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.