Why Atoms Form Bonds
Atoms bond to achieve a stable electron configuration — typically a full outer shell of 8 electrons (octet rule). Noble gases already have filled shells and rarely bond. All other elements bond by losing, gaining, or sharing valence electrons.
Comparison of Bond Types
Key differences between ionic, covalent, and metallic bonding.
| Property | Ionic | Covalent | Metallic |
|---|---|---|---|
| Formed between | Metal + non-metal | Non-metal + non-metal | Metal + metal |
| Electron transfer | Transferred | Shared | Delocalised sea of electrons |
| Structure | Giant ionic lattice | Discrete molecules or giant | Giant metallic lattice |
| Melting point | High | Low (molecular) / High (giant) | Variable (generally high) |
| Conductivity | Yes (molten/dissolved) | Generally no | Yes (solid and liquid) |
| Example | NaCl, MgO | H₂O, CO₂, diamond | Cu, Fe, Al |
VSEPR Theory and Molecular Shapes
The Valence Shell Electron Pair Repulsion (VSEPR) theory states that electron pairs around an atom arrange themselves to minimise repulsion. Lone pairs repel more than bonding pairs. This gives molecules their characteristic shapes: linear (CO₂), bent (H₂O), trigonal pyramidal (NH₃), tetrahedral (CH₄).
Common Molecular Shapes
Shapes predicted by VSEPR theory for common molecules.
| Molecule | Bonding pairs | Lone pairs | Shape | Bond angle |
|---|---|---|---|---|
| BeCl₂ | 2 | 0 | Linear | 180° |
| BF₃ | 3 | 0 | Trigonal planar | 120° |
| CH₄ | 4 | 0 | Tetrahedral | 109.5° |
| NH₃ | 3 | 1 | Trigonal pyramidal | 107° |
| H₂O | 2 | 2 | Bent | 104.5° |
Electronegativity and bond polarity
Exam tip: Polarity
A molecule can have polar bonds but still be non-polar overall if the dipoles cancel (e.g., CO₂ is linear and symmetrical — net dipole = 0). H₂O is bent, so dipoles do NOT cancel — it is polar. This distinction is frequently tested.
Biochemistry Bridge
Water's polarity (bent shape, δ⁻ O and δ⁺ H) allows it to form hydrogen bonds — explaining its unusually high boiling point, surface tension, and ability to dissolve ionic and polar biomolecules. All aqueous biochemistry depends on this.
InstaTest
InstaTest: Chemical Bonding
4-question MCQ sprint on bond types, VSEPR shapes, and polarity.