Which of the following best describes the Lewis structure of the $I_3-$ ion?

• A. It is bent, with three iodine atoms forming a V-shape and one lone pair on each iodine.
• B. It is linear, with three iodine atoms in a row and no lone pairs on any iodine atom.
• C. It is trigonal planar, with the three iodine atoms arranged in a triangle and one lone pair on each iodine.
• D. It is linear, with three iodine atoms in a row and three lone pairs on each terminal iodine and two lone pairs on the central iodine.

Answer: (D)

Verified step by step guidance

1. Identify the central atom and the total number of valence electrons: For the I3- ion, the central atom is iodine, and each iodine atom has 7 valence electrons. Since there are three iodine atoms and an extra negative charge, calculate the total valence electrons as \$3 \times 7 + 1 = 22$ electrons.
2. Draw a skeletal structure: Arrange the three iodine atoms in a row (linear arrangement) with single bonds connecting the central iodine to each terminal iodine atom. Each bond represents 2 electrons.
3. Distribute the remaining electrons as lone pairs: After accounting for bonding electrons, place the remaining electrons as lone pairs around each iodine atom to satisfy the octet rule. The terminal iodines will have three lone pairs each, and the central iodine will have two lone pairs.
4. Check the formal charges: Calculate the formal charge on each iodine atom using the formula \(\text{Formal charge} = \text{Valence electrons} - \text{Nonbonding electrons} - \frac{1}{2} \times \text{Bonding electrons}\). Adjust the structure if necessary to minimize formal charges.
5. Confirm the molecular geometry: With the central iodine having two lone pairs and two bonding pairs, the electron geometry is trigonal bipyramidal, but the molecular shape (considering only atoms) is linear, consistent with the observed structure of the I3- ion.