Final answer:
When 92U238 decays to 90Th234, an alpha particle (He) is produced.
Step-by-step explanation:
When 92U238 decays to 90Th234, an alpha particle (He) is produced. The atomic number has dropped from 92 to 90, indicating a change in the chemical element from uranium to thorium. The equation for the decay is written as 92U238 → 90Th234 + α.