23290Th→ 20882Pb
As mass is also changing along with atomic number it means α particles are also emitted.
No of α particles emitted
1 α = 4 amu
6 α = 24 amu
So atomic number must be decrease by 12 but it is decreasing only by 8
So β particles emitted = 4
Isobars = nuclei having different atomic number but same mass number.
So, if mass number has to be same then it must be β emission.
So, 5 isobars are possible in total.
Aβ⟶Bβ⟶Cβ⟶Dβ⟶E
n+2 = 7