Molecular Structure and Molecular Orbitals Notes

Lewis Structures

  • Lewis structures are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist.

  • Example:

    • CO2 = O=C=O

    • N2 = :N=N:

From Lewis Structure to Molecular Geometry

  • Lewis structures predict three-dimensional molecular shapes based on bonding and lone pairs.

    • Example: H2O has angles of 104.5°.

Localized Electron Model

  • Electrons in a molecule are localized in specific regions around an atom.

  • Each electron pair (bonding or lone pair) occupies its own space.

Electron Domains

  • Electron pairs are categorized as:

    1. Lone pairs

    2. Bonding pairs (single, double, or triple bonds)

  • Count total electron domains to determine molecular geometry.

VSEPR Theory

  • Valence-Shell Electron-Pair Repulsion (VSEPR) model states that electron domains repel each other, and thus, a geometry is formed to minimize repulsion.

  • Formula: ABx, where A = central atom, B = terminal atom, and x = number of bonded atoms (2 to 6).

Examples of Molecular Formulas

  • AB2: BeCl2, SO2, H2O

  • AB3: BF3, NH3, ClF3

  • AB4: CHCl3, SF4

  • AB5: PCl5, IF5

  • AB6: SF6

Geometry of Electron Domains

  • Types of Electron Domain Geometry:

    1. Linear (2 electron domains)

    2. Trigonal Planar (3 electron domains)

    3. Tetrahedral (4 electron domains)

    4. Trigonal Bipyramidal (5 electron domains)

    5. Octahedral (6 electron domains)

Electron Domain vs Molecular Geometry

- Electron domain geometry considers all electron domains while molecular geometry only considers bonded electron pairs.

Bond Angles

  • Define Bond Angles: Angles between adjacent covalent bonds.

    • Linear: 180°

    • Trigonal Planar: 120°

    • Tetrahedral: 109.5°

    • Trigonal Bipyramidal: 90° & 120°

    • Octahedral: 90°

Molecular Geometry and Lone Pairs

  1. Lone pairs occupy more space, affecting bond angles and molecular shapes.

  2. For example, in H2O, the bond angle deviates from 109.5° due to lone pairs pushing the H atoms closer together.

Polarity of Molecules

  • A molecule is polar if it has a net dipole moment. Factors affecting polarity include:

    1. Polarity of individual bonds

    2. Molecular Geometry

Examples of Molecular Polarity:

  • CO2: Polar bonds in a linear geometry result in a nonpolar molecule.

  • H2O:
    Polar bonds and bent geometry make it a polar molecule.

  • BF3: Polar bonds but symmetrical distribution make it nonpolar.

Valence Bond Theory (VBT)

  • Bonds are formed by the overlap of atomic orbitals.

  • Hybridization describes how atomic orbitals mix to form new hybrid orbitals specific to molecular geometry.

  • Electrons in hybrid orbitals allow accounting for observed bond angles.

Hybridization Types:

  1. sp: 2 hybrid orbitals (linear)

  2. sp2: 3 hybrid orbitals (trigonal planar)

  3. sp3: 4 hybrid orbitals (tetrahedral)

  4. sp3d: 5 hybrid orbitals (trigonal bipyramidal)

  5. sp3d2: 6 hybrid orbitals (octahedral)

Molecular Orbital Theory (MOT)

  • Molecular orbitals are formed from the combination of atomic orbitals:

    • Bonding Molecular Orbitals: Stabilizing, electron density between nuclei.

    • Antibonding Molecular Orbitals: Destabilizing, electron density outside between nuclei.

  • Bond order indicates stability:

    • Formula: Bond order=Number of electrons in bonding orbitals−Number of electrons in antibonding orbitals2\text{Bond order} = \frac{\text{Number of electrons in bonding orbitals} - \text{Number of electrons in antibonding orbitals}}{2}

  • Example: H2 bond order is 1 (stable), He2 bond order is 0 (unstable).

Comparison of Bonding Theories

  1. Lewis Theory: Simple visualization, but limited to 2D, fails with bond strength explanations.

  2. VSEPR Model: Predicts shapes well but doesn't explain bond formation.

  3. VBT: Good for overlaps but has limitations in describing certain properties.

  4. Hybridization: Explains bond angles but not all properties (e.g. magnetism).

  5. MOT: Accurately predicts magnetic properties but is complex to understand.

Conclusion

  • Understanding molecular structure involves integrating Lewis structures, VSEPR, Valence Bond Theory, and Molecular Orbital Theory. Each theory offers insights and limitations that complement our understanding of molecular geometry and bonding.