Group VII properties
Diatomic Molecules:
All halogens exist as diatomic molecules (X_2) in their elemental form. This means two atoms of the same halogen are bonded together by a covalent bond.
- Chlorine: Cl_2
- Bromine: Br_2
- Iodine: I_2
General Trends Down the Group:
As you move down Group VII (from fluorine to iodine), the atoms get larger and have more electron shells. This leads to two main trends:
- Density increases: The atoms become heavier, so the density of the element increases.
- Reactivity decreases: Halogens react by gaining an electron to form a negative ion (X^-). As the atom gets larger down the group, the outer shell is further from the nucleus and more shielded by inner electrons. This makes it harder for the nucleus to attract and capture an incoming electron. Therefore, reactivity decreases.
Appearance at Room Temperature and Pressure (r.t.p.):
The physical state changes from gas to liquid to solid as you go down the group due to increasing intermolecular forces.
| Element | State at r.t.p. | Colour |
|---|---|---|
| Chlorine (Cl_2) | Gas | Pale yellow-green |
| Bromine (Br_2) | Liquid | Red-brown |
| Iodine (I_2) | Solid | Grey-black |
A displacement reaction occurs when a more reactive halogen displaces (kicks out) a less reactive halide ion from an aqueous solution.
The Rule: A halogen can only displace another halogen that is below it in Group VII.
- Chlorine (Cl_2) is above Bromine and Iodine. It can displace both Br^- and I^- ions.
- Bromine (Br_2) is below Chlorine but above Iodine. It can only displace I^- ions, not Cl^- ions.
- Iodine (I_2) is at the bottom of the common halogens. It cannot displace Cl^- or Br^- ions because it is less reactive than both.
Prediction: Since chlorine is more reactive than bromine (it is higher in the group), it will displace the bromide ions.
Word Equation:
\text{chlorine} + \text{potassium bromide} \rightarrow \text{bromine} + \text{potassium chloride}
Symbol Equation:
Cl_2 + 2KBr \rightarrow Br_2 + 2KCl
Ionic Equation:
Cl_2 + 2Br^- \rightarrow 2Cl^- + Br_2
Observation: The solution will change colour. Aqueous bromine is orange/brown. Therefore, the colourless potassium bromide solution turns orange/brown as bromine is produced.
Why this happens: Chlorine atoms gain electrons from bromide ions to form chloride ions (Cl^-). The bromide ions lose electrons to become bromine molecules (Br_2). This confirms chlorine is more reactive than bromine.
Correction: Reactivity in Group VII depends on the ability to gain an electron, not on physical state. Iodine is a solid because it has strong intermolecular forces between its large molecules, but it is less reactive than chlorine because its outer shell is further from the nucleus. Always remember: Reactivity decreases down the group.
When to use: When asked to describe the observable change in a displacement reaction.
Why examiners accept this: Examiners look for specific colour changes linked to the product formed. Saying 'a reaction occurs' is insufficient. You must state the colour of the product halogen in solution.
Correct Usage:
- If chlorine displaces bromide, state: 'The solution turns orange/brown' (due to formation of aqueous bromine).
- If chlorine displaces iodide, state: 'The solution turns brown' (aqueous iodine is brown; it may also turn purple in organic solvents like hexane).
- If no reaction occurs (e.g., iodine added to bromide), state: 'No visible change' or 'Solution remains colourless.'