11.1 Molecular geometry

Molecular geometry is all about how atoms arrange themselves in space. It's like a 3D puzzle where electron pairs push each other away, creating different shapes. This topic helps us understand why molecules look the way they do.

We'll learn about VSEPR theory, which predicts molecular shapes based on electron pairs. We'll also explore bond angles and how lone pairs affect a molecule's geometry. It's like understanding the architecture of the molecular world!

Predict the geometry of molecules using the Valence Shell Electron Pair Repulsion (VSEPR) theory

VSEPR theory and molecular geometry

• VSEPR theory states that electron pairs will repel each other and arrange themselves in space to minimize repulsion and maximize distance between them
• Molecular geometry is determined by the number of bonding and nonbonding electron pairs around the central atom
• The AXE method can be used to determine molecular geometry, where A is the central atom, X is the number of bonded atoms, and E is the number of lone electron pairs

Electron pair and molecular geometries

• Electron pair geometries include linear (2 electron pairs), trigonal planar (3 electron pairs), tetrahedral (4 electron pairs), trigonal bipyramidal (5 electron pairs), and octahedral (6 electron pairs)
• Molecular geometries are derived from electron pair geometries and include linear, bent, trigonal planar, trigonal pyramidal, tetrahedral, seesaw, T-shaped, trigonal bipyramidal, square pyramidal, and octahedral
  • Examples of molecular geometries:
    • Linear (CO2)
    • Bent (H2O)
    • Trigonal planar (BF3)
    • Trigonal pyramidal (NH3)
    • Tetrahedral (CH4)

Determine the bond angles in molecules based on their molecular geometry

Bond angles in different geometries

• Bond angles are determined by the arrangement of electron pairs around the central atom
• In a linear geometry, the bond angle is 180°
• In a trigonal planar geometry, the bond angles are 120°
• In a tetrahedral geometry, the bond angles are 109.5°
• In a trigonal bipyramidal geometry, the axial bond angles are 90°, and the equatorial bond angles are 120°
• In an octahedral geometry, all bond angles are 90°

Examples of bond angles in molecules

• H2O (bent): bond angle ≈ 104.5°
• NH3 (trigonal pyramidal): bond angle ≈ 107°
• CH4 (tetrahedral): bond angle = 109.5°
• PCl5 (trigonal bipyramidal): axial bond angles = 90°, equatorial bond angles = 120°
• SF6 (octahedral): bond angles = 90°

Explain the relationship between molecular geometry and the number of bonding and nonbonding electron pairs

Bonding and nonbonding electron pairs

• Molecular geometry is determined by the total number of electron pairs (bonding and nonbonding) around the central atom
• Bonding electron pairs are shared between the central atom and the bonded atoms
• Nonbonding electron pairs (lone pairs) are not shared and belong only to the central atom

Effect of lone pairs on molecular geometry

• The presence of lone pairs affects the molecular geometry and bond angles due to their repulsive effects
• As the number of lone pairs increases, the molecular geometry deviates more from the ideal electron pair geometry
  • Example: In a molecule with tetrahedral electron pair geometry and one lone pair, the molecular geometry is trigonal pyramidal (NH3)

Identify the effect of lone pairs on molecular geometry and bond angles

Lone pairs and molecular geometry

• Lone pairs occupy more space than bonding pairs due to greater repulsion, causing a decrease in bond angles compared to the ideal electron pair geometry
• In molecules with lone pairs, the molecular geometry differs from the electron pair geometry
  • Example: H2O has a tetrahedral electron pair geometry but a bent molecular geometry due to the presence of two lone pairs

Lone pairs and bond angles

• Lone pairs push the bonding pairs closer together, resulting in smaller bond angles than predicted by the ideal electron pair geometry
• For example, in a molecule with tetrahedral electron pair geometry and one lone pair, the molecular geometry is trigonal pyramidal, and the bond angles are slightly less than 109.5° (NH3: bond angle ≈ 107°)
• The greater the number of lone pairs, the more significant the deviation from the ideal bond angles
  • Example: In a molecule with tetrahedral electron pair geometry and two lone pairs, the bond angle is even smaller than in a molecule with one lone pair (H2O: bond angle ≈ 104.5°)