Isotopes share the same atomic number but differ in mass number

Steps:

• Recall that isotopes are variants of the same element, defined by the same number of protons (atomic number).
• Note that the nucleon number is the total of protons and neutrons (mass number).
• For two isotopes of an element, proton numbers are identical, but neutron counts vary, leading to different nucleon numbers.
• Thus, the defining feature is same protons, different nucleons.

Why B is correct:

• By definition, isotopes have the same proton number (same element) but different nucleon numbers due to varying neutrons.

Why the others are wrong:

• A: Different proton numbers means different elements, not isotopes.
• C: Same nucleon and proton numbers implies identical isotopes, not two distinct forms.
• D: Same nucleon but different protons contradicts basic atomic structure, as that would not be the same element.

Final answer: B