In a radioactive decay chain, the initial nucleus is _ 90 ^ 232 Th...

As we know that in alpha decay mass number reduced from for and the atomic number reduced by two

and in beta decay the mass number became unchange and atomic number increase from one

Change in $Z = \left(6 \times 2\right) - 4 = 8$
Change in $A = 6 \times 4 - 4 = 8$
$TH 90232$$\begin{array}{ll}\overset{6\alpha}{\to} \\ 4\beta\end{array}$$PB 82208$