Chlorine is a pale gas, bromine a heavy red-brown liquid, iodine a dark solid that turns to purple vapour. One trend in intermolecular forces explains the change, and the bond energies tell a parallel story.
The halogens exist as diatomic molecules, X2. Down the group the molecules have more electrons, so the instantaneous dipole-induced dipole forces between them get stronger and the boiling points rise: volatility decreases from chlorine (a gas) to bromine (a liquid) to iodine (a solid). The colours deepen too. Within each molecule, the X–X bond energy decreases from Cl–Cl to I–I as the atoms get larger and the bonding pair is further from the nuclei; F–F is anomalously weak because the small atoms' lone pairs repel each other.
Fig. 1: boiling points and appearance of the halogens
Section 01
Colour and volatility.
Compare the three halogens in the simulator at room temperature, then warm each one and watch for the vapour.
Halogen
State at 20 °C
Colour
Vapour
b.p. / °C
Cl2
gas
pale yellow-green
yellow-green
−34
Br2
liquid
red-brown
orange-brown
59
I2
solid
grey-black, shiny
purple
184
Why volatility falls: more electrons per molecule (34, 70, 106) give larger temporary dipoles and stronger id-id forces.
Bond energies / kJ mol−1: Cl–Cl 242, Br–Br 193, I–I 151. Larger atoms mean longer, weaker bonds.
F–F (158): weaker than expected because the very short bond brings the lone pairs on the two small atoms close together, and they repel.
Examiner trap
Boiling a halogen breaks intermolecular forces, not the X–X covalent bond. The rise in boiling point and the fall in bond energy are separate trends with separate explanations: one is about forces between molecules, the other about the bond inside each molecule.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
What is the colour and state of bromine at room temperature?
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Why do the boiling points of the halogens increase down the group?
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What colour is iodine vapour?
Quick check+10 XP
Which halogen bond is the weakest of Cl–Cl, Br–Br and I–I?
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Why is the F–F bond weaker than the Cl–Cl bond?
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Which statement explains the difference in volatility between chlorine and iodine?
Exam style+20 XP
Astatine is below iodine in Group 17. Which prediction is most likely?
Exam style+20 XP
Which correctly orders the bond energies?
Skill unlocked
Halogen properties, mastered.
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