CH 301 Unit 2 Exam 2 Review Notes

1. Bonding

• Bond Polarity:

  • Proportional to the length of the dipole between two atoms.

  • Dipole is directly proportional to ∆EN.

  • Calculate ∆EN from H: 2.1 Li → F (increase 0.5 from 1.0 to 4.0).

  • Given bonds, find ∆EN for each and rank by increasing ∆EN.

• Important: Bond polarity is NOT molecular polarity (e.g., CF4 is nonpolar despite having polar bonds).

2. Lewis Structures

A. Formal Charge

• Key skills:

  • Draw 3-D structures of covalent compounds.

  • Count valence electrons, arrange atoms, place electrons around perimeter.

  • If the central atom lacks 8 electrons, make multiple bonds.

• Practice Problem: Central atom without unpaired electrons - CH4 correct.

B. Drawing Steps

1. Count valence electrons.

2. Arrange perimeter atoms around the central atom.

3. Place electrons around perimeter atoms.

4. Distribute unused valence electrons to central atoms.

C. Example

• Lewis structures for compounds like CHCl3.

• Formal Charge:

  • Formula: Valence e- - (2 * unbonded e-) + (1 * bonded e-).

  • Example: Formal charge on N in NH3 is 0.

3. Multiple Central Atoms

• Organic molecules often have multiple central atoms.

• Use same rules from previous section to construct structures.

• Practice Problem: Lewis structure for ethylene (C2H4) has 2 bonds between carbons.

4. Resonance

• When electron sharing occurs due to insufficient electrons, it causes resonance.

• Identifying resonance in structures like O3, NO3-, CO32-.

• Example: Delocalized π bonds in multiple structures.

5. VSEPR Theory

A. Bond Angles

• Bond angles are affected by polarity (distortion in angles).

• Key Information:

  • 2 e-rich regions: 180°

  • 3 e-rich regions: 120°

  • 4 e-rich regions: 109.5°.

B. Electronic Geometry

• Count electron-rich regions for determining geometry.

• Practice Problem: SF4 has different electronic vs molecular geometry.

6. Molecular Geometries

• Practice Problem: Molecular shape of IF4- is square planar.

• Recognize different molecular geometries from drawings.

7. Molecular Polarity

• Determine if molecules are polar by checking symmetry.

• Important: Non-symmetrical = polar; symmetrical = nonpolar.

8. Hybrid Orbit Theory

• Key Facts:

  • Hybrid orbitals are degenerate.

  • Understand how carbon’s orbitals hybridize in CH4.

9. Molecular Orbitals (MO)

A. Bond Order Calculation

• Bond order affects bond length and energy.

• Key Calculation: B.O. = (bonding e- - antibonding e-) / 2.

• Example: Rank bond length from various species.

B. Magnetism

• Identify paramagnetic species based on unpaired electrons.

• Example: O2 is paramagnetic; Li2- is not.

Summary for Exam Preparation

• Focus on understanding bonding concepts, Lewis structures, and VSEPR theory.

• Practice problems for calculating ∆EN, determining formal charges, bond angles, and recognizing molecular shapes.

• Memorize bond angles, hybridizations, and molecular geometries.