(i) Why is the first ionisation enthalpy of helium maximum among all the elements?
(ii) Arrange thegiven compounds in terms of ascending order of oxidising property: HCl, HBr, HI, HF.

Explanation

(i) Configuration of He is `1s^(2)`, all electrons of He are present in `1s`-orbital. These 1s electrons are attracted very strongly by the nucleus. Further more there are no inner electronic orbitals to shield thse electrons from the pull of the nucleus. so removal of an electron from 1s-orbital requires a large amount of energy. there-fore, first ionisation enthalpy of He is maximum.
(ii) Hydrogen halides do not show oxidisng property. however, the reducing power of the hydrogen halides follow the sequence: `HF lt HC l lt HB r lt HI`.