Chapter 2, Problem 19QAP

Cerium is the most abundant rare earth metal. Pure cerium ignites when scratched by even a soft object. It has four known isotopes: ${}^{\text{136}}\text{Ce}\left(\text{atomic mass}=\text{135}.\text{9}0\text{7 amu}\right){,}^{\text{138}}\text{Ce}(\text{atomic mass}=\text{137}.\text{9}0\text{5 amu}){,}^{\text{14}0}\text{Ce}\left(\text{atomic mass}=\text{139}.\text{9}0\text{5 amu}\right)$, and ${}^{\text{142}}\text{Ce}\left(\text{atomic mass}=\text{141}.\text{9}0\text{9 amu}\right)$. Ce-140 and Ce-142 are fairly abundant. Which is the more abundant isotope?