JEE Main · 2020 · Shift-IIeasyPERI-036

The correct order of the ionic radii of O2-, N3-, F-, Mg2+, Na+ and Al3+ is:

Classification of Elements & Periodicity · Class 11 · JEE Main Previous Year Question

Question

The correct order of the ionic radii of O2−^{2-}, N3−^{3-}, F−^-, Mg2+^{2+}, Na+^+ and Al3+^{3+} is:

Options
  1. a

    \ceN3−\ce{N^{3-}} < \ceO2−\ce{O^{2-}} < \ceF−\ce{F^-} < \ceNa+\ce{Na^+} < \ceMg2+\ce{Mg^{2+}} < \ceAl3+\ce{Al^{3+}}

  2. b

    \ceAl3+\ce{Al^{3+}} < \ceNa+\ce{Na^+} < \ceMg2+\ce{Mg^{2+}} < \ceO2−\ce{O^{2-}} < \ceF−\ce{F^-} < \ceN3−\ce{N^{3-}}

  3. c

    \ceAl3+\ce{Al^{3+}} < \ceMg2+\ce{Mg^{2+}} < \ceNa+\ce{Na^+} < \ceF−\ce{F^-} < \ceO2−\ce{O^{2-}} < \ceN3−\ce{N^{3-}}

    ✓
  4. d

    \ceN3−\ce{N^{3-}} < \ceF−\ce{F^-} < \ceO2−\ce{O^{2-}} < \ceMg2+\ce{Mg^{2+}} < \ceNa+\ce{Na^+} < \ceAl3+\ce{Al^{3+}}

Correct Answerc

\ceAl3+\ce{Al^{3+}} < \ceMg2+\ce{Mg^{2+}} < \ceNa+\ce{Na^+} < \ceF−\ce{F^-} < \ceO2−\ce{O^{2-}} < \ceN3−\ce{N^{3-}}

Detailed Solution

🧠 Six ions, all isoelectronic with 10 electrons — same cloud, only proton count changes \ceN3−\ce{N^{3-}} (Z=7Z=7), \ceO2−\ce{O^{2-}} (Z=8Z=8), \ceF−\ce{F^-} (Z=9Z=9), \ceNa+\ce{Na^+} (Z=11Z=11), \ceMg2+\ce{Mg^{2+}} (Z=12Z=12), \ceAl3+\ce{Al^{3+}} (Z=13Z=13). All 10-electron systems. The only thing pulling them inward is the proton count — more Z = tighter pull = smaller ion.

🗺️ Rank from smallest to largest by decreasing Z \ceAl3+\ce{Al^{3+}} has 13 protons gripping 10 electrons → smallest. Then drop one Z at a time: \ceMg2+\ce{Mg^{2+}} (12) → \ceNa+\ce{Na^+} (11) → \ceF−\ce{F^-} (9) → \ceO2−\ce{O^{2-}} (8) → \ceN3−\ce{N^{3-}} (7) → largest.

Increasing size: \ceAl3+<\ceMg2+<\ceNa+<\ceF−<\ceO2−<\ceN3−\ce{Al^{3+}} < \ce{Mg^{2+}} < \ce{Na^+} < \ce{F^-} < \ce{O^{2-}} < \ce{N^{3-}}.

⚠️ The trap Many students hesitate at the boundary between \ceNa+\ce{Na^+} and \ceF−\ce{F^-} because one is a cation and one is an anion. They think the anion must be huge and the cation must be tiny. But in an isoelectronic set the labels do not matter — only Z matters. \ceNa+\ce{Na^+} has Z=11, \ceF−\ce{F^-} has Z=9, so \ceF−\ce{F^-} is bigger. The "++" or "−-" sign is just bookkeeping.

Answer: (c)\boxed{\text{Answer: (c)}}

Practice this question with progress tracking

Want timed practice with adaptive difficulty? Solve this question (and hundreds more from Classification of Elements & Periodicity) inside The Crucible, our adaptive practice platform.

Practice in The Crucible →Browse Classification of Elements & Periodicity chapter →

More JEE Main Classification of Elements & Periodicity PYQs

2025medium

The correct orders among the following are: Atomic radius: B < Al < Ga < In < Tl Electronegativity: Al < Ga < In < Tl < B Density: Tl < In < Ga < Al < B 1st Ionisation Energy: In < Al < Ga < Tl < B…

2025medium

Given below are two statements: Statement I: The metallic radius of Al is less than that of Ga. Statement II: The ionic radius of Al3+ is less than that of Ga3+. Choose the most appropriate answer:

2025medium

Given below are two statements: Statement I: The radii of isoelectronic species increases in the order: Mg2+ < Na+ < F- < O2- Statement II: The magnitude of electron gain enthalpy of halogen…

2025easy

First ionisation enthalpy values of first four group 15 elements are given below. Choose the correct value for the element that is a main component of apatite family:

2025easy

The incorrect decreasing order of atomic radii is:

View all 82 Classification of Elements & Periodicity PYQs →
The correct order of the ionic radii of O2-, N3-, F-, Mg2+, Na+ and Al3+ is: (JEE Main 2020) | Canvas Classes