There is no change in the number of valence electrons on any of the atoms in this reaction. Both before and after the reaction, each carbon atom shares a total of eight valence electrons and each hydrogen atom shares two electrons. Instead of electrons, this reaction involves the transfer of atoms in this case, hydrogen atoms. There are so many atom-transfer reactions that chemists developed the concept of oxidation number to extend the idea of oxidation and reduction to reactions in which electrons aren't necessarily gained or lost. Oxidation involves an increase in the oxidation number of an atom.
Reduction occurs when the oxidation number of an atom decreases .
During the transformation of ethene into ethane, there is a decrease in the oxidation number of the carbon atom. This reaction therefore involves the reduction of ethene to ethane. Reactions in which none of the atoms undergo a change in oxidation number are called metathesis reactions . Consider the reaction between a carboxylic acid and an amine, for example. Or the reaction between an alcohol and hydrogen bromide.