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Group 7: Halogens

Trends in physical properties of halogens

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.

Trends in chemical properties of halogens

The reactivity of halogens (group 7) decreases as the elements get heavier. Halogen atoms each react to gain one electron.

Lighter halogens have atoms with smaller atomic radii. Electrons are more easily added to atoms with smaller radii.

In lighter halogen atoms, the valence shell (where an electron is added in a reaction) is closer to the positively charged nucleus. As a result, the nucleus is able to exert stronger attractive forces on the electrons.

Chlorine is more reactive than bromine, and bromine is more reactive than iodine. Chlorine gas can displace both bromide and iodide ions when passed through a solution.

$$\ce{2Cl2 {(g)} + 2KBr {(aq)} -> 2KCl {(aq)} + Br2 {(g)}}$$ $$\ce{Cl2 {(g)} + 2KI {(aq)} -> 2KCl {(aq)} + I2 {(g)}}$$

Bromine gas can only displace the iodide ions.

$$\ce{Br2 {(g)} + 2KI {(aq)} -> 2KBr {(aq)} + I2 {(g)}}$$

Iodine is less reactive than both bromine and chlorine and cannot displace either in solution.