In a chemical reaction, only the planetary electrons of an atom are affected. The nucleus does not take part. In the formation of sodium chloride (common salt) from metallic sodium and gaseous chlorine, for example, the single electron in the outermost shell of the sodium atom is transferred to the outermost shell of the chlorine atom. Chemical energy in the form of heat is liberated, and the sodium and chlorine atoms join together.

Chemical reactions produce heat, light, and electrical energy. Nuclear energy can be released only through bombardment by subatomic particles, through the application of extremely high temperatures, or (in small amounts) by radioactivity.

For books about the atom, See book list at the end of Nuclear Energy.