Answer
7 alpha decays and 4 beta decays.
Work Step by Step
In the case of beta decay, the mass number doesn't change. But when an alpha particle is emitted, the mass number decreases by 4. Therefore,
the number of alpha decays=$\frac{Decrease\,in\,mass\,number}{4}=\frac{235-207}{4}=7$
When an alpha particle is emitted, the atomic number decreases by 2. Therefore, when 7 alpha decays occur, the atomic number decreases by 14.
The atomic number (Z) of Uranium= 92
The atomic number after 7 alpha decays is 92-14=78.
Finally, Z= Z of Pb=82.
The atomic number is increased by (82-78)=4.
We know that the atomic number increases by one when a beta particle is emitted. Therefore, the atomic number increased by 4 implies that the number of beta decays occurred is equal to 4.
