The halogens have electronic configuration [NG]ns2np5
(where NG implies the relevant Noble Gas), and the accessible
oxidation states range from -1 to +7. The stability of the highest
oxidation state increases down the group: Fluorine only occurs
in the -1 and 0 oxidation states, because it is the most electronegative
element and so is never found in a positive oxidation state.
The halogens are characterized by high ionization energies, high
electron affinities and high electronegativities. Fluorine here
is anomalous as its electron affinity is lower than that of chlorine:
this results from the greater repulsion between a free electron
and the tightly bound electron cloud on F than on the larger Cl
The high electronegativities and abundances of the halogens lead
to high reactivity and a wide range of compound formation. The
halides of many elements are discussed in the sections corresponding
to those elements.
The halide anions undergo plenty of aqueous redox chemistry,
but there are no simple monatomic cations formed in solution.
The halogens are highly reactive and so are found in compounds
with other elements.
Found predominantly as halides, Iodine is the most easily oxidized
and is also found as the Iodate (IO3-).
The halogens are generally prepared by electrolysis of the halide
ions. The highly positive standard reduction potentials of the
X-/X2 couple mean that strong oxidizing
agents are required to convert X- to X2,
and hence the need for electrolysis; Br2 and I2
have lower reduction potentials and so may be obtained by chemical
oxidation of Br- and I-.
This most abundant isotope is 19F,
with nuclear spin, I, = 0.5, and so is of use in NMR. The isolated
element occurs as F2, which is a light yellow gas.
Chlorine exists as Cl2, a
greenish/yellow gas; Bromine exists
as Br2, a red/brown liquid;
and Iodine exists as I2, a violet solid.
The change from gas to solid as the group is descended reflects
the greater size of the ions, meaning that the valence electron
shell is less bound, and so the van der Waals forces are greater
due to the ability for increased distortion of the electron cloud.
The colours of the elements result from the absorption of energy
for the π*(HOMO) to σ*(LUMO)
transition. The HOMO-LUMO gap decreases down the group, and so
the absorption moves to longer wavelength.
The electronegativity decreases down the group.
The ionization energy decreases down the group.
The enthalpy of formation of the gaseous halide ion decreases down the group.
The bond energy of the X-X bond decreases down the group: the F-F bond is anomalously weak.
The ionic radius increases down the group.
The maximum coordination number increases down the group:
CN(F) = 1,2
CN(Cl) = 1 to 4
CN(Br) = 1 to 5
CN(I) = 1 to 7
As stated above, there are no simple monatomic cations
of the halogens formed in solution. Although the ionization energy
of the halogens is approximately that of hydrogen, the large size
of the halogen cations (X+) means that they are not
stabilized either in a condensed phase or in solution.
To be stabilized in a condensed phase, the lattice
enthalpy of the X+ compound needs to outweigh the ionization
energy. To be stabilized in solution, the enthalpy of solvation
of the X+ ion needs to outweigh the ionization energy.
|The lattice and solvation enthalpies vary inversely with the ionic radius.
The relatively large size of the Hal+ ions means that
the lattice and solvation energies are small, and so they are
Compounds such as FClO4 and BrNO3, with
formal F+ and Br+, are actually oxo-radicals
and are not ionic.
However, polycations do exist, and are most stable for Iodine,
eg. I2+, I3+
As stated the H+ ion is much more stable than X+.
Conversely, the X- ion is much more stable than the
H- ion, and there is a much greater range of stable
halides than hydrides.
The F- ion stabilizes high oxidation state compounds
in both ionic and covalent situations: an excess of F-
will oxidize an element into its highest oxidation state.
The covalency of the halides increases with oxidation state,
and also down a group.
Hydrated halide ions usually have a first coordination sphere
of 6 H2O molecules, ie. X-(H2O)6
Halide ions undergo hydrogen bonding: the H-bonded halide complexes
[X-H-X]- have decreasing stability down the group,
though the fluoride complex is much more stable than the others.
Polyanions are known for Cl, Br and I: the polyiodides are the
ion, I3-, is linear; it is
symmetrical in solution but may be perturbed in the solid