A nucleus of thorium has mass number 232 and atomic number 90. After one alpha decay, what are the mass number and atomic number of the daughter nucleus?

1. List the data: thorium has mass number 232 and atomic number 90, with one alpha decay. 2. Alpha emission reduces the mass number by 4: $232 - 4 = 228$. 3. It reduces the atomic number by 2: $90 - 2 = 88$. 4. So the daughter has mass number 228 and atomic number 88. 5. Confirm both numbers dropped, as alpha decay requires. 6. Trap B applies beta rules; trap C subtracts only 2 from mass; trap D wrongly raises the atomic number. _Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 6 Nuclear Physics "Nuclear Physics", p.84_