Group 7 - physical properties

Group 7 contains non-metal elements placed in a vertical column on the right of the periodic table. The elements in group 7 are called the halogens.

Periodic table with Group 7 in red.Group 7 is on the right-hand side of the periodic table, next to group 0

The halogens show trends in their physical and chemical properties.

Physical properties

The halogens exist as simple molecules. Each molecule is made up of a pair of halogen atoms joined by a single covalent bond. In all groups of the periodic table, the further down the group an element is, the higher its relative molecular mass.

The table shows the colour and physical states of chlorine, bromine and iodine at room temperature.

Chlorine, Cl2Pale greenGas
Bromine, Br2BrownLiquid
Iodine, I2Purple-blackSolid

In group 7, the further down the group an element is, the higher its melting point and boiling point. This is because, going down group 7:

  • the molecules become larger
  • the intermolecular forces become stronger
  • more energy is needed to overcome these forces

The graph shows the melting and boiling points of the first four group 7 elements. Astatine is placed below iodine in group 7. Predict the melting and boiling points of astatine, and its state at room temperature.

Bar chart showing how the melting and boiling points of the halogens increase going down group 7

Astatine should have a melting point of about 300°C and a boiling point of about 340°C. This means that it will be solid at room temperature.

The test for chlorine gas

To test for chlorine, use damp blue litmus paper. The blue colour will turn to red and then to white.

Chlorine gas reacts with water to produce an acidic solution which is also an effective bleach. This explains how the test for chlorine works. The acidic solution formed on the damp litmus paper turns the indicator red. Then the bleach turns the red colour to white.