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Molecular Shapes

VSEPR Theory

• VSEPR Theory: Stands for Valence Shell Electron Pair Repulsion Theory.

Influence of Electron Pairs

• All electron pairs around the central atom affect the shape of the molecule.

Factors Determining Shape

• The shape of molecules depends on:

  • The number of shared pairs of electrons.

  • The number of unshared pairs of electrons around the central atom.

Dot Diagram for Water

• Draw the dot diagram for water (H2O) and label the shared and unshared electron pairs.

Identifying the Central Atom

• The central atom is determined by the element with the least number of atoms in the compound, except when it contains carbon—carbon is ALWAYS the central atom.

Valence Electrons Calculation

• Example for H2O:

  • Total valence electrons = 2(H) + 1(O)

  • 2(1) + 1(6) = 8 valence electrons.

• Identify unshared and shared pairs.

Importance of Unshared Pairs

• Unshared pairs take up more room than shared pairs and are important in predicting the shape of the molecule.

Determining Molecular Shape

• The shape of a molecule is determined by drawing lines connecting the center of the atoms in the molecule.

Shapes to Learn

• There are 6 molecular shapes that you need to learn:

1. Linear

• 2-atom linear

2. Trigonal Planar

• 3-atom linear example: Carbon Dioxide (CO2)

  • 180°

  • Linear Electron Pair Geometry

  • No unshared pairs of electrons on the central atom.

3. Trigonal Planar

• 120° Planar shape.

4. Trigonal Pyramidal

• Contains an unshared pair of electrons.

  • Angle: 107.3°

5. Tetrahedral

• Angle: 109.5°.

6. Bent

• Contains 2 unshared pairs of electrons.

  • Angle: <109.5°.