Isotopes refresher: for neutral atoms of isotopes of the same element, which property is different while the chemical identity remains the same?

Introduction / Context:
Isotopes are variants of the same chemical element that differ in mass but not in elemental identity. This concept explains phenomena from radioisotopic labeling to nuclear stability trends and is basic to both chemistry and nuclear engineering.

Given Data / Assumptions:

• “Isotopes” share the same atomic number Z.
• Neutral atoms of isotopes have the same number of electrons as protons.
• The mass (A) differs because the neutron count differs.

Concept / Approach:
Atomic number Z equals the number of protons and defines the element (e.g., Z = 92 → uranium). Isotopes of an element share Z but have different neutron counts, altering the mass number A, hence the atomic weight. In neutral atoms, electrons equal protons, so electron number is unchanged. Therefore, the property that differs is atomic weight (mass number).

Step-by-Step Solution:
Fix identity: same Z ⇒ same element and same proton count.Neutral atoms: electrons equal protons ⇒ same electron count.Only neutron count varies ⇒ different A and atomic weight.Select “Atomic weights (mass numbers).”

Verification / Alternative check:
Examples: C-12 and C-13; U-235 and U-238. Element is unchanged, chemistry is similar, but nuclear properties and masses differ.

Why Other Options Are Wrong:
Atomic number / protons: same for isotopes.Electrons (neutral): same as protons; therefore unchanged.Element identity: determined by Z, which is unchanged.

Common Pitfalls:
Confusing isotopes (same Z, different A) with isobars (same A, different Z) or isotones (same N, different Z).

Final Answer:
Atomic weights (mass numbers)