(i) (CH3)3P=O exists because phosphorous has vacant d- orbital and so it can extend its covalency beyond 4 but in case of (CH3)3N=O does not exist because nitrogen does not have vacant d- orbital and its covalency cannot become more than 4.
(ii) This is due to very small size of Oxygen atom and the large repulsion between electrons in relatively small 2p sub-shell in comparison to sulphur. So, oxygen has less negative electron gain enthalpy than sulphur
(iii) In H3PO2 there are 2 P–H bonds, whereas in H3PO3 there is 1 P–H bond. So, H3PO2 is a better reducing agent than H3PO3
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