Interhalogen Compounds

Classification

These are classified in mainly four types of interhalogen compounds:

Diatomic interhalogens (AX)

Tetra-atomic interhalogens (AX3)

Hexa-atomic interhalogens (AX5)

Octa-atomic interhalogens (AX7)

All the Interhalogen compounds are covalent in nature, and these are more reactive than halogens because of low bond energy which is because of the dissimilarity in the size of atoms in molecule which makes the molecule a little polar and the boiling point increases due to the inter-molecular attraction forces. These interhalogen compounds are reactive and unstable because of low bond energy.

An interhalogen compound is a molecule which contains two or more different halogen atoms and no different atoms of elements from any other group in the periodic table. The general formula for most of the interhalogen compounds is XYn, where n could be 1, 3, 5 or 7, and X is the less electronegative halogen atom of the two halogens, to be precise the compounds formed by the union of two different halogens are called interhalogen compounds. The interhalogen compounds of type AX and AX3 are formed between the halogens having very less electronegative difference, for an example, ClF and ClF3, whereas the interhalogen compounds of type AX5 and AX7 are formed by larger halogen atoms having low electronegativity difference with the smaller atoms having high electronegativity. This is because it is convenient to fit the greater number of smaller halogen atoms around a larger halogen atom, for example BrF5 or IF7, here bromine and iodine are larger atoms in size as compared to fluorine.

Interhalogens are all more likely to go through hydrolysis and ionize to give rise to polyatomic ions. Except for Fluorine, interhalogens are often more reactive than halogens. This is due to the fact that A-X bonds in interhalogens are weaker than X-X bonds in dihalogen molecules. Interhalogen reactions are comparable to halogen reactions. When interhalogen compounds are hydrolyzed, two products are produced: halogen acid and oxy-acid.

Structures of interhalogen compounds

The structures formed for the various interhalogens compounds confirm to what would be expected based on the VSEPR (valence shell electron pair repulsion) theory, consider for XY3 the shape can be described as T-shaped with 2 lone pairs sitting in equatorial positions of a trigonal bipyramid. For XY5 the shape is a square pyramid with the unpaired electrons sitting in axial positions of an octahedral and XY7 is a pentagonal bipyramid. 

XY diatomic interhalogens

The interhalogens with formula XY have physical properties that are intermediate between those of the two parent halogens. The covalent bond between the two atoms has some ionic character, the larger element, X, gets oxidised and develops a partial positive charge. Most combinations of F, Cl, Br and I are known, but not all are stable, a common example for diatomic interhalogens could be Chlorine monofluoride (ClF), it is the lightest interhalogen, and is also a colourless gas. 

Tetra-atomic Interhalogens (XY3)

Chlorine trifluoride (ClF3) is a tetra-atomic interhalogen that is a colourless gas that condenses into a green liquid and freezes to a white solid. It is formed by reacting chlorine with an excess of fluorine at 250 °C in a nickel tube. It reacts more violently than fluorine, to be precise, in an explosive manner. The molecule is planar and T-shaped and it is used in the manufacture of uranium hexafluoride.

Hexa-atomic Interhalogens (AX5)

Chlorine pentafluoride (ClF5) is a colourless gas, which is formed by reacting chlorine trifluoride with fluorine at high temperatures and high pressures. It reacts violently with water and most metals and non-metals. Its structure is trigonal bipyramidal.

Octa-atomic interhalogens (AX7)

Iodine heptafluoride (IF7) is a colourless gas, and it is formed by the reaction of pentafluoride with fluorine. IF7 is chemically inert, and it mimics sulphur hexafluoride because it lacks a lone pair of electrons in the valence shell. The molecule of Iodine heptafluoride (IF7) is a pentagonal bi-pyramidal structure. This compound is the only interhalogen compound possible where the larger iodine atom is carrying seven of the smaller fluorine atoms

Conclusion

All Interhalogen compounds are volatile at room temperature. All are polar compounds which are mainly due to differences in their electronegativities. These compounds are usually covalent liquids or gases because of small electronegativity differences among them, also all these Interhalogen compounds are more reactive than normal halogens except fluorine. Because they have bond pairs and lone pairs, these interhalogen compounds are diamagnetic in nature; they are also exceedingly reactive. The boiling point of a molecule increases as it becomes more polar due to intermolecular interaction. Because they have less bond energy, they are reactive and unstable.