Physical Properties of Group 17 Elements
This lesson covers:
- Physical properties of the halogens
- Trend in volatility down the group
- Trend in electronegativity down the group
- Trend in bond strength of halogen molecules 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 |
Volatility decreases down group 7
Volatility refers to how readily a substance evaporates.
Volatility decreases 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.
Halogen bond strength generally decreases down group 7
The bond strength of halogen molecules show a decreasing trend down the group, apart from an anomaly at fluorine (F).
Bond | Bond enthalpy (kJ mol-1) |
---|---|
F-F | 156 |
Cl-Cl | 243 |
Br-Br | 193 |
I-I | 151 |
In diatomic halogen molecules, the two atoms form a bond due to a shared electron pair which is attracted towards the nuclei.
The halogen bond strength decreases down the group from Cl2 to I2 because:
- The atomic radius increases
- This means the shared electron pair gets further away from the positive nuclei
- This results in a weaker electrostatic attraction between the shared electron pair and the positive nuclei.
- So the amount of energy needed to break the covalent bond decreases
However, the F-F bond is anomalously weak because the small atomic size enables lone electron pair repulsion between fluorine atoms, which outweighs the attraction between nucleus and shared electron pair to give unexpectedly weak bonding.