Introduction to the Halogens
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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 atom.

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.

Occurrence

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-.

Properties

Fluorine

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.

Other halogens

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.

Trends

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

Cationic Chemistry

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 not stabilized.

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+ and I5+.

Halide ions

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 (aq).

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 most numerous.

The tri-iodide ion, I3-, is linear; it is symmetrical in solution but may be perturbed in the solid state.
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