Contents
1. Ionic Bonding
Electrostatic attraction between oppositely charged ions formed by transfer of electrons from a metal to a non-metal.
Metals lose electrons to form positive ions (cations). Non-metals gain electrons to form negative ions (anions). Both achieve a full outer shell (noble gas configuration).
Na (2,8,1) loses 1 electron → Na⁺ (2,8) — noble gas configuration of neon.
Cl (2,8,7) gains 1 electron → Cl⁻ (2,8,8) — noble gas configuration of argon.
Na⁺ and Cl⁻ attract each other → ionic bond.
Common ions to know
| Ion | Charge | Ion | Charge |
|---|---|---|---|
| Sodium Na⁺ | +1 | Chloride Cl⁻ | −1 |
| Calcium Ca²⁺ | +2 | Oxide O²⁻ | −2 |
| Aluminium Al³⁺ | +3 | Sulfate SO₄²⁻ | −2 |
| Ammonium NH₄⁺ | +1 | Nitrate NO₃⁻ | −1 |
| Iron(II) Fe²⁺ | +2 | Carbonate CO₃²⁻ | −2 |
| Iron(III) Fe³⁺ | +3 | Hydroxide OH⁻ | −1 |
2. Covalent Bonding
A shared pair of electrons between two non-metal atoms. Both nuclei are attracted to the shared electrons, holding the atoms together.
Each atom contributes one electron to the shared pair (single bond). Double bonds share two pairs; triple bonds share three pairs.
| Molecule | Formula | Bond type | Structure |
|---|---|---|---|
| Hydrogen | H₂ | Single (H−H) | Simple molecular |
| Water | H₂O | 2 × Single (O−H) | Simple molecular |
| Oxygen | O₂ | Double (O=O) | Simple molecular |
| Nitrogen | N₂ | Triple (N≡N) | Simple molecular |
| Carbon dioxide | CO₂ | 2 × Double (O=C=O) | Simple molecular |
| Diamond | C | Single (C−C) | Giant covalent |
| Silicon dioxide | SiO₂ | Single (Si−O) | Giant covalent |
3. Metallic Bonding
Electrostatic attraction between a lattice of positive metal ions and a sea of delocalised (free) electrons.
The delocalised electrons are free to move — this is why metals conduct electricity and heat. The regular arrangement of ions allows layers to slide — this is why metals are malleable and ductile.
The delocalised electrons carry charge when a potential difference is applied. This applies to solid and liquid metals. (Ionic compounds only conduct when molten or dissolved — when ions are free to move.)
4. Giant vs Simple Structures
| Structure type | Examples | Melting point | Electrical conductivity |
|---|---|---|---|
| Giant ionic lattice | NaCl, MgO, CaCl₂ | High (strong electrostatic forces) | Only when molten or dissolved |
| Simple covalent (molecular) | H₂O, CO₂, CH₄, I₂ | Low (weak intermolecular forces) | None (no ions or free electrons) |
| Giant covalent | Diamond, Silicon dioxide, Graphite | Very high (many strong covalent bonds) | None (diamond, SiO₂) / Yes (graphite) |
| Giant metallic | Fe, Cu, Al, Na | Generally high | Yes (solid and liquid) |
5. Linking Structure to Properties
Why diamond is hard but graphite is soft
Diamond: each carbon atom is bonded to 4 others in a 3D tetrahedral lattice — no weak points, no free electrons, very hard, non-conductor.
Graphite: each carbon atom is bonded to 3 others in flat hexagonal layers — layers are held together only by weak forces and can slide over each other (lubricant). One delocalised electron per carbon atom → good conductor.
Graphite is a giant covalent structure that conducts electricity. This is the only giant covalent structure that does. In exams, graphite = conductor is a common answer point.
- Ionic: metal + non-metal. Electrons transferred. Giant lattice. High m.p. Conducts when molten/dissolved.
- Covalent: non-metal + non-metal. Electrons shared. Simple molecules = low m.p. Giant covalent = very high m.p.
- Metallic: positive ions in sea of delocalised electrons. Good conductor. Malleable. High m.p.
- Ionic compounds do NOT conduct when solid (ions fixed). DO conduct when molten or in solution.
- Simple molecular covalent (e.g. H2O, CO2): weak intermolecular forces - low m.p./b.p. Strong covalent bonds WITHIN molecule.
- Diamond: every C bonded to 4 others in giant covalent lattice. Very hard, very high m.p., does not conduct.
6. Common Exam Traps
CO₂, H₂O, CH₄ all have covalent BONDS that are strong — but the forces BETWEEN molecules are weak. It is the intermolecular forces that determine melting point, not the covalent bonds. Students lose marks writing "CO₂ has a high melting point because it has strong covalent bonds."
In solid ionic compounds, ions are fixed in the lattice and cannot move — no conduction. They only conduct when molten or dissolved in water, where ions are free to move.
These are two completely different things. Covalent bonds hold atoms together within a molecule. Intermolecular forces hold molecules together. Boiling breaks intermolecular forces, not covalent bonds.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.