Concept: The shape and bond angles of a molecule or ion are determined by the VSEPR (Valence Shell Electron Pair Repulsion) theory, which considers the number of electron domains (bonding pairs and lone pairs) around the central atom.

Why (A) is correct:

1. Determine the central atom: In I₃⁻, the central atom is one of the iodine atoms.

2. Calculate total valence electrons: Each iodine atom has 7 valence electrons. For I₃⁻, total valence electrons = (3 × 7) + 1 (for the negative charge) = 21 + 1 = 22 electrons.

3. Determine electron domains around the central atom: The central iodine atom forms two single bonds with the other two iodine atoms. This accounts for 4 electrons (2 bonding pairs). The remaining electrons are 22 - 4 = 18 electrons. These 18 electrons form 9 lone pairs. However, we need to consider lone pairs on the central atom. The central iodine atom will have 3 lone pairs (6 electrons) to complete its octet (or expanded octet in this case). So, the central iodine atom has 2 bonding pairs and 3 lone pairs.

4. Apply VSEPR theory: With 2 bonding pairs and 3 lone pairs, the total number of electron domains around the central iodine atom is 5. According to VSEPR theory, 5 electron domains correspond to a trigonal bipyramidal electron geometry. The lone pairs occupy the equatorial positions to minimize repulsion. The two bonding iodine atoms will occupy the axial positions.

5. Determine molecular geometry and bond angle: When the lone pairs are in the equatorial positions and the bonding atoms are in the axial positions, the resulting molecular geometry is linear. The I–I–I bond angle is 180°.

Option Analysis:

• A) Linear; 180°: This is consistent with VSEPR theory for a central atom with 2 bonding pairs and 3 lone pairs.
• B) T-shaped ; 180° and 90°: T-shaped geometry occurs with 3 bonding pairs and 2 lone pairs (e.g., ClF₃).
• C) Trigonal planar; 120°: Trigonal planar geometry occurs with 3 bonding pairs and 0 lone pairs (e.g., BF₃).
• D) Distorted trigonal planar; 135° and 90°: This is not a standard VSEPR geometry for I₃⁻.

Correct Answer: (A)