During -decay, atomic mass of parent nuclei is

Step-by-step solution

An -particle is a helium nucleus, denoted as ₂⁴ He . In an -decay process, a parent nucleus _ Z ^ A X emits an -particle to form a daughter nucleus _ Z-2 ^ A-4 Y . The atomic mass number A of the parent nucleus is reduced by the mass number of the -particle, which is 4. Therefore, the atomic mass of the parent nucleus is decreased by 4 units. Answer: decreased by 4 units