Account for the following:(i) Helium is used in diving apparatus.(ii) Flourine does not exhibit positive oxidation state.(iii) Oxygen shows catenation behaviour less than sulphur.

i. The main reason to use helium in the deep sea diving breathing mix (Heliox, 0-100% He) is its very low solubility in body fluids. At high pressure, nitrogen (in normal air) dissolves in the blood and tissues. When the diver ascends, this nitrogen comes out of solution, forming bubbles that can cause the painful and dangerous condition known as decompression sickness ('the bends'). Helium's low solubility means that significantly less gas dissolves in the blood and tissues at high pressures, reducing the risk of decompression sickness. Its low density also reduces the work of breathing at depth.

ii. Fluorine is the most electronegative element. It has a very strong tendency to gain an electron to achieve a stable octet, resulting in a -1 oxidation state. It lacks low-lying vacant orbitals to accommodate electrons in higher oxidation states. The energy required to remove electrons from fluorine is extremely high, making it unlikely to exhibit positive oxidation states.

iii. Catenation is the ability of an atom to form bonds with other atoms of the same element to create chains or rings. Oxygen's ability to catenate is limited compared to sulfur due to several factors:
* Bond Strength: The O-O bond (in peroxide) is weaker than the S-S bond. This is attributed to stronger electron-electron repulsions in the smaller oxygen atom. The weaker O-O bond makes oxygen less likely to form long chains.
* Bond Angle: The small size of oxygen leads to a much smaller O-O-O bond angle, increasing electron-electron repulsion and further destabilizing longer chains. The larger size of sulfur allows for a larger S-S-S angle, reducing repulsion and making longer chains more stable.
* d-orbital Participation: While not significant for sulfur, sulfur's larger size allows for increased participation of d-orbitals in bonding which enhances its catenation ability. Oxygen does not readily use d-orbitals in bonding.
In essence, the weaker O-O bond, smaller bond angle leading to greater electron repulsion, and less d-orbital participation combine to greatly limit oxygen's catenation compared to sulfur.