Explanation:
Reaction equation showing alpha decay in Uranium-238 is as follows.
[tex]^{238}_{92}U \rightarrow ^{234}_{90}Th + ^{4}_{2}He[/tex]
It is known that an alpha particle is basically a helium nucleus and it contains 2 protons and 2 neutrons.
Symbol of an alpha particle is [tex]^{4}_{2}\alpha[/tex].
As atomic mass or weight is the sum of total number of protons and neutrons present in an atom.
Hence, the atomic weight of the alpha particle is (2 + 2) = 4.
