Trends in properties of the halogensAQA A-Level Chemistry: Revision notes
Section 1
Electronegativity down Group 7
Electronegativity is the ability of an atom to attract the bonding pair of electrons in a covalent bond. In Group 7 it decreases down the group: fluorine (4.0) > chlorine (3.0) > bromine (2.8) > iodine (2.5) on the Pauling scale.
Explanation: going down the group the atomic radius increases and the number of inner shells (shielding) increases. The nuclear charge also increases, but the extra shielding and distance outweigh it. The bonding pair is therefore further from the nucleus and attracted less strongly.
A larger electronegativity difference between the two atoms in a bond gives a more polar bond, so H–Cl is more polar than H–I.
Do not say electronegativity falls because there are fewer protons. The nuclear charge increases down the group; it is distance and shielding that win.
Section 2
Boiling points of chlorine, bromine and iodine
Halogens exist as diatomic simple molecules (Cl₂, Br₂, I₂). In the solid and liquid states the molecules are held together by London (induced dipole-dipole) forces.
Boiling point increases down the group: Cl₂ is a gas (about 239 K), Br₂ a liquid (about 332 K) and I₂ a solid (boils at about 457 K).
Explanation: going down the group each molecule has more electrons, so larger temporary dipoles form and the London forces between molecules are stronger. More energy is needed to overcome them.
When a halogen boils or melts only the intermolecular forces are overcome. The covalent X–X bonds are not broken.
Never write that covalent bonds break when a halogen boils. Boiling Br₂ gives Br₂ molecules in the gas, not Br atoms.
Section 3
Oxidising ability of the halogens
An oxidising agent gains electrons (and is itself reduced). Halogen atoms gain one electron to form halide ions: X₂ + 2e⁻ → 2X⁻.
Oxidising ability decreases down the group: Cl₂ > Br₂ > I₂.
Explanation: going down the group the atoms are larger and have more shielding, so the nucleus attracts an incoming electron less strongly and it is less easily gained.
The same trend is seen in reverse for halide ions, which become better reducing agents down the group (I⁻ > Br⁻ > Cl⁻).
Section 4
Displacement reactions of halide ions
A more powerful oxidising halogen oxidises the halide ions of a less powerful one, displacing that halogen from solution.
- Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ (orange solution)
- Cl₂ + 2I⁻ → 2Cl⁻ + I₂ (brown solution)
- Br₂ + 2I⁻ → 2Br⁻ + I₂ (brown solution)
- No reaction between Br₂ and Cl⁻, or I₂ and Br⁻ or Cl⁻.
Adding cyclohexane and shaking makes the colours clearer: the halogen dissolves in the upper organic layer, which is orange for bromine and violet for iodine.
In each reaction the halide ion is oxidised and the halogen reduced.
Colours in cyclohexane: chlorine pale green or colourless, bromine orange, iodine violet. In water bromine is orange and iodine brown.
Must Know
- Electronegativity decreases down Group 7 because atomic radius and shielding increase
- Boiling point rises Cl₂ < Br₂ < I₂ because London forces strengthen with more electrons; covalent bonds are not broken
- Oxidising ability falls Cl₂ > Br₂ > I₂
- A halogen displaces any halide below it in the group
- Cyclohexane layer: orange for Br₂, violet for I₂
That's the notes covered.
Carry on to the next subtopic.
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
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).