Think about the halogens — fluorine, chlorine, bromine, iodine. They are all non-metals with similar properties, but they differ in size and electronegativity. Now imagine what happens when two different halogens combine with each other. That is exactly what an interhalogen compound is: a molecule formed by the direct combination of two distinct halogen atoms.
The general formula is XYn, where X is the larger and less electronegative halogen (the central atom), and Y is the smaller and more electronegative halogen. Here n can be 1, 3, 5, or 7 — always an odd number. The central atom X can be Cl, Br, or I, while Y is always F or sometimes Cl.
Why does this happen? The larger halogen has empty d-orbitals that can accommodate electrons from the smaller, more electronegative halogen. Fluorine, being the smallest and most electronegative, can oxidise the central atom to its highest oxidation state. That is why you see compounds like IF7 (iodine in +7 state) but never ClF7 — chlorine is too small to hold seven fluorine atoms.
XYn(n=1,3,5,7)
X = larger halogen (central atom), Y = smaller halogen
Types and examples
Type
Formula
Examples
Geometry
XY
Diatomic
ClF, BrF, ICl, IBr
Linear
XY3
T-shaped
ClF3, BrF3
Bent T-shaped
XY5
Square pyramidal
BrF5, IF5
Square pyramidal
XY7
Pentagonal bipyramidal
IF7
Pentagonal bipyramidal
Reactivity — the key point
Interhalogen compounds are more reactive than the parent halogens (except fluorine itself). The reason is straightforward: the bond between two different halogens is weaker than the bond between two identical halogen atoms. In Cl2, the bond is strong because both atoms are identical. In ClF, the bond is polar and weaker, making it easier to break. This makes ClF3 a stronger fluorinating agent than F2 itself in many reactions.
Watch out
Do not confuse reactivity with stability. ClF3 is highly reactive (it can ignite glass, asbestos, and even water) but it is a stable compound under controlled conditions. Reactivity means it reacts readily, not that it decomposes easily.
Why fluorine is special
Fluorine is the only halogen that can form interhalogen compounds with all other halogens. It is also the only one that can appear as the Y atom in XY5 and XY7 compounds. Chlorine can appear as Y only in ICl and ICl3 — it is too large to fit around iodine in higher numbers. Bromine and iodine never appear as Y in any interhalogen compound because they are too large and not electronegative enough.
Preparation
Most interhalogen compounds are made by direct combination of the elements under controlled conditions. For example:
Cl2+3F2573K2ClF3
I2+7F22IF7
The reaction conditions matter. Too much heat and the compound decomposes; too little and no reaction occurs.
Iodine in IF7 has 7 bond pairs and 0 lone pairs around it (sp3d3 hybridisation), and the VSEPR arrangement for 7 electron domains with no lone pairs …
IF7 has iodine surrounded by 7 fluorine atoms with no lone pairs, giving sp3d3 hybridisation and a pentagonal bipyramidal molecular geometry.
Iodine in IF7 uses all 7 of its valence electrons to form 7 I–F sigma bonds, leaving no lone pair on the central atom. By VSEPR theory, 7 identical bonding domains around a central atom with zero lone pairs arrange themselves to minimise repulsion in a pentagonal bipyramidal shape: 5 fluorine atoms occupy the equatorial pentagonal plane (at 72° to each other) and 2 fluorine atoms occupy the axial positions perpendicular to that plane. Square pyramidal and octahedral geometries correspond to 6 or fewer effective …