Zumdahl • DeCoste chemistry world of Chapter 12

Steps for Predicting Molecular Structure Using VSEPR Model

• Draw the Lewis structure for the molecule
• Count electron pairs & arrange them to minimize repulsion (far apart)
• Determine position of atoms from the way the electron pairs are shared
• Determine name of molecular structure from the positions of atoms

Example 12.5: Predicting Molecular Structure using VSEPR

• Predict structure of ammonia, NH3
• Draw Lewis Structure
• Count pairs of electrons & arrange them to minimize repulsions (see next)

• H N H
• H

Figure 12.13: Tetrahedral arrangement of electron pairs.

• Example 12.5:
• NH3 has four pairs of electrons around the N atom (3 bonding) – best arrangement for 4 pairs is tetrahedral

Figure 12.13: Hydrogen atoms occupy only three corners of the tetrahedron.

• Step 3: Determine the positions of the atoms

Step 4: Determine name of structure

• Name based on positions of the atoms
• Placement of electron pairs determines the structure, but name based on positions of atoms
• NH3 has tetrahedral arrangement of electron pairs, but is not tetrahedral
• Structure is trigonal pyramid (one side different from other three)

Figure 12.13: The NH3 molecule has the trigonal pyramid structure.

Figure 12.14: Tetrahedral arrangement of four electron pairs around oxygen.

• Example 12.6: describe molecular structure of water
• Step 1: draw Lewis structure
• Step 2: Count electron pairs & arrange to minimize repulsions

Figure 12.14: Two electron pairs shared between oxygen and hydrogen atoms.

• Look at only atoms to determine structure
• Step 3: tetrahedral arrangement of electron pairs, but not atoms – atoms form V-shape

Figure 12.14: V-shaped molecular structure of the water molecule.

• Step 4: Molecule is V-shaped or bent

Table 12.4

Molecular Structure Involving Double Bonds

• When using the VSEPR model to predict the molecular geometry of a molecule, a double bond is counted the same as a single electron pair
• Four electrons involved in double bond do not act as two independent pairs, but are “tied together” for form one effective repulsive unit