Question

many radioactive atoms that have large masses undergo radioactive decay by releasing a particle that is identical to a helium-4 nucleus. what changes in the original atom are expected as a result of this natural phenomenon? the atomic number and the mass number will decrease. the atomic number and the mass number will increase. the atomic number will increase, and the mass number will decrease. the atomic number will decrease, and the mass number will increase.

Answer 1

The answer is the atomic number and the mass number will decrease.

When atoms undergo radioactive decay, alpha particles release.

Alpha particles consist of two protons and two neutrons. So, mass is 4 (2+2) amu$^{4} _2 \alpha$.
When an atom of emits an alpha particle, its atomic number will be reduced by 2 and a mass number will be reduced by 4:

 $^{A}_ZX$ →  $^{A-4}_{Z-2}X$ + $^{4}_{2} \alpha$

Answer 2

The answer to your question is option A.