Colours and states of the halogens at room temperature
Definition
Halogen
An element in Group 7 of the periodic table, which has seven electrons in its outer shell.
- Chlorine is a pale green gas.
- Bromine is a red-brown liquid that gives off an orange vapour.
- Iodine is a dark grey solid that forms a purple vapour when warmed.
- All three exist as molecules of two atoms, written Cl2\text{Cl}_2Cl2, Br2\text{Br}_2Br2 and I2\text{I}_2I2.
- The colours deepen down the group, from pale green through red-brown to almost black.
Key Idea
Gas, liquid and solid in one group is itself the pattern: the state changes as you go down.
Melting and boiling points rise down the group
- Chlorine boils at about −34 ∘C-34\ ^{\circ}\text{C}−34 ∘C, well below room temperature.
- Bromine boils at about 59 ∘C59\ ^{\circ}\text{C}59 ∘C, so it is a liquid at room temperature.
- Iodine melts at about 114 ∘C114\ ^{\circ}\text{C}114 ∘C, so it is a solid at room temperature.
- Both melting point and boiling point increase steadily down the group.
- The change of state from gas to liquid to solid is a direct consequence of that rise.
Example
- Room temperature: chlorine gas, bromine liquid, iodine solid.
- The reason: room temperature lies above chlorine's boiling point and below iodine's melting point.
Larger molecules are held to one another more strongly
Definition
Intermolecular force
A force of attraction between neighbouring molecules, much weaker than the covalent bonds inside a molecule.
- A halogen is a simple molecular substance, so melting separates whole molecules.
- The intermolecular forces between molecules are what has to be overcome.
- Molecules get larger down the group, from Cl2\text{Cl}_2Cl2 to Br2\text{Br}_2Br2 to I2\text{I}_2I2.
- Larger molecules attract one another more strongly, so more energy is needed to separate them.
- The covalent bonds inside each molecule are not broken when a halogen melts or boils.
Common Mistake
Melting a halogen separates its molecules, so the strength of the covalent bond is not what matters.
Predicting fluorine and astatine
- Fluorine sits above chlorine, so its melting and boiling points are lower still.
- Fluorine is therefore a gas at room temperature, and its colour is paler than chlorine's.
- Astatine sits below iodine, so its melting and boiling points are higher.
- Astatine is therefore a solid at room temperature, and darker than iodine.
- Each prediction extends the trend rather than recalling a measured value.
Note
A prediction is stated as a direction along the trend, such as higher or darker, rather than as a figure.
Reading a table of halogen data
- Data are usually laid out in group order, from fluorine at the top downwards.
- Checking that a quantity rises or falls steadily confirms which trend is present.
- A state at room temperature is deduced by comparing 20 ∘C20\ ^{\circ}\text{C}20 ∘C with the melting and boiling points.
- A value above the boiling point means a gas, and below the melting point means a solid.
- A missing entry is filled by placing the element in the order and reading the neighbouring values.
Self review
- Give the colour and state of chlorine, bromine and iodine at room temperature.
- What happens to boiling point down Group 7?
- Why does iodine have a higher melting point than chlorine?
- Predict the state and colour of astatine.
- How do you decide from a data table whether an element is a liquid at room temperature?
