Study Notes on Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals

Chapter Overview: Chemical Bonding II: Molecular Geometry and Hybridization of Atomic Orbitals

10.1 Introduction to The Structures of Compounds

• Focus on the three-dimensional structure of compounds.

• Discusses how atoms are distributed in space.

• Introduction to the hybridization of atomic orbitals to form bonds between atoms in compounds.

10.1 Valence Shell Electron Pair Repulsion (VSEPR) Model

• Purpose: To predict molecular geometry from electron pair interactions.

• Key Features:

  • Visual representation of 3D shapes of molecules.

  • Based on electrostatic repulsions between bonding and non-bonding pairs of electrons.

  • Electron pairs (bonding and non-bonding) are positioned as far apart as possible due to repulsion.

  • Non-bonding pairs (lone pairs) are generally larger than bonding pairs, leading to stronger repulsion.

  • Termed as Electron Domains: binding sites which include single, double, or triple bonds counted as one domain.

  • To determine electron arrangement in a molecule's Lewis structure, count all electron domains (bonding + non-bonding).

10.1 Electron Domain Arrangements

Molecular Geometry for Given Electron Domain Configurations:

• AB2 Configuration:

  • Geometry: Linear

  • # of Bonded Atoms: 2; # of Lone Pairs: 0

• AB3 Configuration:

  • Geometry: Trigonal Planar

  • # of Bonded Atoms: 3; # of Lone Pairs: 0

• AB4 Configuration:

  • Geometry: Tetrahedral

• AB5 Configuration:

  • Geometry: Trigonal Bipyramidal

• AB6 Configuration:

  • Geometry: Octahedral

• Example molecules include PCl5 and SF6.

Different Variants with Lone Pairs

• AB3E: Trigonal pyramidal (with 1 lone pair)

• AB2E2: Bent (2 lone pairs)

• AB4E: Distorted tetrahedral
  e.g., Water (H2O) has a bent shape due to 2 lone pairs on oxygen.

10.1 Bonding Pair vs. Lone Pair Repulsion

• Lone-lone pair repulsion > Lone-bonding pair repulsion > Bonding-bonding pair repulsion.

10.1 Predicting Molecular Geometry Steps:

1. Draw the Lewis structure for the molecule.

2. Count lone pairs on the central atom and number of atoms bonded to it:

   • The least electronegative atom is usually placed in the center of the molecule.

3. Complete the octet for the central atom, except for hydrogen.

4. Compare theoretical vs. actual number of electrons; form double/triple bonds if necessary.

5. Utilize VSEPR to predict geometrical shape.

Example Predictions:

• SO2 has Bent geometry (AB2E).

• SF4 has distorted tetrahedral geometry (AB4E).

10.2 Dipole Moments and Polar Molecules

Definitions:

• Dipole Moment (m): A measure of the separation of positive and negative charge; calculated as:
  $m = Q imes r$
  where ( Q ) = charge (C) and ( r ) = distance between charges (m).

• Expressed in Debye unit (D): ( 1 D = 3.36 imes 10^{-30} Cm ).

Identifying Dipole Moments in Molecules:

• Examples:

  • H2O (Polar)

  • CO2 (Nonpolar)

  • SO2 (Polar)

  • CH4 (Nonpolar)

• Question: Does BF3 have a dipole moment? No.

• Question: Does CH2Cl2 have a dipole moment? Yes.

10.3 Hybridization of Atomic Orbitals

• Definition: Mixing of two or more atomic orbitals (e.g., s and p) creates new hybrid orbitals used for bonding.

• Process involves:

  1. The number of hybrid orbitals relates directly to the number of atomic orbitals mixed.

  2. Hybrid orbitals differ in shape from original atomic orbitals.

Types of Hybridization:

• sp: Linear geometry

• sp2: Trigonal planar geometry

• sp3: Tetrahedral geometry (e.g., CH4).

• sp3d: Trigonal bipyramidal geometry.

• sp3d2: Octahedral geometry.

Valence Bond Theory Example - NH3:

• Hybridization occurs by overlapping 2p orbitals from nitrogen with 1s orbitals from hydrogen.

• Predicted angle from 2p overlap would be 90° but is actually 107° due to lone pair-bond pair interactions.

10.4 Sigma and Pi Bonds

• Sigma bond (s): Electron density exists between two atoms.

• Pi bond (p): Electron density exists above and below the plane of the nuclei.

• Bond types based on hybridization:

  • Single bond: 1 sigma bond

  • Double bond: 1 sigma + 1 pi bond

  • Triple bond: 1 sigma + 2 pi bonds

Example - Acetic Acid (CH3COOH):

• Total sigma bonds: 7 (counting H, O, C bonds; includes all connectivity)

• Total pi bonds: 1 (due to the double bond between C=O).

10.5 Summary of Bond Angles:

• Example bond angles:

  • HCN bond angle

  • HCH bond angle in propane

  • CCO bond angle in propane

Questions to Review:

1. Which compounds possess a dipole moment?

2. Identify the hybridization state of given central atoms from structural diagrams.