the single bond enthalpy of oxygen and sulphur are +142 and +264 respectively, why oxygen atom has a much smaller single bond enthalpy than sulphur atom ?may anyone please expalin the large difference?

I am confused by your question. The bond enthalpies of elements vary depending on what other element the atom bonds to. For example, the enthalpy (i.e. bond strength) of an O-H bond is going to be different than an O-C bond.

 

Also, oxygen should have a stronger bond enthalpy than sulfur because it has a much smaller radius than sulfur.

 

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