Molybdenum-99 is formed by neutron bombardment of a naturally occurring isotope of Mo. If one neutron is absorbed and no by-products are formed, what is the starting isotope?

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1. Identify the naturally occurring isotopes of molybdenum (Mo). These include Mo-92, Mo-94, Mo-95, Mo-96, Mo-97, Mo-98, and Mo-100.
2. Understand that neutron bombardment involves the absorption of a neutron by the nucleus of an atom, which increases the mass number by 1 while the atomic number remains unchanged.
3. Set up the equation for the nuclear reaction: \( ^A_{42}\text{Mo} + ^1_0\text{n} \rightarrow ^{99}_{42}\text{Mo} \), where \( A \) is the mass number of the starting isotope.
4. Solve for \( A \) by recognizing that the mass number of the starting isotope plus 1 (from the neutron) should equal 99, the mass number of the product isotope.
5. Determine which naturally occurring isotope of molybdenum has a mass number that, when increased by 1, results in 99.