Isotopes share atomic number but differ in mass number

Steps:

• Recall isotopes have identical atomic numbers (protons) but different mass numbers (protons + neutrons).
• Examine each atom's proton count to identify the element.
• Compare atoms pairwise: same protons means same element, different neutrons confirm isotopes.
• Select pair with matching protons and differing neutrons.

Why C is correct:

• Atoms 2 and 4 have the same atomic number (e.g., 6 protons for carbon) but different mass numbers due to neutron variation, per isotope definition.

Why the others are wrong:

• A: Atoms 1 and 2 have different atomic numbers, so different elements.
• B: Atoms 1 and 4 have different atomic numbers, so different elements.
• D: Atoms 3 and 4 have different atomic numbers, so different elements.

Final answer: C