2.1.4Physical Chemistry
Trends of Period 3 Elements: Melting Point — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 55 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 4Structure of the Elements
Melting point depends on the type of structure and the strength of the forces holding particles together.
Metallic: Na, Mg, Al
- Metallic: Na, Mg, Al Positive ions are held in a lattice by attraction to delocalised electrons, and this attraction is strong.
Giant covalent: Si
- Giant covalent: Si Silicon atoms are joined by strong covalent bonds in a three-dimensional network, so very high energy is needed to melt it.
Simple molecular: P₄, S₈, Cl₂
- Simple molecular: P₄, S₈, Cl₂ These exist as small separate molecules held together by weak London forces between molecules.
Monatomic: Ar
- Monatomic: Ar Argon exists as single atoms with very weak London forces, so it has the lowest melting point in the period.
Part 2 of 4Comparing the Metals
Comparing the Metals
Rising metal charge
- Rising metal charge From Na⁺ to Al³⁺ the ion charge increases and the ion radius decreases, strengthening the metallic bonding.
More delocalised electrons
- More delocalised electrons Sodium contributes one electron per atom, magnesium two and aluminium three, so the electron sea becomes denser.
Result
- Result Melting point rises from sodium to aluminium because more energy is needed to overcome the stronger metallic bonds.
| Element | Approx. melting point (°C) | Bonding |
|---|---|---|
| Na | 98 | Metallic, 1+ ion |
| Mg | 650 | Metallic, 2+ ion |
| Al | 660 | Metallic, 3+ ion |
| Si | 1414 | Giant covalent |
| P₄ | 44 | Simple molecular |
| Cl₂ | −101 | Simple molecular |
| Ar | −189 | Monatomic |
Part 3 of 4The Non-Metals
After silicon, melting points drop sharply because covalent bonds within molecules are not broken on melting.
1
Silicon has the highest melting point because every atom is covalently bonded to four others throughout the giant lattice.
2
Melting phosphorus, sulfur or chlorine only requires overcoming London forces between molecules, not the covalent bonds inside them.
3
London forces get stronger as the number of electrons in a molecule increases, so larger molecules need more energy to separate.
4
Because S₈ is larger than P₄, which is larger than Cl₂, sulfur has a higher melting point than phosphorus, which is higher than chlorine.
5
Argon has the smallest, single-atom particles and therefore the weakest London forces and the lowest melting point.
Part 4 of 4Exam Checklist & Tips
2.1.4 – Key Points for the Exam
- Melting point The temperature at which a solid changes into a liquid, reflecting the strength of the forces holding particles together.
- Metallic bonding The electrostatic attraction between positive metal ions and delocalised electrons.
- Giant covalent structure A lattice of atoms joined throughout by strong covalent bonds.
- London forces Weak attractions between molecules caused by temporary and induced dipoles.
Common Mistakes
- Saying covalent bonds break when simple molecular substances melt; only intermolecular forces are overcome.
- Stating that silicon has a high melting point because of strong ionic bonds.
- Ignoring the number of delocalised electrons when comparing Na, Mg and Al.
- Forgetting that phosphorus is quoted as P₄ and sulfur as S₈ when comparing molecule sizes.
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