Physical properties of the halogens
Physical properties of the halogens
- The halogens are the Group 17 elements; they exist as diatomic molecules ($\text{Cl}_2$, $\text{Br}_2$, $\text{I}_2$).
- They change colour and state going down the group.
- Their volatility falls down the group.
Practice
The halogens exist as diatomic molecules.
They form X₂ molecules: Cl₂, Br₂, I₂.
Colour, state and volatility
| Element | At room temperature |
|---|---|
| chlorine | pale green gas |
| bromine | red-brown liquid |
| iodine | grey-black solid |
- Volatility decreases down the group: larger molecules with more electrons have stronger London (dispersion) forces, so boiling points rise.
Practice
At room temperature, the states of chlorine, bromine and iodine are:
Chlorine is a gas, bromine a liquid and iodine a solid — volatility falls down the group.
Practice
Volatility decreases down Group 17 because:
More electrons give stronger instantaneous/induced dipole (London) forces, raising the boiling point.
Bond energy
- The $\text{X}\text{–}\text{X}$ bond energy generally falls from $\text{Cl}_2$ to $\text{I}_2$.
- The shared electrons are further from the nuclei in the larger atoms.
Practice
The X–X bond energy generally falls from Cl₂ to I₂ because:
In bigger atoms the bonding electrons are further from the nuclei, so the bond is weaker.
You've got it
Key idea
- halogens are diatomic Group 17 elements: Cl₂ (green gas), Br₂ (red-brown liquid), I₂ (grey-black solid)
- volatility decreases down (more electrons → stronger London forces → higher boiling point)
- the X–X bond energy falls down the group