Isotopes Defined by Neutron Variation

Steps:

• Recall isotopes are atoms of the same element with identical protons but varying neutrons.
• Note atomic number (protons) defines the element, while mass number (protons + neutrons) differs in isotopes.
• Compare each option to this definition.
• Select the option matching same element, different neutrons.

Why D is correct:

• Isotopes are atoms of the same element (same protons) with different numbers of neutrons, as per the standard atomic structure definition.

Why the others are wrong:

• A: Different elements have different protons, not isotopes.
• B: Different elements cannot be isotopes, regardless of neutrons.
• C: Different electrons in the same element create ions, not isotopes.

Final answer: D