Why does electronegativity decrease down Group 7?

Atomic radius and shielding increase, so the nucleus attracts the bonding pair less strongly.

Which is more polar, H–Cl or H–I, and why?

H–Cl, because chlorine is more electronegative so the electronegativity difference with hydrogen is greater.

What are the physical states of Cl₂, Br₂ and I₂ at room temperature?

Cl₂ is a gas, Br₂ a liquid and I₂ a solid.

Why does boiling point increase from Cl₂ to I₂?

More electrons per molecule give stronger London forces, so more energy is needed to overcome them.

Are covalent bonds broken when bromine boils?

No. Only the London forces between Br₂ molecules are overcome.

What is the trend in oxidising ability down Group 7?

It decreases: Cl₂ Br₂ I₂.

Why is chlorine a better oxidising agent than iodine?

Its smaller atoms have less shielding, so the nucleus attracts an incoming electron more strongly.

Write the ionic equation for chlorine with bromide ions.

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

What is observed when chlorine water is added to potassium iodide with cyclohexane?

The solution turns brown and the cyclohexane layer is violet as iodine forms.

Does bromine react with aqueous chloride ions?

No, bromine is a weaker oxidising agent than chlorine.

Trends in properties of the halogensAQA A-Level Chemistry: Flashcards

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Define electronegativity.

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Define electronegativity.: The ability of an atom to attract the bonding pair of electrons in a covalent bond.
What is the trend in electronegativity down Group 7?: It decreases: F > Cl > Br > I.
Why does electronegativity decrease down Group 7?: Atomic radius and shielding increase, so the nucleus attracts the bonding pair less strongly.
Which is more polar, H–Cl or H–I, and why?: H–Cl, because chlorine is more electronegative so the electronegativity difference with hydrogen is greater.
What are the physical states of Cl₂, Br₂ and I₂ at room temperature?: Cl₂ is a gas, Br₂ a liquid and I₂ a solid.
Why does boiling point increase from Cl₂ to I₂?: More electrons per molecule give stronger London forces, so more energy is needed to overcome them.
Are covalent bonds broken when bromine boils?: No. Only the London forces between Br₂ molecules are overcome.
What is the trend in oxidising ability down Group 7?: It decreases: Cl₂ > Br₂ > I₂.
Why is chlorine a better oxidising agent than iodine?: Its smaller atoms have less shielding, so the nucleus attracts an incoming electron more strongly.
Write the ionic equation for chlorine with bromide ions.: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
What is observed when chlorine water is added to potassium iodide with cyclohexane?: The solution turns brown and the cyclohexane layer is violet as iodine forms.
Does bromine react with aqueous chloride ions?: No, bromine is a weaker oxidising agent than chlorine.
What colour is bromine in cyclohexane?: Orange.

Exam questions on Trends in properties of the halogens

A student is comparing the halogens using data from a textbook. The Pauling electronegativity values given are fluorine 4.0, chlorine 3.0, bromine 2.8 and iodine 2.5.
Hydrogen has a Pauling electronegativity of 2.2. State and explain which is the more polar bond, H–Cl or H–I.2 marks
Chlorine, bromine and iodine are a gas, a liquid and a solid respectively at room temperature. Their boiling points are approximately 239 K, 332 K and 457 K.
Explain why the boiling point of iodine is higher than that of chlorine.2 marks
A student investigates displacement reactions of the halogens in aqueous solution. In each test she adds a few drops of a halogen water to 1 cm³ of an aqueous potassium halide, adds 1 cm³ of cyclohexane, shakes the tube and allows the layers to separate.
Chlorine water is added to aqueous potassium bromide. State the colour of the cyclohexane layer, write an ionic equation for the reaction and identify the species that is oxidised.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).