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- Fluorine F2 - yellow gas
- Chlorine Cl2 - yellow-green gas
- Bromine Br2 - red-brown liquid
- Iodine I2 - black (dark purple) solid - sublimes to a purple vapour.
They are highly reactive non-metals. They exist as diatomic molecules containing a covalent bond. Due to their reactivity the elements do not occur in the free state but are always combined with other elements.
Melting point, boiling point, density and latent heats of fusion and vaporisation are all dependent on the strength of the intermolecular forces.
Values of these properties are relatively low and show a steady increase down the group. This is because the molecules are non-polar and so the only intermolecular attractions are weak van der Waals. These increase down the group with molecular mass. This is observed with the changes of state as we descend the group (gas to solid).
These properties all depend upon the effective nuclear charge felt by the outer electrons. The halogen atoms have a maximum nuclear charge for the irrespective periods (apart from the noble gases) and hence the greatest pull on their outer electrons.
- The atoms and ions have relatively small radii that increase down the group. Ionic radii are larger than atomic radii.
- Ionisation energies and electron affinities are relatively high and decrease down the group.
- Electronegativity decreases down the group. Fluorine is the most electronegative element. This trend decreases down the group and can be explained in terms of increase in atomic radius and increase in shielding effect which more than offset the increase in nuclear charge. Hence, the nuclear pull on the bonding electrons decreases.
They have an outer electron structure of ns2np5 and need to gain one electron to complete their octet. The most common oxidation state -1 and they commonly form both covalent and ionic bonds.
a) Halide ions F-, Cl-, Br-, I- are formed from bonding with electropositive metals.
b) Covalent bonds are formed with non-metals and weakly electropositive metals (e.g. HCl).
The halogens are able to expand their valency shell by making use of vacant 'd' orbitals.
They do this by unpairing the s and p electrons and promoting them to vacant 'd' orbitals.
This means that the halogens can show oxidation states greater than -1.
The maximum number of unpaired electrons is 7 and so the maximum possible oxidation state is 7.
Note: Fluorine can only exhibit the one oxidation state of -1 since it has no available 'd' orbitals.
The tendency to form ionic bonds decreases down the group as the electronegativity decreases. The tendency to form covalent bonds increases as you descend the group.