Things are simpler in such a world. The same pieces that make up all atoms - protons, neutrons, and electrons - also make up hydrogen and helium, but in an especially simple way. In the following chapter, we will begin the study of atomic structure with a detailed discussion of hydrogen and helium. The reactions that these elements by themselves can undergo are simple and few. Four hydrogen atoms can fuse to make a helium atom, and the stars are fueled by the energy from this reaction. If the temperature at the center of a star is high enough, hydrogen fusion can be followed by helium fusion, and successive reactions, to produce the heavier elements. The heaviest of these elements have a tendency to break down again spontaneously, in the process of atomic fission.

These examples all are nuclear reactions, in which one element is changed into another element by altering the structure of its nucleus. Nuclear reactions ordinarily are considered as part of the realm of physics, not chemistry. At far lower temperatures, closer to those of our own planet, the first true chemical reactions can take place, in which atoms come together, separate, and associate with other atoms, without altering their nuclear structures and their own identities.