Answer
$He^4_2$
Work Step by Step
We calculate the required atomic number as follows:
$90=88+Z$
$\implies Z=2$, hence the element is helium He.
Now we find the atomic mas
$234=230+A$
$\implies A=4$
Thus, the complete nuclear reaction is
$Th^{234}_{90}\rightarrow Ra^{230}_{88}+He^4_2$
