Explain the following:
(i)    Most of the known noble gas compounds are those of Xenon.
(ii)    ClF₃ exists but FCl₃ does not.
(iii)    Among the hydrides of elements of Group 16, water shows unusual physical
properties.
(iv)    Unlike phosphorus, nitrogen shows little tendency for catenation.
(v)    Despite lower electron affinity, fluorine is a stronger oxidising agent than chlorine.

i) Only xenon is form compound in noble gas because energy level of the 5p orbital, 6s and 4f orbitals are very close and hence energyetically accessible.
Only a small amount of energy is needed to excite the pair of electron form the 5p orbital to the 65 and 4f electrons where they can then form covalent bonds with other atom.

ii) The electronic configuration of chlorine atom 1s² 2s² 3s² 3p⁶ 3d⁰ it has vacant d orbitals it, therefore, can expand it valence shell and show +3 oxidation state, Hence ClF₃ is formed. 
On other hand, flourine atom 1s² 2s² 2p⁵ has no d - orbital and, therefore, it can not expand its valence shell. It only shows -1 oxidation state. Hence FCl₃ does not exist.

iii) The oxygen has high electronegativity. Hence the O-H bond in H₂O forms strong intermolecular hydrogen bonds. As a result, water exist as an associated molecule. But the other hydrides of group 16 do not form hydrogen bonds. Therefore they exist as discrete molecules. Thus, water shows unusual physical properties such as high thermal stability, high b.p., weaker acidic character as compared to the other hydrides of group 16.

iv) Nitrogen is small in size and have unique ability to form multiple bonding; single N–N bond is weaker than the single P–P bond because of high interelectronic repulsion of the non-bonding electrons, owing to the small bond length. As a result the catenation tendency is weaker in nitrogen.

v) Halogens have high electronegativity and electron affinity. They have greater tendency to accept electrons or easily reduced, therefore they are strong oxidizing agent.