Which species is not known?

Options

A. `XeF_(6)`
B. `XeF_(4)`
C. `XeO_(3)`
D. `KrF_(6)`

Explanation

To determine which species is not known among the given options, we will analyze each species one by one.

### Step 1: Analyze XCF6 (Xenon Hexafluoride)
- **Existence**: Xenon hexafluoride (XeF6) is known to exist.
- **Preparation**: It can be prepared by reacting xenon (Xe) with fluorine (F2) at a temperature of 573 K and a pressure of 60 to 70 bar.
- **Conclusion**: XeF6 is a known compound.

### Step 2: Analyze XCF4 (Xenon Tetrafluoride)
- **Existence**: Xenon tetrafluoride (XeF4) is also known.
- **Preparation**: It is prepared by reacting xenon gas with 2 moles of fluorine at a temperature of 873 K and a pressure of 7 bar.
- **Conclusion**: XeF4 is a known compound.

### Step 3: Analyze XeO3 (Xenon Trioxide)
- **Existence**: Xenon trioxide (XeO3) is known.
- **Preparation**: It can be formed by the hydrolysis of xenon fluorides.
- **Conclusion**: XeO3 is a known compound.

### Step 4: Analyze KrF6 (Krypton Hexafluoride)
- **Existence**: Krypton hexafluoride (KrF6) is not known.
- **Reason**: While xenon can form several fluorides, compounds of krypton are limited. Although KrF2 and KrF4 are known, KrF6 has not been observed.
- **Conclusion**: KrF6 is not a known compound.

### Final Conclusion
Among the four species analyzed, KrF6 (Krypton Hexafluoride) is the only one that is not known.

### Answer
The species that is not known is KrF6 (Krypton Hexafluoride).

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