Hey there!:
β-decay:
Unstable nucleus undergo β-decay in which a neutron is converted into proton , so
¹₀ n ----> ₁¹p + ₋₁⁰e ( Z + 1 )
The complete reaction is as follows:
²²⁷₈₉AC -----> ²²⁷₉₀ Th + ₋₁⁰e
Therefore , the daughter nucleus is : ²²⁷₉₀ Th
Hope that helps!
