Final answer:
After Uranium 238 undergoes alpha decay, the resulting element will have a mass number of 234 and an atomic number of 90, which identifies it as thorium (Th).
Step-by-step explanation:
When Uranium 238 undergoes alpha decay, an alpha particle, which has a mass number of 4 and an atomic number of 2, is emitted. According to the nuclear equation for alpha decay, the mass number and atomic number of the resultant element must be accounted for to maintain balance. Since an alpha particle has the symbol 42 He, emitting an alpha particle causes the atomic number to decrease by 2 and the mass number to decrease by 4. Therefore, the resulting element from the decay of Uranium 238 will have a mass number of 234 (238 - 4) and an atomic number of 90 (92 - 2), which is identified as the element thorium (Th).
Using this information, the complete reaction for the alpha decay of Uranium 238 is written as:
23892U → 42He + 23490Th