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[tex] ^{4} _2 \alpha [/tex].
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:
[tex] ^{A}_ZX[/tex] → [tex] ^{A-4}_{Z-2}X[/tex] + [tex] ^{4}_{2} \alpha [/tex]
