Question

Oxygen reacts with fluorine to form only `OF_2`, but sulphur which is in the same group 16 as oxygen, reacts with fluorine to form `SF_2`, `SF_4` and `SF_6`. Explain?

Answer 1

There's a very important differen between oxygen and sulfur in terms of orbitals available for bonding.

Explanation:

Indeed, oxygen can only bond with fluorine to form oxygen difluoride, `"OF"_2`.

On the other hand, sulfur can easily form sulfur difluoride, `"SF"_2`, sulfur tetrafluoride, `"SF"_4`, and sulfur hexafluoride, `"SF"_6`.

Sulfur is located in the same group as oxygen, but one period lower, which implies that its is a larger atom than oxygen. This is a drirect consequence of the fact that sulfur's outermost electrons are further away from the nucleus.

Electrons that are located further away from the nucleus have higher energies than those located closer to the nucleus.

In oxygen's case, its valence electrons are located in the 2p-subshell, one full energy level lower than where the valence electrons of sulfur are located, which is in the 3p-subshell.

This is important because the 3p-subshell is close in energy to the 3d-subshell, which means that, under the right conditions, the energy cost of promoting an electron from the 3p-subshell, or even from the 3s-subshell, to the 3d-subshell is not very significant.

This ultimately means that sulfur has access to its empty 3d-orbitals, something that cannot be said of oxygen, for which the energy difference between the 2s or 2p-subshell and the 3d-subshell is very, very significant.

So, as a conclusion, sulfur can use its 3d-orbitals to expand its octet and accomodate more than 8 valence electrons in its outermost shell (think `sp^3d` and `sp^3d^2` hybridization), whereas oxygen simply cannot do that (it cannot have more than `sp^3` hybridization).