However, in the much more powerful nuclear reactions, these principles of separate conservation of mass and energy must be combined into the conservation of the total of mass and energy. Mass can be converted into energy and energy into mass according to Einstein's relationship E = mc, in which E is energy, m is mass, and c is the velocity of light. In the last half of this chapter we will discuss nuclear reactions for which this mass-energy conversion is important. The conversion of mass into energy is central to both nuclear fission and nuclear fusion, on this planet and in the sun.

In principle, if a reaction gives off energy, the products formed must have lower energy and be lighter than the reactants. But a release of 100 kcal by a typical chemical reaction corresponds (via the Einstein relationship) to a mass loss of only 5 x 10 amu per molecule, or one hundred thousandth the mass of an electron. This amounts to only 5 x 10 gram per mole, which is far less than we can measure. This is why we can say that, for chemical reactions, mass and energy are conserved independently.

Many properties other than mass are not conserved in chemical reactions: volume, density, shape, thermal conductivity, hardness, color, and others. It was Antoine Lavoisier, the brilliant French chemist who revolutionized chemistry before he went to the guillotine in 1794, who realized that mass was more fundamental than any of these properties. When you ask "How much?" in chemistry, you basically are asking "What mass?"