JEE Main 2021 · 18 March · morning · Single correct · +4 / −1
The ionic radius of Na⁺ ions is 1.02 Aᵒ. The ionic radii (in Aᵒ) of Mg²⁺ and Al³⁺, respectively, are
- (a) 1.05 and 0.99
- (b) 0.72 and 0.54
- (c) 0.85 and 0.99
- (d) 0.68 and 0.72
Answer and solution
Answer (b) 0.72 and 0.54
Solution
Chemistry ma’am · 3 steps
All three ions carry 10 electrons, so the one with more protons pulls them in tighter.
- Same electron count. Na⁺, Mg²⁺ and Al³⁺ each have 10 electrons, the neon configuration. Ions with the same number of electrons are called isoelectronic.
- Compare nuclear charge. In an isoelectronic series the electron cloud is the same, so size depends only on how many protons pull on it. More protons give a stronger pull and a smaller ion.
Ion Protons Electrons Radius trend Na⁺ 11 10 largest, 1.02 Å (given) Mg²⁺ 12 10 smaller than 1.02 Å Al³⁺ 13 10 smallest - So. The order must be Na⁺ > Mg²⁺ > Al³⁺. Only (b), with 0.72 Å and 0.54 Å, fits: both values are below 1.02 Å, and the Al³⁺ value is the smaller one.
The idea to keep. For isoelectronic ions, more protons means a smaller ion. Line them up by nuclear charge, and the radii fall in exactly the reverse order.
Answer from the NTA final answer key. Solution by PrepWiser.