Molecular Geometry, Resonance, and Chemical Structure: Study Notes for General Chemistry

Covalent Bond Properties

Key Properties of Covalent Bonds

Covalent bonds are fundamental to molecular structure and chemical reactivity. Their properties influence molecular geometry, polarity, and physical behavior.

• Directionality: Covalent bonds form specific angles, leading to defined molecular shapes.

• Bond Strength: The energy required to break a bond; stronger bonds are more stable.

• Bond Order: The number of shared electron pairs between two atoms (single, double, triple bonds).

• Bond Length: The distance between the nuclei of bonded atoms; higher bond order generally means shorter bond length.

• Polarity: Unequal sharing of electrons leads to polar bonds, affecting molecular properties.

Example: In H2O, the O-H bonds are polar due to oxygen's higher electronegativity.

Calculating Reaction Energy

The energy change in a chemical reaction can be estimated using bond energies:

• Sum the energies of bonds broken (reactants).

• Sum the energies of bonds formed (products).

• Calculate reaction energy: $\Delta H = \sum E_{\text{bonds broken}} - \sum E_{\text{bonds formed}}$

Bond Polarity and Dipole Moment

Bond polarity arises from differences in electronegativity. The bond dipole moment quantifies the separation of charge:

• $\mu = q \times r$

• Where $q$ is the charge and $r$ is the distance between charges.

Percent Ionic Character: Measures how much a bond resembles an ionic bond.

Electronegativity (EN)

Electronegativity is an atom's ability to attract electrons in a bond. The difference in EN between atoms determines bond polarity.

• Large EN difference: More polar bond.

• Small EN difference: Less polar or nonpolar bond.

Exceptions to the Octet Rule & Formal Charge

Some molecules do not follow the octet rule. Formal charge helps identify the most stable Lewis structure:

• $\text{Formal charge} = \text{valence electrons} - \text{nonbonding electrons} - \frac{1}{2} \times \text{bonding electrons}$

Resonance Structures

Some polyatomic species require multiple Lewis structures to represent electron distribution. These are called resonance structures.

• Resonance structures have the same atom connectivity but different electron arrangements.

• Non-equivalent resonance structures differ by the number of bonds of the same type.

• "Better" or "major" resonance structures contribute more to the actual electron distribution.

Example: Carbon monoxide (CO) has resonance structures with different formal charges and bond orders.

Chemical Structure and Properties

Relationship Between Structure and Properties

The chemical and physical properties of a substance depend on its composition and structure.

• Composition: Types and ratios of atoms present.

• Structure: Connectivity, electron distribution, and spatial arrangement of atoms.

Example: Diamond and graphite are both forms of carbon but have different structures and properties:

Allotrope	Structure	Properties
Diamond	3D network of covalent bonds	Very hard, electrical insulator
Graphite	Layers of planar hexagonal rings	Very soft, electrical conductor

Valence Shell Electron Pair Repulsion (VSEPR) Theory

Major Ideas of VSEPR Theory

VSEPR theory predicts molecular geometry based on electron group repulsions around a central atom.

• Electron groups (EG) include lone pairs and bonds to terminal atoms.

• Electron groups arrange themselves to minimize repulsion.

• Number of electron groups: $(\#\text{EG}) = (\#\text{lone pairs}) + (\#\text{bonds})$

Electron Group Geometries

Common electron group arrangements:

# Electron Groups	Geometry	Bond Angles
2	Linear	180°
3	Trigonal Planar	120°
4	Tetrahedral	109.5°
5	Trigonal Bipyramidal	90°, 120°
6	Octahedral	90°, 180°

Stereochemical Formulas

Stereochemical formulas use solid and hashed wedges to indicate the spatial arrangement of atoms:

• Solid line: In the plane of the drawing.

• Solid wedge: Above the plane.

• Hashed wedge: Below the plane.

Predicting Molecular Geometry

Steps to predict molecular geometry:

1. Draw the Lewis structure.

2. Count electron groups around the central atom.

3. Determine electron group geometry.

4. Place lone pairs and terminal atoms to maximize distances.

5. Identify the molecular geometry.

Example: CO2 is linear; SO2 is bent due to lone pairs.

Distortions and Exceptions

Ideal geometries can be distorted by lone pairs, multiple bonds, or different atom sizes:

• Lone pairs occupy more space than bonding pairs.

• Double/triple bonds occupy more space than single bonds.

• Smaller atoms occupy less space.

VSEPR theory may not accurately predict geometries for species with d- or f-block central atoms or for some heavy p-block elements.

Molecular Polarity

Defining Molecular Polarity

Molecular polarity is determined by the vector sum of bond dipole moments. It affects boiling point, solubility, and other physical properties.

• A molecule is polar if it has polar bonds and a non-zero molecular dipole moment.

• Geometry can cause bond dipoles to cancel (nonpolar) or reinforce (polar).

Example: Water (H2O) is polar due to its bent geometry and polar O-H bonds.

Predicting Molecular Polarity

1. Draw the Lewis structure.

2. Determine molecular geometry.

3. Identify polar bonds.

4. Find the vector sum of all bond dipole moments.

Dipole Moments of Selected Molecules

Molecule	Geometry	Dipole Moment (D)
HF	Linear	1.78
HCl	Linear	1.05
NH3	Trigonal Pyramidal	1.47
CH4	Tetrahedral	0
CHCl3	Tetrahedral	1.60

Limitations of Lewis and VSEPR Theories

Lewis Theory Limitations

Lewis theory does not account for paramagnetism or the correct structure of some molecules (e.g., O2).

• Fails to predict unpaired electrons in O2.

• Multiple resonance structures may be needed for polyatomic species.

VSEPR Theory Limitations

VSEPR theory is most accurate for s- and p-block elements. It may fail for species with d- or f-block central atoms or heavy p-block elements.

• Cannot predict geometry for some transition metal complexes.

• Exceptions occur for elements in periods 5 and higher.

Practice Questions

• Compare the molecular geometry of CO2 and SO2.

• Compare the molecular geometry of NF3 and IF3.

Additional info: These notes cover core concepts from General Chemistry, including molecular structure, resonance, VSEPR theory, and polarity, suitable for exam preparation.