Physical properties of the halogens
Introduced| English | Meaning |
|---|---|
| halogen/ˈhælədʒn/ | The elements in group 17 of the periodic table. |
| diatomic/ˌdaɪəˈtɒmɪk/ | diatomic |
| volatility/ˌvɒləˈtɪlɪti/ | The tendency of a substance to vaporise readily under given conditions. |
| bond energy/bɒnd ˈenədʒi/ | The energy required to break one mole of a specified covalent bond in gaseous molecules. |
| reactivity/rɪəkˈtɪvɪti/ | The tendency or readiness of a substance to undergo chemical reactions. |
Meet the salt-formers
- The halogens 卤素 are the Group 17 elements; they exist as diatomic 双原子 molecules ($\text{Cl}_2$, $\text{Br}_2$, $\text{I}_2$).
- They change colour and state going down the group.
- Their volatility 挥发性 falls down the group.
Halogen physical trend lab
Follow halogens down the group and link state, colour and volatility.
The halogens exist as diatomic molecules.
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They form X₂ molecules: Cl₂, Br₂, I₂.
The halogens become more reactive down Group 17.
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False. They become less reactive: the outer shell is further from the nucleus and more shielded, so gaining an electron is harder.
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.
The halogens change from gas to liquid to solid down the group as volatility falls
At room temperature, the states of chlorine, bromine and iodine are:
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Chlorine is a gas, bromine a liquid and iodine a solid — volatility falls down the group.
Volatility decreases down Group 17 because:
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More electrons give stronger instantaneous/induced dipole (London) forces, raising the boiling point.
Match each halogen to its state at room temperature.
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Each item links the term to its correct meaning.
Bond energy 键能
- 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.
Bromine is a Group 17 halogen: a dark liquid giving an orange vapour
The X–X bond energy generally falls from Cl₂ to I₂ because:
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In bigger atoms the bonding electrons are further from the nuclei, so the bond is weaker.
The trend in reactivity 反应活性
- Halogen reactivity decreases down the group: the larger atom attracts an extra electron less strongly.
- This is why chlorine displaces bromine and iodine, but not the other way round.
You've got it
- 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