Q.20 Iodine forms an anionic species Q in aqueous solution of iodide (I−). The number of lone pair(s) of electrons on the central atom of Q is __________.
Iodine forms the triiodide anion, I₃⁻, in aqueous iodide (I⁻) solutions. The central iodine in I₃⁻ has 2 lone pairs of electrons.
Detailed Solution: Iodine Anionic Species Q
In aqueous solutions containing iodide ions (I⁻), added iodine (I₂) reacts to form the polyhalide anion I₃⁻ (triiodide ion):
I₂ + I⁻ ⇌ I₃⁻
This is the anionic species Q. I₃⁻ is linear (D∞h symmetry), with the central iodine bonded to two terminal iodines via sigma bonds.
Lewis Structure and Electron Count on Central Iodine
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Central I: 7 valence electrons.
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Forms 2 single bonds (uses 2 electrons).
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Total electrons around central I: 7 (own) + 2 (from each I–I bond) + 4 (from two lone pairs) = 14 electrons (octet expanded).
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Lone pairs on central I: 2 (four electrons in two pairs).
VSEPR: AX₂E₂ → Linear geometry, two bonding pairs, two lone pairs.
Answer: 2
(No options provided; it’s a fill-in-the-blank. Common distractors: 1 or 3 lone pairs from miscounting bonds.)
Introduction to Iodine Anionic Species in Aqueous Solution
The iodine anionic species forms as I₃⁻ (triiodide) when I₂ dissolves in aqueous I⁻ solutions via I₂ + I⁻ ⇌ I₃⁻. The lone pairs on central atom of this Q species is a key GATE Life Sciences/Chemistry question testing coordination chemistry and VSEPR.
Formation of I₃⁻: Reaction in Aqueous Iodide
Iodine (I₂, brown) + excess I⁻ (colorless) yields deep yellow-brown I₃⁻, stable in water. Equilibrium constant K ≈ 700 M⁻¹. Central I oxidation state: 0.
Lewis Structure: Counting Lone Pairs on Central Iodine
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Central I: Bonds to two I atoms (2 bond pairs).
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Remaining: Two lone pairs (E₂ in AX₂E₂).
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Total: 10 electrons around central I (hypervalent).
Visualize: I←I→I (linear, ~180° bond angle).
VSEPR Analysis for I₃⁻ Central Atom
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Steric number: 5 (2 bonds + 3 lone pairs? No—central I uses 3 orbitals for bonds/lone pairs effectively).
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Actual: AX₂E₂ (bent electron geometry, linear molecular). Lone pairs: 2.
| Species | Central Atom | Bonds | Lone Pairs | Geometry |
|---|---|---|---|---|
| I₃⁻ | I (central) | 2 | 2 | Linear |
| I₂ | I | 1 | 3 | Linear |
| IF₅ | I | 5 | 1 | Square pyramidal |
Common Mistakes in Iodine Anionic Species Questions
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Counting 3 lone pairs (ignores hypervalency).
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Confusing with IO₃⁻ (iodate, 3 lone pairs on I).
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Forgetting aqueous context (I₃⁻ vs. I⁅).
Exam Tips for Lone Pairs Questions
Ideal for GATE XL Chemistry or CSIR NET. Practice: Br₃⁻ (same, 2 lone pairs). Draw structures first!
Master iodine anionic species lone pairs for top scores.
