Question

Which among the following compounds is NOT a colourless gas?

• A. ClF
• B. $\text{IF}_7$
• C. $\text{IF}_3$
• D. $\text{ClF}_3$

Hint:

As a general trend in p-block chemistry, iodine-containing compounds with lower fluorine ratios (like $\text{IF}_3$ or ICl) tend to exhibit solid states and distinct dark or vibrant colors (yellows, browns, purples) due to the large, easily polarizable electron cloud of the heavy Iodine atom.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question expects us to recall the physical states and colors of standard interhalogen compounds to pinpoint the one that fails to be a colorless gas at room temperature.

Step 2: Key Formula or Approach:
Interhalogen compounds ($\text{XX}'_n$) showcase varied physical states (gas, liquid, solid) and colors depending heavily on molecular weight and polarizability as you move down Group 17.

Step 3: Detailed Explanation:
Let's analyze the physical properties of the options listed:

ClF (Chlorine monofluoride): This is a light interhalogen compound that exists as a

colorless gas under standard conditions.

$\text{IF}_7$ (Iodine heptafluoride): Despite having a high molecular weight, its highly symmetrical pentagonal bipyramidal structure prevents strong intermolecular packing, rendering it a

colorless gas at room temperature.

$\text{ClF}_3$ (Chlorine trifluoride): This highly reactive and dangerous halogen compound exists as a

colorless gas (condensing into a pale green-yellow liquid at cold temperatures).

$\text{IF}_3$ (Iodine trifluoride): This compound is structurally unstable at ambient temperatures and behaves as a

bright yellow solid. It does not exist as a gas under standard laboratory conditions.

Step 4: Final Answer:
The compound that is NOT a colorless gas is $\text{IF}_3$, which corresponds to option (C).