times10^{-15}m)A^{1/3}}{(1.2\times10^{-15}m)238^{1/3}}$$
$$A=238(\frac{1}{2})^{3}=29.75\approx 30$$
Look in Appendix B to find a stable nucleus with an atomic mass number close to 30. The answer is $^{31}_{15}P$.
