Lecture Notes: VSEPR Theory for 5 Electron Clouds

Overview

• Continued exploration of VSEPR theory for 5 electron clouds.
• Focus on determining the shape of molecules with examples: Chlorine Trifluoride (ClF₃) and Triiodide Ion (I₃⁻).

Example 1: Chlorine Trifluoride (ClF₃)

Steps to Determine Molecular Shape

1. Draw Dot Structure

   • Calculate total valence electrons:
     • Chlorine (Cl): 7 valence electrons (Group 7)
     • Fluorine (F): 3 atoms x 7 = 21
     • Total: 28 valence electrons
   • Chlorine is central (less electronegative than Fluorine).
   • Bond Cl to 3 F atoms, use 6 electrons, 22 electrons remain.
   • Assign remaining electrons to terminal atoms (F) to fulfill octet rule.
   • Remaining 4 electrons assigned as lone pairs on central Cl.
   • Chlorine exceeds octet rule due to its position in the third period.

2. Count Electron Clouds

   • Electron clouds = regions of electron density (bonding & non-bonding).
   • Cl is surrounded by 5 electron clouds: 3 bonding pairs, 2 lone pairs.

3. Predict Geometry

   • 5 electron clouds form a trigonal bipyramidal geometry.
   • Lone pairs take equatorial positions to minimize repulsion.
   • Final shape (ignoring lone pairs): T-shaped.

4. Bond Angles

   • Ideal bond angles: 90° and 180° (actual may vary experimentally).

Key Points

• Lone pairs take up more space than bonding pairs, influencing molecular shape.
• Position lone pairs in equatorial positions to minimize repulsion.

Example 2: Triiodide Ion (I₃⁻)

Steps to Determine Molecular Shape

1. Draw Dot Structure

   • Calculate total valence electrons:
     • Iodine (I): 7 x 3 = 21
     • Negative charge: +1 electron, total = 22 electrons
   • Bond 3 I atoms, use 4 electrons, 18 electrons remain.
   • Assign remaining electrons to terminal atoms and central atom as lone pairs.
   • Central Iodine accommodates extra electrons (expands valence shell).
   • Formal charge of central Iodine: -1.

2. Count Electron Clouds

   • Total of 5 electron clouds around central Iodine: 2 bonding, 3 lone pairs.

3. Predict Geometry

   • Electron clouds form trigonal bipyramidal arrangement.
   • Lone pairs in equatorial positions.
   • Final shape (ignoring lone pairs): Linear.

4. Bond Angles

   • Predicted bond angle is approximately 180°.

Key Points

• Dot structures help visualize electron distribution and formal charges.
• Predicting shape involves considering only the bonded atoms after placing lone pairs.

Summary

• Discussed two different molecules with 5 electron clouds and how lone pairs affect geometry.
• Respective geometries depend on electron cloud arrangements and lone pair positions.
• Examples illustrate how molecular shape is determined using VSEPR theory.