Isotopes of the same element differ in which fundamental quantity while sharing the same atomic number and (for neutral atoms) electronic configuration?

Introduction / Context:
Isotopes are central to nuclear science and analytical chemistry. They are atoms of the same element (same atomic number Z) that vary in nucleon count, leading to different masses but essentially identical electronic structures for neutral atoms—hence similar chemistry.

Given Data / Assumptions:

• Comparison at identical charge state (neutral).
• Chemical properties judged by electron configuration.
• Atomic number Z defines nuclear charge.

Concept / Approach:
Because isotopes have the same Z, they have the same number of electrons when neutral, so their electronic configuration and valence behavior are the same to first order. The distinguishing nuclear property is the number of neutrons, which changes the mass number A = Z + N. Small isotope effects in chemistry exist but do not alter the fundamental definition.

Step-by-Step Solution:
State isotope definition: same Z, different N.Identify differing quantity: mass number A changes with N.Check invariants: nuclear charge (Z) and neutral-atom electronic configuration remain the same.Conclude: “Mass number” is the correct distinguishing quantity.

Verification / Alternative check:
Periodic trends and chemical behavior are set by valence electrons; isotopes line up identically in the table because Z—hence valence configuration—does not change.

Why Other Options Are Wrong:
Electronic configuration: same for neutral isotopes of an element.Nuclear charge: by definition equal for isotopes.Chemical properties: essentially the same; only minor isotope effects occur.Valence electron count: identical for neutral atoms of the same element.

Common Pitfalls:
Confusing isotopes with ions (which do alter electron counts).Mixing up isotopes and isobars (same A, different Z).

Final Answer:
Mass number