Suppose we wanted to use one of the more massive atoms, like \({ }_{92}^{235} \mathrm{U}\), as the standard reference for the atomic mass scale. (a) What would be the universally agreed-upon atomic mass of the \({ }^{235} \mathrm{U}\) isotope? (b) Why don't we use \({ }_{92}^{235} \mathrm{U}\) instead of \({ }_{6}^{12} \mathrm{C}\) ?

If we were to use uranium-235 (\({ }_{92}^{235}\mathrm{U}\)) as the standard reference for the atomic mass scale, then by definition, the atomic mass of uranium-235 would be exactly 235 atomic mass units (amu). This is because the reference isotope acts as a benchmark for determining the atomic mass of other elements. So in this case, the universally agreed-upon atomic mass of \({ }^{235}\mathrm{U}\) isotope would be: \(235\,\textrm{amu}\)

There are several reasons for not using uranium-235 (\({ }_{92}^{235}\mathrm{U}\)) as the standard reference for the atomic mass scale: 1. Carbon-12 (\({ }_{6}^{12}\mathrm{C}\)) is the most abundant and stable isotope of carbon, while uranium-235 is less abundant and less stable. It is easier to measure atomic masses relative to a stable and abundant isotope. 2. Uranium-235 is a radioactive isotope used as fuel in nuclear reactors and nuclear weapons. Handling radioactive materials, especially on a standard scale for mass, would be dangerous and impractical for laboratory use. 3. Carbon is a lighter element and is present in many organic compounds and familiar substances. It is easier and more practical to use a lighter element with simple and well-understood chemistry as a reference for atomic masses. 4. Measuring atomic masses relative to a lighter element helps reduce uncertainties in the measurement, as heavier elements tend to introduce more significant deviations due to isotope differences. In summary, we do not use uranium-235 (\({ }_{92}^{235}\mathrm{U}\)) as the standard reference for atomic mass measurements due to its scarcity, radioactivity, complex chemistry, and potential measurement uncertainties. Carbon-12 (\({ }_{6}^{12}\mathrm{C}\)) is preferred as the standard reference for its abundance, stability, simplicity, and convenience.