Final answer:
The daughter element's atomic number after alpha decay is less than that of the parent element due to the emission of an alpha particle. The atomic number decreases by two and the mass number decreases by four, which can be exemplified by uranium-238 decaying into thorium-234.
Step-by-step explanation:
In alpha decay, a heavy nucleus emits an alpha particle, which consists of two protons and two neutrons. As a result of alpha decay, the daughter element's atomic number is less than that of the parent element since it loses two protons. Consequently, the atomic number decreases by two and the mass number decreases by four. For example, when Radium-226 undergoes alpha decay, it transforms into Radon-222.
Following alpha decay, the daughter nucleus will have a mass number that is four units smaller (A-4) and an atomic number that is two units smaller (Z-2) than the parent nucleus. Alpha decay tends to move a nuclide closer to the band of stability if it initially lies below it, due to the increase in the neutron to proton (n:p) ratio in the daughter nuclide.
An example of alpha decay is:
238 U → 234 Th + 4 He
This shows uranium-238 decaying to thorium-234 and emitting an alpha particle (4He).