• The group 17 elements are called halogens
• The halogens have uses in water purification and as bleaches agents (chlorine), as flame-retardants and fire extinguishers (bromine) and as antiseptic and disinfectant agents (iodine)

Colours

• All halogens have distinct colours which get darker going down the group

The colours of the Group 17 elements get darker going down the group

Volatility

• Volatility refers to how easily a substance can evaporate
  • A volatile substance will have a low boiling point

The boiling points of the Group 17 elements increase going down the group which indicates that the elements become less volatile

• Going down the group, the boiling point of the elements increases which means that the volatility of the halogens decreases
  • This means that fluorine is the most volatile and iodine the least volatile

• Halogens are non-metals and are diatomic molecules at room temperature
  • This means that they exist as molecules which are made up of two similar atoms, such as F₂
• The halogens are simple molecular structures with weak van der Waals’ forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces

The diagram shows that a sudden distribution of electrons in a nonpolar molecule can cause an instantaneous dipole. When this molecule gets close to another non-polar molecule it can induce a dipole as the cloud of electrons repel the electrons in the neighbouring molecule to the other side

• The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces
• Therefore, the larger the molecule the stronger the van der Waals’ forces between molecules
• This is why as you go down the group, it gets more difficult to separate the molecules and the melting and boiling points increase
• As it gets more difficult to separate the molecules, the volatility of the halogens decreases going down the group

Going down the group, the van der Waals’ forces increase due to an increased number of electrons in the molecules which means that the volatility decreases