4.2 - Group 7 - The Halogens
The halogens and their trends
- The elements in Group 7 are non-metals called the halogens.
- Every halogen atom has seven electrons in its outer shell, so they all have similar chemical properties.
- Halogens exist as diatomic molecules (pairs of atoms joined by a single covalent bond): F2, Cl2, Br2, I2.
- All halogens are toxic and are handled in a fume cupboard.
Appearance at room temperature
- Fluorine (F2) - Pale yellow gas.
- Chlorine (Cl2) - Pale yellow-green gas.
- Bromine (Br2) - Red-brown liquid that gives off an orange vapour.
- Iodine (I2) - Dark grey crystalline solid that sublimes to a purple vapour when heated.
Trends going down the group
- Melting and boiling points increase - The state at room temperature changes from gas (fluorine, chlorine) to liquid (bromine) to solid (iodine).
- Relative molecular mass and density increase - Each halogen molecule is heavier than the one above it.
- Colour darkens - From pale yellow to dark grey/black.
- Reactivity decreases - Fluorine at the top is the most reactive, and astatine at the bottom is the least reactive.
Predicting properties of an unknown halogen
Because properties change steadily down the group, you can predict unknown properties from the trend: Astatine is below iodine, so it is predicted to be a black solid with a higher melting point and lower reactivity than iodine.
Displacement reactions of the halogens
A displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide salt.
Testing halogen reactivity
Adding an aqueous halogen solution to an aqueous halide solution produces a colour change if displacement occurs:
| Reactants | KCl solution (colourless) | KBr solution (colourless) | KI solution (colourless) |
|---|---|---|---|
| Chlorine water | No reaction | Orange solution (Br2 formed) | Brown solution (I2 formed) |
| Bromine water | No reaction | No reaction | Brown solution (I2 formed) |
| Iodine solution | No reaction | No reaction | No reaction |
Key equations
- Chlorine displaces bromine:
- chlorine + potassium bromide → bromine + potassium chloride
- Cl2(aq) + 2KBr(aq) → Br2(aq) + 2KCl(aq)
- Chlorine displaces iodine:
- chlorine + potassium iodide → iodine + potassium chloride
- Cl2(aq) + 2KI(aq) → I2(aq) + 2KCl(aq)
- Bromine displaces iodine:
- bromine + potassium iodide → iodine + potassium bromide
- Br2(aq) + 2KI(aq) → I2(aq) + 2KBr(aq)
These results confirm that reactivity decreases down the group: chlorine > bromine > iodine.
TermDefinition
HalogensThe non-metal elements in Group 7 of the periodic table, which each have seven electrons in their outer shell.
Diatomic moleculeA molecule made of two atoms covalently bonded together, such as H2, O2 or Cl2.
Displacement reactionA reaction in which a more reactive element takes the place of a less reactive element in a compound or in a solution of its salt.