The total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom is known as the Oxidation Number.
In H2SO3
Let the oxidation number of S be x.
2(+1)+x+3(−2)=0x=+4
Similarly the +4oxidation state of sulphur is shown by SOCl2andSF4.
Compounds O.S.of Sulphur: SO3+6H2SO3+4SOCl2+4SF4+4BaSO4+6H2S2O7+6