Among U-235, U-238, U-239, and Pu-239, which nucleus contains the largest number of neutrons?

Introduction / Context:
Counting neutrons from nuclide notation A/Z X is a fundamental skill. The number of neutrons N equals A − Z, where A is mass number and Z is atomic number. Uranium has Z = 92 and plutonium has Z = 94, so comparing A values quickly reveals which has more neutrons.

Given Data / Assumptions:

• Z(U) = 92 and Z(Pu) = 94.
• Mass numbers given: 235, 238, 239.
• We assume the common isotopes listed.

Concept / Approach:
Compute neutrons for each candidate with N = A − Z. Larger A at the same Z means more neutrons. Even when Z differs (U vs. Pu), a simple subtraction provides the neutron count directly.

Step-by-Step Solution:
U-235: N = 235 − 92 = 143.U-238: N = 238 − 92 = 146.U-239: N = 239 − 92 = 147.Pu-239: N = 239 − 94 = 145.Largest neutron count is 147 for U-239.

Verification / Alternative check:
Periodic knowledge confirms U has fewer protons than Pu; therefore, for the same A = 239, uranium must have more neutrons than plutonium (147 vs. 145), supporting the choice.

Why Other Options Are Wrong:
U-235: 143 neutrons—less than 147.U-238: 146 neutrons—still less than 147.Pu-239: 145 neutrons—less than U-239.U-233: 141 neutrons—much less than 147.

Common Pitfalls:
Mixing up A and Z or forgetting that Z differs between U and Pu.Assuming higher A always means more neutrons without checking Z.

Final Answer:
U-239