Physics I / Chemical periodicity
Practice question · Put in order

Rank these ions and atom from smallest to largest radius: they all have exactly 10 electrons.

Hints
  1. Same electron cloud, different nuclear pull. More protons squeeze the identical 10-electron cloud tighter.
  2. Order by proton count, descending: the strongest squeeze gives the smallest ion.
Show the answer
  1. Mg2+Mg^{2+} (12 protons)
  2. Na+Na^{+} (11 protons)
  3. Ne (10 protons)
  4. F−F^{-} (9 protons)
  5. O2−O^{2-} (8 protons)
Why

An isoelectronic series is a controlled experiment: the cloud is fixed at 10 electrons, so size is set purely by nuclear charge. Mg2+Mg^{2+} (12 p⁺) crushes the cloud smallest; O2−O^{2-} (8 p⁺) barely contains it. Size is a tug-of-war, not a fixed property.

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