Isotopes share atomic number but differ in mass number

Steps:

• Recall that isotopes are nuclei with the same atomic number (Z, protons) but different mass numbers (A, protons + neutrons).
• Examine Z and A values for nuclei P, Q, R, S from the given data.
• Compare Z for each pair: P-Q differ in Z; P-S differ in Z; Q-R differ in Z; R-S share Z but differ in A.
• Select the pair matching the isotope definition: R and S.

Why D is correct:

• R and S have identical Z (same element) but different A (different neutron count), per the isotope definition.

Why the others are wrong:

• A: P and Q have different Z, so different elements.
• B: P and S have different Z, so different elements.
• C: Q and R have different Z, so different elements.

Final answer: D