A third, rather subtle quantity also is conserved during propane combustion. This quantity, the oxidation number, is a measure of the location of the electrons.

Carbon and hydrogen atoms are oxidized, because they begin by sharing electrons equally with neighboring atoms, but end by forming CO and HO bonds in which oxygen exerts the greater pull on the electrons.

Conversely, oxygen atoms are reduced because they begin by sharing electrons equally in O=O molecules and end by monopolizing electrons in their bonds with C and H. The sum of changes in oxidation numbers of all the atoms in propane combustion is zero, because every atom that loses its grip on an electron must be matched by another atom that pulls the electron toward it. In this chapter we consider the conservation of mass and oxidation number; the following chapters are devoted to energy.

If we look at mass and energy closely enough, the principles that they individually are conserved turn out to be only approximately true. Mass and energy actually are interconvertible, and are different manifestations of the same thing. We can uncouple them in thinking about chemical reactions only because the quantities of energy involved in chemical processes correspond to infinitesimal amounts of mass.