2.3.1Physical Chemistry
Physical Properties of Group 7 — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 60 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.
Section 1 of 4Appearance and State
Diatomic molecules
- Diatomic molecules Each halogen exists as a pair of atoms joined by a single covalent bond, written X₂.
Colour trend
- Colour trend The colour becomes darker down the group, from pale yellow to grey-black.
State trend
- State trend The state changes from gas to liquid to solid as the group is descended at room temperature.
Group number
- Group number All have seven outer electrons, so they need one more electron to complete the outer shell.
| Halogen | Colour and state at room temperature | Boiling point (°C) |
|---|---|---|
| Fluorine, F₂ | Pale yellow gas | −188 |
| Chlorine, Cl₂ | Pale green gas | −34 |
| Bromine, Br₂ | Red-brown liquid | 59 |
| Iodine, I₂ | Grey-black solid, purple vapour | 184 |
Section 2 of 4Volatility Trend
Volatility is how easily a substance evaporates, and it decreases down Group 7.
1
Halogen molecules are simple molecular and held together in the solid or liquid by London forces.
2
Going down the group, molecules have more electrons, so the temporary dipoles are larger.
3
Larger induced dipoles mean stronger London forces between molecules.
4
More energy is therefore needed to separate molecules, so melting and boiling points increase down the group.
5
Fluorine is the most volatile halogen and iodine is the least volatile.
Section 3 of 4Bond Strength and Reactivity Link
Bond enthalpy of X–X bonds in kJ mol⁻¹: F–F 158, Cl–Cl 242, Br–Br 193, I–I 151.
Bond Strength and Reactivity Link
Chlorine to iodine
- Chlorine to iodine Bond strength decreases from Cl₂ to I₂ because the bonding pair is further from both nuclei and more shielded.
Fluorine anomaly
- Fluorine anomaly The F–F bond is weaker than Cl–Cl because the small atoms bring non-bonding lone pairs close together, causing repulsion.
Electronegativity
- Electronegativity The ability to attract a bonding pair decreases down the group as atoms get larger.
Why it matters
- Why it matters Bond strength is one factor affecting reactivity, but the oxidising power of the halogen is also related to electron-gaining ability.
Section 4 of 4Exam Checklist & Tips
2.3.1 – Key Points for the Exam
- Halogen An element in Group 7, existing as diatomic molecules and having seven outer-shell electrons.
- Volatility
- How readily a substance evaporates;
- volatile substances have low boiling points.
- How readily a substance evaporates;
- London forces Weak intermolecular forces caused by temporary and induced dipoles.
- Electronegativity The ability of an atom to attract the bonding pair of electrons in a covalent bond.
Common Mistakes
- Saying covalent bonds in the halogen molecules break when halogens boil; only London forces are overcome.
- Stating that bond strength increases down the group; it decreases from chlorine to iodine.
- Forgetting that fluorine is an exception to the bond strength trend.
- Describing boiling points in terms of "bigger atoms" without mentioning electron number and London forces.
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