Question

When uranium-235 undergoes alpha decay, what is the new atom produced?

neptunium-235
Thorium-231
protactinium-235
Uranium-231

Answer 1

When uranium-235 (U-235) undergoes alpha decay, it emits an alpha particle (which is equivalent to a helium-4 nucleus) and transforms into a new element. The atomic number of uranium (U) is 92, so when it emits an alpha particle, its atomic number decreases by 2, resulting in an atomic number of 90. The new element with atomic number 90 is thorium (Th).

The mass number of U-235 is 235, and when it loses 4 (the mass of the alpha particle), the new mass number will be 231.

Thus, the product of the alpha decay of uranium-235 is thorium-231 (Th-231).

Therefore, the correct answer is:
**Thorium-231**.