The Halogens
This lesson covers:
- Physical properties of the halogens
- Trend in boiling point down the group
- Trend in electronegativity down the group
Physical properties and trends
The halogens make up group 7 of the periodic table. This family consists of 5 elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).
As highly reactive non-metals, they tend to form diatomic molecules with one covalent bond between two atoms (e.g. F2). Halogens have some notable trends down the group.
| Halogen | Formula | Appearance at room temperature | Boiling point (°C) |
|---|---|---|---|
| Fluorine | F2 | Pale yellow gas | -188 |
| Chlorine | Cl2 | Green gas | -35 |
| Bromine | Br2 | Red-brown liquid | 59 |
| Iodine | I2 | Grey solid | 184 |
Boiling points increase down group 7
The boiling points increase down the group because:
- The molecules become larger in size and relative mass
- This allows for stronger induced dipole-dipole forces to develop between molecules
- These stronger intermolecular forces require increasing amounts of energy to overcome
- So the the halogens boil at higher temperatures down the group
Electronegativity decreases down group 7
Related to atomic size, the electronegativity of halogens shows an overall decrease going down the group. Electronegativity refers to the tendency of an atom to attract a bonding pair of electrons.
The electronegativity decreases down the halogens because:
- Nuclear charge - Increases down the group as more protons are added, increasing attraction for electrons
- Atomic radius - Increases down the group as more electron shells are added, moving electrons away from nucleus
- Electron shielding - Inner electron shells reduce the atom’s ability to attract electrons
Although the halogens have high electronegativities compared to other elements, the atomic radius and shielding effects down the group are greater than the nuclear charge effect. This leads to an overall decrease in electronegativity on descending group 7.