The compound that violates the octet rule is AsF3.
Here’s why: The octet rule states that atoms tend to bond in such a way that they have eight electrons in their valence shell, achieving a stable electronic configuration similar to that of noble gases. In the case of AsF3, arsenic (As) is the central atom and has five valence electrons. When it forms bonds with three fluorine atoms, it shares its electrons but only achieves a total of eight electrons around the fluorine atoms, not around itself.
On the other hand:
- MgF2: Magnesium (Mg) donates two electrons to two fluorine atoms, achieving an octet for both the Mg and the F.
- OF2: Oxygen (O) can achieve an octet by sharing its electrons with two fluorine atoms.
- SiF4: Silicon (Si) shares its four valence electrons with four fluorine atoms, thus completing an octet for all involved atoms.
Thus, AsF3 is the exception that violates the octet rule due to its ability to have more than eight electrons surrounding the arsenic atom without achieving a complete octet.