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Trends in physical properties of halogens

Chlorine (left), bromine (centre) and iodine (right) are the first few members of group 7.
Chlorine (left), bromine (centre) and iodine (right) are the first few members of group 7.

Halogens are coloured non-metals with seven valence electrons in group 7. Halogens tend to exist as diatomic molecules (e.g. chlorine exists as $$\ce{Cl2}$$).

The colour intensity of these elements increases with increasing atomic mass. The physical appearances of the elements are listed below.

  • Chlorine ($$\ce{Cl2}$$): Yellow-green gas
  • Bromine ($$\ce{Br2}$$): Red-brown liquid
  • Iodine ($$\ce{I2}$$): Blue-black solid (that can sublimate to form a purple vapour)

The melting point increases from chlorine to iodine. This is due to increasing strength of intermolecular forces as we go down the group.

The heavier halogens have stronger intermolecular forces than the lighter ones as molecules of halogens get larger.

When molecule size increases, there is more surface area for contact with neighbouring molecules. This results in greater extent of intermolecular forces of attraction.