Chemistry

Problem

A chemist is investigating a set of five ions that each contain ten electrons. The ions are listed below in no particular order:

N^{3-}

O^{2-}

F^{-}

Na^{+}

Mg^{2+}

Assume all ions are in their ground state and are not interacting with any other species. (a) Identify the ion in the set that has the largest ionic radius. Provide a detailed justification that references the relevant periodic property or properties. (b) Rank the five ions from smallest to largest ionic radius. In your explanation, clearly relate each step in the ordering to the underlying factors that determine ionic size. (c) Predict qualitatively how the ionic radius of

Al^{3+}

would compare with the radii of the ions listed above, and explain the reasoning for your prediction.

Intuitive explanation

These five ions are isoelectronic — they all have the same electron count (10), but different nuclear charges. The key insight is that nuclear charge determines how strongly electrons are pulled inward: more protons mean smaller, more tightly bound orbitals and higher ionization energies. To order these ions by size or energy, count the protons in each nucleus and recognize that the ion with the most protons will be smallest and hardest to ionize, while the one with fewest protons will be largest and easiest to ionize.

Intuitive explanation

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