47 . Oxoacids of halogens :
Fluorine forms only one oxoacid HOF ( Fluoric ( I ) acid or hypofluorous acid ) due to high electronegativity .
Acid strength : HOCl < HClO2 < HClO3 < HClO4 Reason : HClO4 → H + + ClO4 - most stable
Acid strength : HOF > HOCl > HOBr > HOI
This is because Fluorine is most electronegative .
GROUP 18 ELEMENTS :
48 . Ionisation enthalpy : They have very high ionization enthalpy because of completely filled orbitals .
Ionisation enthalpy decreases down the group because of increase in size .
49 . Atomic radii : Increases down the group because number of shells increases down the group .
50 . Electron gain enthalpy : They have large electron gain enthalpy because of stable electronic configuration .
51 . Melting and boiling point : Low melting and boiling point because only weak dispersion forces are present .
52 . XeF2 is linear , XeF4 is square planar and XeF6 is distorted octahedral . KrF2 is known but no true compound of He Ne and Ar are known .