Definition of isotopes:\nIsotopes are atoms that have the same ________ but differ in mass number because of differing neutron counts.

Introduction / Context:
Isotopes play a key role in nuclear chemistry, radiometric dating, and analytical methods. They are variants of the same chemical element and therefore share chemical behavior while differing in nuclear properties that depend on neutron count.

Given Data / Assumptions:

• Isotopes share the same element identity.
• Chemical identity is defined by proton number (atomic number Z).
• Mass number A = Z + N, where N is number of neutrons.

Concept / Approach:
All atoms of the same element must have the same atomic number (Z). Isotopes differ in neutron number N, which changes the mass number A but not the element’s chemical identity. Average atomic masses on periodic tables are weighted by natural isotopic abundances and are not integers. Electron configurations can vary by ionization/excitation but ground-state configurations are element-specific and not a defining criterion for isotopes.

Step-by-Step Solution:

Verification / Alternative check:
Nuclear notation A_ZX explicitly encodes Z as the element identity; changing Z changes the element, so isotopes cannot share mass number or neutron number necessarily, only Z.

Why Other Options Are Wrong:

• Mass number: by definition different among isotopes.
• Number of neutrons: differs among isotopes.
• Average atomic mass: a tabulated, weighted value differs per sample.
• Electron configuration: not the defining feature for isotope identity.

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

Final Answer:
Atomic number (number of protons).