Atomic Nucleons and Isotopes

Steps:

• Nucleon number (A) equals protons (Z) plus neutrons (N), so A > Z indicates neutrons.
• Difference A - Z gives the number of neutrons in the nucleus.
• Atoms with same Z but different A have varying neutrons, defining isotopes.
• Gaps filled by "neutrons" (gap 1) and "isotopes" (gap 2), matching option D.

Why D is correct:

• By definition, nucleon number A = Z + N, so neutrons fill gap 1; isotopes are atoms with identical Z but different A, filling gap 2.

Why the others are wrong:

• A: Electrons orbit outside nucleus and aren't nucleons; isomers describe molecular compounds, not atomic variants.
• B: Neutrons correct for gap 1, but isomers wrong—applies to molecules, not atoms with same protons.
• C: Isomers incorrect for gap 2; describes structural variants in molecules, not nuclear differences.

Final answer: D