In both nuclear and chemical reactions, two physical quantities are seen to be conserved and unchanging: the number of particles and the total charge. A constant number of particles in nuclear reactions does not imply that mass is conserved. Three nuclei and one nucleus each have twelve nucleons (protons and neutrons), yet the three helium nuclei weigh 12.0078 amu, whereas the carbon nucleus weighs only 12.0000 amu. The difference represents the conversion of some of the mass into energy. In contrast, the energies involved in ordinary chemical reactions are so small that mass-energy conversions can be neglected, so equal numbers of atoms of each kind before and after a reaction takes place do imply conservation of mass.

Charge conservation in nuclear reactions means that no net positive or negative charge is created in the course of the reaction. If a positive charge appears somewhere, either another positive charge must disappear somewhere else, or a negative charge must appear to counterbalance it. For chemical reactions, charge conservation means that electrons in molecules are neither created nor destroyed. Whenever an electron moves away from one atom during an oxidation, it must move closer to another atom, which therefore is reduced. Conservation of oxidation number in chemical reactions is analogous to conservation of particle charge in nuclear reactions. In this chapter we have seen two quantities that always are conserved in chemical reactions: mass and oxidation number. In the next chapter we shall see a third and very important conservation principle: the conservation of energy.