2.5.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Trends of the Group 7 Elements — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 33 of 53 Unit quiz
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 4Physical Trends
The halogens are diatomic molecules whose properties change steadily down the group.
Appearance
- Appearance Fluorine is a pale yellow gas, chlorine a pale green gas, bromine a red-brown liquid and iodine a grey-black solid.
Colour
- Colour The colour becomes darker down the group, from pale yellow to purple-black vapour in the case of iodine.
Volatility
- Volatility Volatility falls down the group because the boiling points rise as the molecules get larger.
Boiling points
- Boiling points Approximate boiling points are -188 °C for F₂, -34 °C for Cl₂, 59 °C for Br₂ and 184 °C for I₂.
Explanation
- Explanation Molecules with more electrons form stronger London forces, so more energy is needed to separate them.
Astatine
- Astatine Astatine, below iodine, is radioactive and is expected to be darker and less volatile than iodine.
| Halogen | State at room temperature | Colour | Boiling point / °C |
|---|---|---|---|
| F₂ | Gas | Pale yellow | -188 |
| Cl₂ | Gas | Pale green | -34 |
| Br₂ | Liquid | Red-brown | 59 |
| I₂ | Solid | Grey-black | 184 |
Section 2 of 4Electronegativity and Oxidising Power
A halogen reacts by gaining one electron to form X⁻, so it acts as an oxidising agent.
1
Each element down the group has an extra electron shell, so the outer shell is further from the nucleus.
2
Shielding by inner electrons also increases, so the nucleus attracts an incoming electron less strongly.
3
Electronegativity therefore decreases from fluorine (about 4.0) to iodine (about 2.7).
4
Because gaining an electron becomes less favourable, oxidising power and reactivity decrease down the group.
5
Fluorine is the strongest oxidising agent and iodine the weakest of the four common halogens.
Section 3 of 4Reactions with Hydrogen
Reactions with Hydrogen
Fluorine
- Fluorine It reacts explosively with hydrogen even in cold, dark conditions: H₂(g) + F₂(g) → 2HF(g).
Chlorine
- Chlorine It reacts explosively with hydrogen in sunlight, forming HCl(g).
Bromine
- Bromine It reacts with hydrogen only on heating, so the reaction is much less vigorous.
Iodine
- Iodine It reacts slowly and incompletely with hydrogen on strong heating because the reaction is reversible.
Reactivity order
- Reactivity order Reactivity falls in the order F₂ > Cl₂ > Br₂ > I₂, which is the basis for displacement reactions.
Section 4 of 4Exam Checklist & Tips
2.5.1 – Key Points for the Exam
- Halogen An element in Group 7, existing as a diatomic molecule.
- Volatility
- How easily a substance turns into a gas;
- high volatility means a low boiling point.
- How easily a substance turns into a gas;
- Electronegativity The ability of an atom to attract the bonding electrons in a covalent bond.
- Oxidising agent A species that gains electrons and is reduced.
Common Mistakes
- Saying the halogens become more reactive down the group; reactivity decreases.
- Explaining the trend in boiling point by breaking covalent bonds within molecules.
- Forgetting that the halogens exist as diatomic molecules such as Cl₂.
- Describing iodine as a liquid at room temperature when it is a solid.
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