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General Chemistry Study Guide: Molecular Geometry, Bonding Theories, and Gas Properties

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Chapter 9: Molecular Geometry and Bonding Theories

VSEPR Theory (Valence Shell Electron Pair Repulsion)

The VSEPR Theory explains the three-dimensional shapes of molecules by considering the repulsion between electron groups around a central atom. Electron groups arrange themselves to minimize repulsion and maximize their distance from each other.

  • Electron Groups include single bonds, double bonds, triple bonds, and lone pairs.

  • Multiple bonds (double/triple) count as one electron group in VSEPR.

Electron Geometry vs. Molecular Geometry:

Electron Groups

Electron Geometry

Bonding Groups

Lone Pairs

Molecular Shape

2

Linear

2

0

Linear

3

Trigonal planar

3

0

Trigonal planar

3

Trigonal planar

2

1

Bent

4

Tetrahedral

4

0

Tetrahedral

4

Tetrahedral

3

1

Trigonal pyramidal

4

Tetrahedral

2

2

Bent

5

Trigonal bipyramidal

5

0

Trigonal bipyramidal

6

Octahedral

6

0

Octahedral

Additional info: For more complex molecules, consult a comprehensive molecular geometry chart.

Molecular Shapes (Small Molecules: ≤ 7 Atoms)

To determine the shape of a molecule, follow these steps:

  1. Draw the Lewis structure.

  2. Count the electron groups around the central atom.

  3. Determine the electron domain geometry.

  4. Determine the molecular geometry (actual shape).

Common Shapes and Angles:

  • Linear: 180°

  • Trigonal planar: 120°

  • Tetrahedral: 109.5°

  • Trigonal pyramidal: ~107°

  • Bent: ~104.5°

  • Trigonal bipyramidal: 90°, 120°

  • Octahedral: 90°

Practice Molecules: CO2, BF3, CH4, NH3, H2O, SO2, PCl5, SF6

Molecular Shape and Polarity

Bond Polarity is determined by the difference in electronegativity (ΔEN) between atoms:

  • Small ΔEN (< 0.5): Nonpolar covalent

  • Moderate ΔEN: Polar covalent

  • Large ΔEN: Ionic character

Molecular Polarity depends on:

  1. Presence of polar bonds

  2. Whether dipoles cancel due to molecular geometry

Symmetry Rule: If identical atoms surround the central atom symmetrically, the molecule is usually nonpolar.

Examples:

  • Nonpolar: CO2 (linear), BF3 (trigonal planar), CH4 (tetrahedral)

  • Polar: H2O (bent), NH3 (trigonal pyramidal), SO2 (bent)

Additional info: Review symmetric/asymmetric geometries for predicting polarity.

Valence Bond Theory (Orbital Overlap)

Valence Bond Theory describes covalent bond formation as the overlap of atomic orbitals. The strength of a bond depends on the amount of overlap and the distance between nuclei.

  • Types of Overlap: s–s, s–p, p–p

Hybrid Orbitals

Hybridization explains observed molecular geometries by combining atomic orbitals into new hybrid orbitals.

Electron Groups

Hybridization

Geometry

2

sp

Linear

3

sp2

Trigonal planar

4

sp3

Tetrahedral

How to Determine Hybridization: Count the electron groups around the central atom.

Examples:

  • CH4, NH3, H2O: 4 groups → sp3

  • CO2: 2 groups → sp

  • BF3: 3 groups → sp2

Multiple Bonds and Resonance

Sigma (σ) and Pi (π) Bonds:

  • Single bond: 1 σ

  • Double bond: 1 σ + 1 π

  • Triple bond: 1 σ + 2 π

σ bonds result from head-to-head overlap; π bonds from side-to-side overlap.

Resonance: Occurs when multiple valid Lewis structures exist for a molecule, differing only in electron placement. The true structure is a resonance hybrid.

  • Resonance equalizes bond lengths, delocalizes charge, and increases stability.

  • Examples: O3, NO3-, SO2

Chapter 10: Basic Properties of Gases

Properties of Gases

Gases are characterized by their lack of fixed shape or volume, high compressibility, low density, and ability to mix uniformly.

  • No fixed shape or volume; assume the volume of their container.

  • Highly compressible and expandable.

  • Low density compared to solids and liquids.

  • Mix uniformly with other gases.

Pressure

Pressure is defined as the force exerted per unit area.

  • Formula:

  • Common units: atm, mmHg (Torr), kPa, Pa

Atmospheric Pressure:

  • Standard atmospheric pressure: 1 atm = 760 mmHg = 760 Torr = 101.325 kPa = 101,325 Pa

Pressure Conversions

Pressure units can be converted using dimensional analysis.

  • Example: Convert 0.85 atm to mmHg:

Key Skills to Practice

  • Drawing accurate Lewis structures

  • Determining molecular shape from VSEPR

  • Predicting polarity from shape

  • Identifying hybridization

  • Counting σ and π bonds

  • Recognizing resonance structures

  • Performing pressure unit conversions

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