Ion Comparison: Protons, Electrons, and Neutrons Differ

Steps:

• Atomic number (Z) defines protons: Fe has Z=26, Co has Z=27, so different protons.
• For +2 ions, electrons = Z - 2: Fe^{2+} has 24 electrons, Co^{2+} has 25 electrons, so different electrons.
• Neutrons = mass number (A) - Z; common isotopes are ^{56}Fe (30 neutrons) and ^{59}Co (32 neutrons), so different neutrons.
• Thus, all three (protons, neutrons, electrons) differ.

Why D is correct:

• Ions of different elements with +2 charge have distinct Z, leading to unequal protons and electrons; neutrons vary by isotope.

Why the others are wrong:

• A: Electrons differ (24 vs. 25) due to different Z.
• B: Neutrons differ based on common isotopes (30 vs. 32).
• C: Protons equal Z, which differs (26 vs. 27).

Final answer: D