3.3.1 The Halogens

Trend in boiling points

• Boiling point increases down the group
• We can explain this trend by looking at the forces between the molecules
• Halogens are non-metals and are diatomic molecules at room temperature
• This means that they exist as molecules that are made up of two similar atoms, such as F₂
• The halogens are simple molecular structures with weak London forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces

• The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces
• Therefore, the larger the molecule the stronger the London forces between molecules
• This is why as you go down the group, it gets more difficult to separate the molecules and the  boiling point increases

• The colour of the solution in the test tube shows which free halogen is present in solution.
  • Chlorine = very pale green solution (often colourless),
  • Bromine = yellow solution
  • Iodine = brown solution(sometimes black solid present)
• The colour of the organic solvent layer in the test tube shows which free halogen is present in solution
  • Chlorine = colourless
  • Bromine = yellow
  • Iodine = purple

Observations with an organic solvent

Reaction of bromide ions and chlorine

• Chlorine is above bromine in Group 7 so it is more reactive
• Chlorine will therefore displace bromine from an aqueous solution of a metal bromide

Trend in Reactivity

• The oxidising power of the halogens decreases going down the group (the halogens get less reactive)
• This can be explained as follows:
  • Atomic radius increases
  • There are more inner shells so shielding increases
  • There is less nuclear attraction to attract an electron from another species

• Each halogen has 7 electrons in their outer shell with two electrons in the outer s sub-shell and 5 in the outer p sub-shell
  • __s²__p⁵

• Halogens react with metals by accepting an electron from the metal atom to become an ion with 1- charge
  • For example:

Ca (s) + Cl₂ (g) → CaCl₂ (s)  

consisting of Ca²⁺ and 2Cl^- ions

• Halogens are therefore oxidising agents:
  • Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases)
  • Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases)