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#. Why is this?

The reason for the disparity in the ways that sulfur and oxygen react with fluorine is that sulfur has six valence electrons, whereas oxygen has two. This difference causes the molecular structures and bonding of the compounds that are produced to differ. For example, sulfur's higher valence electron capacity allows it to accept more fluorine atoms, which leads to the formation of SF2, SF4, and SF6 while sulfur forms OF2 through the formation of two covalent bonds.

Oxygen and sulfur are both in Group 16 of the periodic table, meaning they share some chemical properties due to their similar electron configurations. However, their reactions with fluorine differ due to their atomic sizes, electron affinities, and the availability of d-orbitals for bonding, leading to the formation of different fluorides.

1. Atomic Size and Electronegativity: Oxygen is smaller than sulfur and has a higher electronegativity. This makes oxygen less likely to share its electrons with many fluorine atoms due to repulsion between the closely packed electron clouds. Consequently, oxygen can only form OF_2, as adding more fluorine atoms would result in too much electron repulsion in the molecule.

2. Availability of d-Orbitals: Sulfur, being larger, can expand its octet because it has access to 3d orbitals in addition to the 3s and 3p orbitals. This allows sulfur to accommodate more than eight electrons around it, forming SF_2, SF_4, and SF_6 by utilizing its d-orbitals for bonding with fluorine atoms. Oxygen, on the other hand, does not have access to d-orbitals in its valence shell (being in the second period of the periodic table), limiting it to form only OF_2.

3. Bond Strength and Stability: The bond strength between oxygen and fluorine in OF_2 is quite high due to the small size and high electronegativity of both atoms, making the OF_2 molecule relatively stable with only two fluorine atoms. In contrast, sulfur can form stable molecules with two, four, or six fluorine atoms due to its ability to distribute the electron density over a larger volume and through the use of d-orbitals, making multiple stable configurations possible.

In summary, the differences in atomic size, electronegativity, and the availability of d-orbitals between oxygen and sulfur lead to their differing reactivities with fluorine, resulting in the formation of different fluorides.