Physical Properties
- The Group 7 elements are called halogens
- The halogens have uses in water purification and as bleaching agents (chlorine), as flame-retardants and fire extinguishers (bromine) and as antiseptic and disinfectant agents (iodine)
Colours
- All halogens have distinct colours which get darker going down the group
The colours of the Group 7 elements get darker going down the group
Volatility
- Volatility refers to how easily a substance can evaporate
- A volatile substance will have a low boiling point
The melting and boiling points of the Group 7 elements increase going down the group which indicates that the elements become less volatile
- Going down the group, the boiling point of the elements increases which means that the volatility of the halogens decreases
- This means that fluorine is the most volatile and iodine the least volatile
Trend in melting and boiling points
- Halogens are non-metals and are diatomic molecules at room temperature
- This means that they exist as molecules which are made up of two similar atoms, such as F2
- The halogens are simple molecular structures with weak London dispersion forces between the diatomic molecules caused by instantaneous dipole-induced dipole forces
The diagram shows that a sudden imbalance of electrons in a nonpolar molecule can cause an instantaneous dipole. When this molecule gets close to another non-polar molecule it can induce a dipole as the cloud of electrons repel the electrons in the neighbouring molecule to the other side
- The more electrons there are in a molecule, the greater the instantaneous dipole-induced dipole forces
- Therefore, the larger the molecule the stronger the London dispersion forces between molecules
- This is why as you go down the group, it gets more difficult to separate the molecules and the melting and boiling points increase
- As it gets more difficult to separate the molecules, the volatility of the halogens decreases going down the group
Trend in electronegativity
- The electronegativity of the halogens decreases down the group
The electronegativity of the halogens decreases going down the group
- The electronegativity of an atom refers to how strongly it attracts electrons towards itself in a covalent bond
- The decrease in electronegativity is linked to the size of the halogens
- Going down the group, the atomic radii of the elements increase which means that the outer shells get further away from the nucleus
- An ‘incoming’ electron will therefore experience more shielding from the attraction of the positive nuclear charge
- The halogens’ ability to accept an electron (their oxidising power) therefore decreases going down the group
With increasing atomic size of the halogens (going down the group) their electronegativity, and therefore oxidising power, decreases