Reduction reactions are characterized by a gain of electrons whereas oxidation reactions are characterized by a loss of electrons. Recall that elements on the right side of the periodic table have the tendency to gain electrons to complete the octet. Groups such as chalcogens (group VI) and halogens (group VII) have six and seven valence electrons, respectively; therefore, they only need a few electrons to have eight valence electrons and complete the octet. Since halogens tend to gain electrons, they typically undergo reduction.

Alkaline earth metals (group II) and transition metals have few valence electrons; therefore, they tend to lose electrons and undergo oxidation. Noble gases are nonreactive elements and do not undergo reduction or oxidation.

For this problem, we need to figure out the oxidation states of manganese in each compound. Oxygen typically has an oxidation number of  and potassium has an oxidation number of . For Compound A, there are two oxygen molecules; therefore, oxygen contributes a total charge of . Since manganese dioxide is a neutral molecule, the manganese in it will have an oxidation number of . For Compound B, four oxygen atoms contribute a charge of  whereas one potassium atom contributes a charge of ; therefore, the manganese in potassium permanganate will have an oxidation number of . Conversion of manganese from Compound A to manganese in Compound B involves the removal of electrons—three electrons, specifically, because its charge has to change from  to ; therefore, this reaction is characterized as an oxidation reaction.

A reaction that has both reduction and oxidation half reaction is called a redox reaction. It involves one or more atoms gaining electrons (reduction) and one or more atoms losing electrons (oxidation). Recall that single displacement reactions involve the replacement of an element in a compound with another element. An example of single replacement reaction is shown below:

In this reaction, a sodium atom replaces a calcium atom in calcium sulfate. If we calculate the oxidation numbers for sodium and calcium, we can see that sodium loses an electron (the oxidation state goes from  to ) whereas the calcium ion gains two electrons (goes from  to ); therefore, this is a redox reaction. All single displacement reactions follow this general trend and are characterized as redox reactions.

Reaction of sodium chloride and calcium sulfate is as follows:

If we calculate the oxidation state of each atom we will notice that oxidation number doesn’t change; therefore, this isn’t a redox reaction.

Oxidation involves a loss of electrons whereas reduction involves a gain of electrons. This means that oxidation generates electrons as products whereas reduction consumes electrons as reactants. Note that oxidation and reduction reactions can be distinguished by looking at oxidation states. If the oxidation state of an atom becomes more positive, then it underwent an oxidation reaction, and if the oxidation state becomes more negative, then that atom underwent a reduction reaction.