Step 1: Understanding the Concept:
Interhalogen compounds are formed when two different halogens react. Their physical state depends on the size of the atoms and the resulting Van der Waals forces.
As we move down the halogen group (F \(\to\) Cl \(\to\) Br \(\to\) I), the atomic size and number of electrons increase.
An increase in electrons leads to stronger London dispersion forces, which increases the boiling and melting points of the compounds.
Step 2: Detailed Explanation:
Let's examine the physical states of the given interhalogens at room temperature (\(25^{\circ}\text{C}\)):
- ClF (Chlorine monofluoride): This is a small molecule. It is a colorless gas at room temperature.
- BrF (Bromine monofluoride): Bromine is heavier than chlorine. BrF is typically an unstable gas that disproportionates into \(Br_2\) and \(BrF_3\), but in the context of pure chemical classification, it is considered gaseous.
- ClF$_3$ (Chlorine trifluoride): This is a highly reactive compound. It is a colorless gas (though it condenses to a pale green liquid at \(12^{\circ}\text{C}\), so at \(25^{\circ}\text{C}\), it is definitely gaseous).
- IF$_3$ (Iodine trifluoride): Iodine is the heaviest common halogen. As the central atom gets larger and more polarizable, the intermolecular forces become significantly stronger. \(IF_3\) is a yellow solid that is stable only at very low temperatures (below \(-28^{\circ}\text{C}\)). At room temperature, it is not a gas; it is a solid (or decomposes).
In general, interhalogens with small central atoms (Cl) tend to be gases, those with medium atoms (Br) are liquids, and those with large central atoms (I) are solids at room temperature.
Step 3: Final Answer:
\(IF_3\) is not in the gaseous phase at \(25^{\circ}\text{C}\).