(a) Account for the following :
(i) Helium is used in diving apparatus.
(ii) Fluorine does not exhibit positive oxidation state.
(iii) Oxygen shows catenation behaviour less than sulphur.
(b) Draw the structures of the following molecules.
(i) `XeF_2` (ii) `H_2S_2O_8`

Answer

(a)(i)Due to small size of fluorine atom, there are strong interelectronic repulsions in the relatively small 2p-orbitals of fluorine and thus, the incoming electron does not experience much attraction.
(ii)unlike `NH_(3),PH_(3)` molecules are not associated through hydrogen bonding in liquid state. That is why the boiling point of `PH_(3)` is lower than `NH_(3)`.
(b)(i)
`(##RES_INO_CHM_XII_V02_C03_E01_187_A01##)`
OR
(a)(i)A mixture of helium and oxgen does not cause pain due to very low solubility of helium in blood as compared to nitrogen.
(ii)The fluorine atom has no d-orbitals in its valence shell and therefore cannot expand its octet.Being the most electronegative, it exhibits only-1 oxidation state.
(iii)The single O-O bond is weaker than the single S-S 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 oxygen.
(b)(i)
`(##RES_INO_CHM_XII_V02_C03_E01_187_A02##)`