Objective: Understand 3D shapes of molecules and determine molecular polarity.
Method: Use the A B system
A: Central atom
B: Number of atoms bonded
E: Unshared pairs of electrons
Molecular Shapes
AB System
AB2 Example: Beryllium Chloride
Beryllium has 2 bonds, no lone pairs.
Shape: Linear, 180 degrees.
AB3 Example: Boron Trifluoride
Boron has 3 bonds, no lone pairs.
Shape: Trigonal Planar.
AB4 Example: CH4 (Methane)
Carbon has 4 bonds, no lone pairs.
Shape: Tetrahedral.
AB5 Example: Trigonal Bipyramidal
AB6 Example: Octahedral
Central atom (e.g., sulfur) shares all valence electrons, no lone pairs.
Electron Pair Geometry vs. Molecular Geometry
Electron Pair Geometry: Considers all electron pairs (bonded and lone pairs).
Molecular Geometry: Considers only bonded atoms.
Examples with Unshared Pairs:
AB2E: Bent instead of linear.
AB4E: Trigonal Pyramidal.
Exceptions to the Octet Rule
AB5 with Lone Pairs:
1 Lone Pair: Distorted Tetrahedron (Seesaw)
2 Lone Pairs: T-shaped
3 Lone Pairs: Linear
AB6 with Lone Pairs:
1 Lone Pair: Square Pyramidal
2 Lone Pairs: Square Planar
Examples
CBr4: Tetrahedral, no lone pairs.
SCl2: Bent due to 2 lone pairs.
ClO3: Trigonal Pyramidal.
PCl3: Trigonal Pyramidal.
Dipole Moments and Polar Molecules
Dipole Moment: Temporary electron-rich and electron-poor regions.
Polar Molecule: Has a dipole moment (e.g., HF, NH3).
Examples of Polarity
BrCl: Polar due to electronegativity difference.
BF3: Nonpolar, symmetrical electron distribution.
CH2Cl2: Polar, chlorine atoms create an uneven distribution.
Key Concepts
Non-polar Molecules: Same atoms around the central atom (e.g., BeCl2).
Polar Bonds but Non-polar Molecule: BF3 due to symmetrical shape.
Electronegativity: Fluorine is most electronegative.
Both molecules with polar bonds can have dipole moments, thus being polar.
Conclusion
Understanding the shape and electron distribution of molecules is crucial for determining polarity. Shape influences molecular geometry and how unshared electron pairs affect overall molecular shape and polarity.