# Chemical Bonding and Structure (IB)
Chemical bonding explains how atoms combine to form compounds. The IB covers ionic, covalent, and metallic bonding; Lewis structures; VSEPR for molecular geometry; and intermolecular forces including hydrogen bonding.
1. Types of Bonding
| Type | Description | Between | Properties |
|---|---|---|---|
| Ionic | Electron transfer | Metal + non-metal | High MP, conducts when molten/dissolved |
| Covalent | Electron sharing | Non-metal + non-metal | Can be simple molecular or giant |
| Metallic | Delocalised electrons | Metals | Conducts, malleable, high MP |
2. Lewis Structures
Steps to draw Lewis structures:
- Count total valence electrons
- Draw skeleton (central atom bonded to outer atoms)
- Complete octets on outer atoms
- Place remaining electrons on central atom
- Form double/triple bonds if central atom needs more electrons
Exceptions to the Octet
- Expanded octet: Period 3+ elements can exceed 8 (e.g. PCl₅, SF₆)
- Incomplete octet: BF₃ (only 6 electrons around B)
Resonance
Some molecules have equivalent Lewis structures (e.g. O₃, CO₃²⁻, NO₃⁻). The real structure is a hybrid — electrons are delocalised.
Formal Charge
where V = valence electrons, N = non-bonding electrons, B = bonding electrons
The best Lewis structure minimises formal charges.
3. VSEPR and Molecular Geometry
| Electron Domains | Bonding | Lone Pairs | Shape | Angle |
|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° |
| 3 | 3 | 0 | Trigonal planar | 120° |
| 3 | 2 | 1 | Bent | ~117° |
| 4 | 4 | 0 | Tetrahedral | 109.5° |
| 4 | 3 | 1 | Trigonal pyramidal | ~107° |
| 4 | 2 | 2 | Bent | ~104.5° |
| 5 | 5 | 0 | Trigonal bipyramidal | 90°, 120° |
| 6 | 6 | 0 | Octahedral | 90° |
Lone pairs repel more than bonding pairs → compressed bond angles.
4. Polarity
Bond polarity: unequal sharing due to electronegativity difference → /
Molecular polarity: depends on bond polarity AND molecular geometry
- Symmetrical molecules (CO₂, CCl₄): non-polar (dipoles cancel)
- Asymmetrical molecules (H₂O, NH₃, CHCl₃): polar
5. Intermolecular Forces
| Force | Strength | Occurs In |
|---|---|---|
| London dispersion | Weakest | All molecules |
| Dipole-dipole | Medium | Polar molecules |
| Hydrogen bonding | Strongest IMF | H bonded to F, O, or N |
Hydrogen bonding explains:
- High BP of water
- Ice less dense than liquid water
- Solubility of ethanol in water
6. Hybridisation (HL)
| Hybridisation | Geometry | Example |
|---|---|---|
| sp | Linear | BeCl₂, CO₂ |
| sp² | Trigonal planar | BF₃, C₂H₄ |
| sp³ | Tetrahedral | CH₄, NH₃, H₂O |
7. Practice Questions
- Draw Lewis structures for SO₂, PCl₅, and NO₃⁻.
- Predict the shape and bond angle of SF₆.
- Explain why BF₃ is non-polar but NF₃ is polar.
- Compare the boiling points of CH₄, NH₃, and H₂O.
- Determine the hybridisation of carbon in CO₂ and CH₄.
Want to check your answers and get step-by-step solutions?
Summary
- Lewis structures show valence electrons and bonding
- VSEPR predicts geometry from electron pair repulsion
- Polarity depends on bond polarity + molecular geometry
- IMFs: London < dipole-dipole < hydrogen bonding
- HL: hybridisation (sp, sp², sp³) determines geometry