Interhalogen Compounds

What Is an Interhalogen Compound?

When two different halogens combine directly, they produce an interhalogen compound. Because halogens differ in size and electronegativity, these compounds always pair a larger, more electropositive halogen $X$ with a smaller, more electronegative one $X'$. Depending on how many atoms of $X'$ attach to $X$, four general families are possible:

The key trend to remember is that the bigger the size gap between $X$ and $X'$, the more atoms of $X'$ can pack around the central atom $X$. Since fluorine is the smallest and most electronegative halogen, it forms the interhalogens with the highest $X'$ count. This is exactly why the radius ratio between iodine and fluorine is the largest among all halogen pairs, which lets iodine accommodate a full seven fluorine atoms — giving the formula $IF_7$, the interhalogen compound with the maximum number of atoms in a single molecule.

Preparation

Interhalogen compounds are made either by direct combination of the two halogens or by reacting a halogen with an already-formed lower interhalogen compound. Crucially, the exact product obtained (mono-, tri-, penta-, or heptahalide) depends on the reaction conditions — especially the ratio in which the two halogens are mixed and whether one of them is present in excess. A few representative examples:

Notice the pattern: keeping the heavier halogen ($Cl_2$, $Br_2$, $I_2$) in excess tends to stop the reaction at a lower interhalogen, while flooding the mixture with fluorine drives it toward the higher fluorides.

Physical Properties

The properties of some common interhalogens are summarised below.

All of these are covalent, diamagnetic molecules. Except for $ClF$, which stays gaseous at 298 K, the rest exist as volatile liquids or solids. As a family, their melting and boiling points sit roughly between the values of the two parent halogens, though slightly higher than a simple average would predict.

Reactivity and Hydrolysis

Interhalogens tend to be more reactive than the halogens themselves (except fluorine). The reason lies in bond strength: the $X$–$X'$ bond in an interhalogen is weaker than the $X$–$X$ bond within a pure halogen molecule, fluorine's own $F$–$F$ bond being the one exception. A weaker bond breaks more readily, so interhalogens react faster. …