Isotopes Defined by Neutron Variation

Steps:

• Recall that isotopes are atoms of the same element, sharing the same atomic number (protons).
• Note that atomic number defines the element, while mass number (protons + neutrons) can vary.
• Evaluate options: A and B involve different elements, which contradicts the same-element requirement.
• Confirm D matches the standard definition of differing neutrons in the same element.

Why D is correct:

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, as per the IUPAC definition.

Why the others are wrong:

• A: Different elements have different proton counts, not just electron matches; electrons relate to ions, not isotopes.
• B: Different elements cannot be isotopes; neutron equality doesn't define isotopes.
• C: Different electron numbers describe ions, not isotopes, which have neutral atoms with varying neutrons.

Final answer: D