Answer:
[tex]\rm_{90}^{231}\text{Th}[/tex]
Explanation:
The unbalanced nuclear equation is
[tex]\rm _{92}^{235}\text{U} \longrightarrow \, _{2}^{4}\text{He} + X[/tex]
Let's write X as a nuclear symbol.
[tex]\rm _{92}^{235}\text{U} \longrightarrow \, _{2}^{4}\text{He} + _{Z}^{A}\text{X}[/tex]
The main point to remember in balancing nuclear equations is that the sums of the superscripts and of the subscripts must be the same on each side of the reaction arrow.
Then
235 = 4 + A , so A = 235 - 4 = 231, and
92 = 2 + Z , so Z = 92 - 2 = 90
And your nuclear equation becomes
[tex]\rm _{92}^{235}\text{U} \longrightarrow \, _{2}^{4}\text{He} +\, _{90}^{231}\text{X}[/tex]
Element 90 is thorium, so
[tex]\rm X = _{90}^{231}\text{Th}[/tex]
