Final answer:
Alpha radiation causes the mass of the original atom to decrease by 4, due to the emission of an alpha particle consisting of two protons and two neutrons.
Step-by-step explanation:
The question pertains to the effect of alpha radiation on the mass of the original atom. Alpha decay involves the emission of an alpha particle, which corresponds to a helium nucleus. In this process, the mass number of the original atom decreases because the emitted alpha particle consists of two protons and two neutrons. Therefore, when an atom undergoes alpha decay, its mass number is reduced by the mass of these four nucleons. As described in the given information, the emission of an alpha particle from uranium-238 is shown as 238U → 4He + 234Th, indicating that the mass number of uranium decreases by 4 units to become thorium (234Th).