The triiodide ion `I_(3)^(-)` formed by dissolving iodine in aqueous potassium iodide has which one of the following structures/ geometries ? 

Options

A. Triangular
B. Tetrahedral with one corner occupied by a lone pair of electrons
C. Trigonal bipyramidal with three lone pairs, one occupying the equatorial and two axial positions
D. Linear with bond angle of exactly 180°

Answer

To determine the structure or geometry of the triiodide ion \( I_3^- \) formed by dissolving iodine in aqueous potassium iodide, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components**:
The triiodide ion \( I_3^- \) is formed from the reaction of iodine (\( I_2 \)) with potassium iodide (\( KI \)). The reaction can be represented as:
\[ KI + I_2 \rightarrow K^+ + I_3^- \]

2. **Count the Valence Electrons**:
Each iodine atom contributes 7 valence electrons. For three iodine atoms:
\[ 3 \times 7 = 21 \text{ valence electrons} \]
The triiodide ion has a charge of -1, which means we add one more electron:
\[ 21 + 1 = 22 \text{ total valence electrons} \]

3. **Draw the Lewis Structure**:
In the Lewis structure of \( I_3^- \), we can arrange the three iodine atoms. One iodine atom will be central, and the other two will be bonded to it. The central iodine atom will have a lone pair of electrons, and the two terminal iodine atoms will each have three lone pairs:
\[ I - I - I \]
The central iodine atom will have one bond to each of the terminal iodine atoms and one lone pair.

4. **Determine the Hybridization**:
The central iodine atom has:
- 2 bond pairs (from the two \( I \) atoms)
- 1 lone pair
This gives a total of 3 regions of electron density, which corresponds to \( sp^3 \) hybridization.

5. **Identify the Geometry**:
The arrangement of the electron pairs is trigonal bipyramidal due to the \( sp^3 \) hybridization. However, since there are lone pairs, the actual shape of the molecule is determined by the positions of the bonded atoms. The two iodine atoms will be in a linear arrangement, while the lone pair will occupy an equatorial position, resulting in a linear shape for the ion:
\[ \text{Shape: Linear} \]

6. **Conclusion**:
Thus, the geometry of the triiodide ion \( I_3^- \) is linear.

### Final Answer:
The triiodide ion \( I_3^- \) has a **linear geometry**.