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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

11 Group 17

11.1 Physical properties of the Group 17 elements 11.2 The chemical properties of the halogen elements and the hydrogen halides 11.3 Some reactions of the halide ions 11.4 The reactions of chlorine

Physical Properties of the Group 17 Elements

Specification Reference Inorganic Chemistry, Group 17 11.1

Quick Notes

  • Chlorine (Cl2): pale green gas
  • Bromine (Br2): red-brown liquid
  • Iodine (I2): grey-black solid with purple vapour
    all at room temperature and pressure (RTP)
  • Volatility decreases down the group: Cl2 > Br2 > I2
    • Stronger instantaneous dipole–induced dipole forces between larger molecules
  • Bond strength of X–X bonds decreases from Cl2 to I2
    • Larger atoms give longer bond lengths

Full Notes

Colours and Volatility of Chlorine, Bromine, and Iodine

The Group 17 elements (halogens) are diatomic molecules (X2) that become darker and less volatile down the group.

CIE A-Level Chemistry diagram showing the volatility trend of halogens decreasing down the group.

Trend: Volatility decreases down the group due to stronger intermolecular forces between larger molecules.

Element Appearance at Room Temperature Volatility
Cl2 Pale green gas High
Br2 Red-brown liquid Medium
I2 Grey-black solid, purple vapour Low

Trend in Bond Strength of Halogen Molecules

The bond strength of the X–X single bonds decreases from Cl2 to I2.

Molecule Bond Enthalpy (kJ mol−1)
Cl–Cl +243
Br–Br +193
I–I +151

Why:
Atomic radius increases down the group and the bond length increases. CIE A-Level Chemistry diagram showing decreasing bond strength from Cl2 to I2 due to increasing atomic size.

Instantaneous Dipole–Induced Dipole Forces

The halogen molecules are non-polar, so the only intermolecular forces between molecules are London dispersion forces.

CIE A-Level Chemistry graph showing boiling point trends for halogens, increasing down the group due to stronger London dispersion forces.

London Dispersion Force (Instantaneous Dipole–Induced Dipole Forces) strength depends on the number of electrons and size and shape of molecules.

Trend Explanation:

As the size and electron cloud of the halogen molecule increases down the group:

For Example

Summary