Nuclear Decay Rules for Mass and Atomic Number

Steps:

• Recall alpha decay subtracts 4 from mass number and 2 from atomic number.
• Recall β⁻ decay keeps mass number the same and adds 1 to atomic number.
• Recall β⁺ decay keeps mass number the same and subtracts 1 from atomic number.
• Check each option against these rules to match parent to product.

Why D is correct:

• ²³⁴Th (mass 234, Z=90) undergoing β⁻ decay yields ²³⁴Pa (mass 234, Z=91), as β⁻ increases Z by 1 while mass stays constant.

Why the others are wrong:

• A: ²³⁸U (Z=92) α-decay should yield Z=90 (²³⁴Th), not ²³⁴Rn (Z=86).
• B: ²²⁶Ra (Z=88) β⁻ decay should yield Z=89 (²²⁶Ac), not ²²⁶Pa (Z=91).
• C: ²³⁸Th (Z=90) β⁺ decay should yield Z=89 (²³⁸Ac), not ²³⁸Pa (Z=91).

Final answer: D