The $\alpha$ particle is represented as $_{2}^{4} \mathrm{He}$. The sum of atomic and mass numbers should be equal on both sides. So, we have:
$^{238}_{92} \mathrm{U} + _{2}^{4} \mathrm{He} \rightarrow _{94}^{239} \mathrm{Pu} + X$
Here, X represents the unknown
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