Structural difference between U-238 and U-235: by how many neutrons do these uranium isotopes differ?

Introduction / Context:
Nuclide notation allows quick comparison of isotopes. Uranium’s atomic number is Z = 92 for all uranium atoms, so any difference between U-238 and U-235 must arise from the neutron count, not the proton count or electron configuration (for neutral atoms).

Given Data / Assumptions:

• Uranium atomic number Z = 92.
• Mass numbers: 238 and 235.
• Neutral atoms considered for electron counts.

Concept / Approach:
The number of neutrons N is found by N = A − Z. Because Z is fixed for uranium, changes in A directly indicate changes in neutron number. Electrons adjust only with ionization, not isotope identity; protons define the element and therefore do not change among isotopes.

Step-by-Step Solution:
Compute for U-238: N = 238 − 92 = 146.Compute for U-235: N = 235 − 92 = 143.Difference: 146 − 143 = 3 neutrons.Thus, the isotopes differ by three neutrons, not by protons or electrons.

Verification / Alternative check:
All isotopes of uranium share Z = 92; any periodic table confirms this. Consequently, differing mass numbers must reflect neutron differences.

Why Other Options Are Wrong:
Protons: would change the element (not uranium anymore).Electrons: equal to protons in neutral atoms; isotope identity does not set ionization.Combined changes in electrons/protons: contradict isotope definition.

Common Pitfalls:
Confusing isotopes (same Z) with isobars (same A but different Z).Assuming electron count differs for isotopes; it does not for neutral atoms.

Final Answer:
Neutrons