Group 7 Halogens
Overview
The Group 7 elements are called the halogens: fluorine, chlorine, bromine, iodine and astatine. They are:
- Non-metals
- Poisonous
- Diatomic: they exist as molecules of pairs of atoms (F2, Cl2, Br2, I2)
- All have 7 electrons in their outer shell, giving them similar chemical properties
Physical Properties
| Halogen | State at room temperature | Appearance | Colour in solution |
|---|---|---|---|
| Fluorine (F2) | Gas | Pale yellow | Very reactive; not tested in solution |
| Chlorine (Cl2) | Gas | Pale yellow-green | Pale green |
| Bromine (Br2) | Liquid | Red-brown, volatile | Orange |
| Iodine (I2) | Solid | Grey, shimmery crystals; sublimes to purple vapour | Dark brown |
Trends going down Group 7:
- Melting and boiling points increase (gas, liquid, solid)
- Colours become darker
- Density increases
The melting and boiling points increase because the molecules become larger, leading to stronger intermolecular forces that require more energy to overcome.
Reactions with Metals
Halogens react with metals to form ionic metal halide salts. The halide ion carries a -1 charge.
Example: Chlorine with sodium: 2Na(s) + Cl2(g) → 2NaCl(s)
Example: Chlorine with calcium: Ca(s) + Cl2(g) → CaCl2(s)
Reactivity decreases down the group, so iodine reacts less vigorously with metals than chlorine.
Reactions with Non-metals
Halogens react with non-metals to form simple molecular covalent structures.
Example: Chlorine with hydrogen: H2(g) + Cl2(g) → 2HCl(g)
Fluorine is the most reactive (reacts with hydrogen even at low temperatures in the absence of light). Iodine reacts least vigorously with hydrogen.
Displacement Reactions
A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt.
Reactivity order (most to least reactive): F2 > Cl2 > Br2 > I2
You need to know the displacement reactions involving chlorine, bromine and iodine.
| Reaction | Equation | Observation |
|---|---|---|
| Chlorine + potassium bromide | Cl2(aq) + 2KBr(aq) → 2KCl(aq) + Br2(aq) | Solution turns orange (bromine formed) |
| Chlorine + potassium iodide | Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq) | Solution turns brown (iodine formed) |
| Bromine + potassium iodide | Br2(aq) + 2KI(aq) → 2KBr(aq) + I2(aq) | Solution turns brown (iodine formed) |
Displacement Summary Table
| Added to | KCl | KBr | KI |
|---|---|---|---|
| Cl2 | No reaction | Orange (Br2 formed) | Brown (I2 formed) |
| Br2 | No reaction | No reaction | Brown (I2 formed) |
| I2 | No reaction | No reaction | No reaction |
These displacement reactions provide evidence for the trend in reactivity: a more reactive halogen can displace a less reactive one.
Trend in Reactivity
Reactivity decreases going down Group 7. Fluorine is the most reactive; astatine is the least reactive.
Evidence:
- Chlorine can displace bromine and iodine; bromine can displace iodine; iodine cannot displace either.
- Fluorine is so reactive it reacts with most substances it contacts.
Predicting Properties of Other Halogens
Using the trends, we can predict:
- Halogens above chlorine would be gases at room temperature with even lower boiling points.
- Halogens below iodine would be solids at room temperature with higher melting points.
- Astatine would be the least reactive halogen and the darkest in colour.
Higher Tier
Explaining the trend in reactivity using electronic configurations:
When halogens react, they gain an electron into their outer shell to form a -1 halide ion.
Going down Group 7:
- The outer shell is further from the nucleus
- There is more electron shielding from inner shells
- The force of attraction between the nucleus and the outer shell is weaker
- It becomes harder for the atom to attract an incoming electron
Therefore, the ability to gain an electron decreases down the group, so reactivity decreases.
Fluorine is the smallest halogen, so its outer shell is closest to the nucleus and the attraction is strongest, making it the most reactive.
Exam Questions
Q: State the physical state and colour of bromine at room temperature.
Bromine is a red-brown liquid at room temperature.
Q: What would you observe when chlorine solution is added to potassium iodide solution?
The solution would turn brown, as iodine is formed. Chlorine displaces iodine from potassium iodide because chlorine is more reactive than iodine.
Q: Explain why iodine cannot displace chlorine from potassium chloride solution.
Iodine is less reactive than chlorine. A halogen can only displace a less reactive halogen from a solution of its salt. Since iodine is less reactive, it cannot displace chlorine.
Q: Explain why the reactivity of halogens decreases going down Group 7.
Going down the group, the outer electron shell is further from the nucleus and there is more electron shielding. The attraction between the nucleus and the outer shell decreases, so it becomes harder for the atom to attract an incoming electron. Therefore, the halogens become less reactive going down the group.
Q: Predict the colour and state of astatine (below iodine in Group 7) at room temperature.
Based on the trend down Group 7 (darker colour, higher melting point), astatine would be a very dark (possibly black) solid at room temperature.
Exam Tip
Remember: halogens show the opposite reactivity trend to Group 1 metals. Group 1 metals become MORE reactive going down; halogens become LESS reactive going down. The reasoning is also opposite: metals lose an electron (easier further from nucleus), halogens gain an electron (harder further from nucleus).