Isotopes: Atoms of the same element with varying neutrons

Steps:

• Recall that elements are defined by atomic number, which is the number of protons.
• Isotopes share the same atomic number but differ in mass number due to neutron count.
• Examine choices: Focus on "same element" meaning same protons, and variation in neutrons.
• Eliminate options that mismatch protons or elements.

Why C is correct:

• By definition, isotopes are nuclei of the same element (same protons) with different neutrons, as per atomic structure principles.

Why the others are wrong:

• A: Isotopes are same element, not different; different elements have different protons.
• B: Same nucleons would mean identical mass, not isotopes; different elements can't share all nucleons.
• D: Different protons define different elements, not isotopes.

Final answer: C