Consider the following reaction: #CaCO_3+SO_2 to CaSO_3+CO_2#. Here #SO_2# is oxidized to #CaSO_3#, so is #SO_2# a reducing agent or oxidizing agent? Kindly give your reason.
I have assigned oxidation numbers to all the elements involved and you can see that none are changed, so this is not a redox reaction.
However, this is not the case since you are, in effect, transferring an oxide ion to make the sulfate(IV) ion:
Sulfur remains +4.
The idea of adding/removing electrons and the use of oxidation numbers was introduced so that the concept of redox could be extended to other systems that did not involve oxygen or hydrogen.