2.2.8Additional lessons
Giant Covalent Structures — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 32 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Network Solids
Some covalent substances form continuous lattices rather than small molecules.
Definition
- Definition A giant covalent structure is a huge network of atoms joined by covalent bonds in every direction.
No molecules
- No molecules The formula gives only the ratio of atoms, not a separate molecule.
Strong bonds
- Strong bonds Many strong covalent bonds must be broken to melt, so melting points are very high.
Insolubility
- Insolubility They do not dissolve in common solvents because the bonds are stronger than any attraction to solvent.
Section 2 of 5Carbon Allotropes
Carbon Allotropes
Diamond
- Diamond Each carbon forms four bonds in a tetrahedral network, making it extremely hard and a non-conductor.
Graphite
- Graphite Each carbon bonds to three others in flat layers, leaving one delocalised electron per atom.
Graphene
- Graphene A single layer of graphite, one atom thick, which is strong and conducts electricity.
Fullerene C₆₀
- Fullerene C₆₀ This is a simple molecular substance of 60 atoms, so it is soft and has a low melting point compared with diamond.
| Property | Diamond | Graphite |
|---|---|---|
| Hardness | Very hard | Soft, layers slide |
| Conductivity | None | Conducts along layers |
| Bonds per atom | 4 | 3 |
Section 3 of 5Explaining Graphite
1
Within each layer, carbon atoms form strong covalent bonds in a hexagonal pattern.
2
The fourth electron on each atom is delocalised across the layer, so graphite conducts electricity.
3
Between layers there are only weak London dispersion forces.
4
The layers slide over each other easily, so graphite is soft and acts as a lubricant.
5
It still has a very high melting point because breaking the covalent bonds within layers is hard.
Section 4 of 5Silicon and Silicon Dioxide
Silicon
- Silicon It has a diamond-like structure of Si–Si bonds and a melting point near 1410 °C.
Semiconductor
- Semiconductor Silicon conducts only slightly, which is why it is used in electronic devices.
Silicon dioxide
- Silicon dioxide Each silicon bonds to four oxygen atoms and each oxygen to two silicon atoms, so the empirical formula is SiO₂.
Properties of SiO₂
- Properties of SiO₂ It is hard, has a very high melting point near 1700 °C and does not conduct electricity.
Section 5 of 5Exam Checklist & Tips
2.2.8 – Key Points for the Exam
- Giant covalent structure A continuous network of atoms linked by covalent bonds.
- Allotropes Different structural forms of the same element.
- Delocalised electron An electron that is not tied to one atom and can move through a structure.
- Graphene A single sheet of carbon atoms in a hexagonal arrangement.
Common Mistakes
- Saying graphite is soft because its covalent bonds are weak; the weak forces are between layers.
- Stating that diamond conducts electricity.
- Calling silicon dioxide a molecule of SiO₂.
- Confusing C₆₀ with giant covalent structures.
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