Isotopes share protons but differ in neutrons

Steps:

• Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
• The proton number (atomic number) equals the number of protons and identifies the element.
• The nucleon number (mass number) is the total of protons plus neutrons.
• Different isotopes thus have identical proton numbers but varying nucleon numbers due to neutron differences.

Why C is correct:

• By definition, isotopes of the same element have the same atomic number (protons) but different mass numbers (protons + neutrons).

Why the others are wrong:

• A: Proton numbers are the same for isotopes of one element; different protons mean different elements.
• B: Proton numbers cannot differ for the same element, as that changes the element.
• D: Identical proton and nucleon numbers describe the same isotope, not different ones.

Final answer: C