(a) In ozone, the 3 O-atoms are arranged to form a bent shaped structure where the central O-atom makes a single bond with one of the terminal O and a double bond with the other terminal O.
But the electrons of double bond are delocalized over all the three oxygen atoms. So due to resonance, the single and the double bond are not entirely pure and both the bonds have partial double bond character which makes their length identical.
(b) If the metal exhibit more than one oxidation state then the compound in which metal is in higher oxidation state will be more covalent than the one in lower oxidation state because more the oxidation state of a metal, smaller will be its size and more will be the polarizing power which means more covalent character.
In PbCl4 and PbCl2 the charge on Pb is +4 and +2 respectively. Therefore, Pb in PbCl4 will polarize Cl ion to higher extent and will be more covalent than PbCl2.
Xenon does not form fluorides such as XeF3 and XeF5 because Xe has a complete filled 5p configuration - [Kr] 4d105s25p6
If it shows bonding with odd number of F-atoms such as 3 or 5 then one unpaired electron will be left.
For e.g. if Xe bonds with 3F atoms then 1 lone pair and 1 unpaired electron on Xe will be left and that unpaired electron causes the molecule to become unstable. As a result XeF3 and XeF5 do not exist.
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