1.4.3Physical Chemistry
Covalent Bonding — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 19 of 102
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.
Part 1 of 4The Shared Pair
Covalent bond
- Covalent bond A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of the two bonded atoms.
Non-metals
- Non-metals Covalent bonding usually occurs between non-metal atoms, which share electrons so each can achieve a stable outer shell.
Single bond
- Single bond One shared pair, as in H–H or H–Cl, gives the longest and weakest bond between a given pair of atoms.
Double bond
- Double bond Two shared pairs, as in O=O and C=C, make a bond that is shorter and stronger than the corresponding single bond.
Triple bond
- Triple bond Three shared pairs, as in N≡N, give a very short and strong bond, which explains the low reactivity of nitrogen gas.
Part 2 of 4Types of Covalent Structure
Types of Covalent Structure
Simple molecular
- Simple molecular Small separate molecules such as H₂O, CO₂ and I₂ are held in the solid by weak intermolecular forces, giving low melting points.
Giant covalent
- Giant covalent Diamond, graphite and silicon dioxide have atoms joined throughout by covalent bonds in a continuous lattice,
- giving very high melting points.
Diamond
- Diamond Each carbon forms four covalent bonds in a tetrahedral arrangement, making diamond extremely hard.
Graphite
- Graphite Each carbon bonds to three others in layers, leaving delocalised electrons that allow graphite to conduct electricity.
Part 3 of 4Bond Strength Trends
1
Bond length is the distance between the nuclei of two bonded atoms, and it falls as the number of shared pairs increases.
2
A shorter bond brings the nuclei closer to the shared electrons, so the attraction and therefore bond strength increase.
3
Breaking a covalent bond requires energy, which is measured as a bond enthalpy in kJ mol⁻¹.
4
For example, the strength order is C–C less than C=C less than C≡C, with bond length decreasing in the same order.
5
Strong covalent bonds inside molecules do not affect the melting point of simple molecular substances, since only intermolecular forces are overcome.
| Bond | Pairs shared | Example |
|---|---|---|
| Single | 1 | Cl–Cl |
| Double | 2 | O=C=O |
| Triple | 3 | N≡N |
Part 4 of 4Exam Checklist & Tips
1.4.3 – Key Points for the Exam
- Covalent bond A shared pair of electrons attracting the nuclei of two atoms.
- Bond length The distance between the nuclei of two covalently bonded atoms.
- Simple molecular A structure made of discrete small molecules with weak forces between them.
- Giant covalent lattice A network of atoms joined by covalent bonds throughout the whole structure.
Common Mistakes
- Believing that boiling a molecular liquid breaks the covalent bonds;
- only the weak intermolecular forces are overcome.
- Saying graphite has no free electrons; each carbon has one delocalised electron between the layers.
- Stating that double bonds are twice as strong as single bonds; they are stronger but not exactly double.
- Treating all covalent substances as low-melting; giant covalent lattices melt at very high temperatures.
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