Q.Chlorine and fluorine combine to form interhalogen compounds. The halide ion will be of chlorine or fluorine ?

Interhalogen Compounds

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 $XY_n$, 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 $IF_7$ (iodine in +7 state) but never $ClF_7$ — chlorine is too small to hold seven fluorine atoms.

Types and examples

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 $Cl_2$, the bond is strong because both atoms are identical. In $ClF$, the bond is polar and weaker, making it easier to break. This makes $ClF_3$ a stronger fluorinating agent than $F_2$ itself in many reactions.

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 $XY_5$ and $XY_7$ compounds. Chlorine can appear as $Y$ only in $ICl$ and $ICl_3$ — 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:

The reaction conditions matter. Too much heat and the compound decomposes; too little and no reaction occurs.

Hydrolysis

All interhalogen compounds hydrolyse (react with water) to give a mixture of products. The pattern is that the central halogen ends up in a higher oxidation state and the terminal halogen in a lower one. For example:

The big picture

Interhalogen compounds are a beautiful example of how subtle differences in atomic size and electronegativity create a whole family of molecules with predictable structures and properties. They are more reactive than the parent halogens because the $X-Y$ bond is weaker than the $X-X$ or $Y-Y$ bond. Fluorine, being the smallest and most electronegative, plays a unique role as the oxidising agent that drives the formation of these compounds.

The final takeaway: Interhalogen compounds are $XY_n$ molecules where two different halogens combine, with the larger halogen at the centre. They are more reactive than the parent halogens (except $F_2$) because of weaker, polar bonds. Fluorine is the only halogen that can form all types ($XY$, $XY_3$, $XY_5$, $XY_7$) with other halogens.

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