Introduction

Atoms are the smallest particles of ordinary matter. Every atom has a central core, or nucleus, of protons and neutrons that provide nearly all the atom's mass. Moving around the nucleus are the much lighter electrons, the same in number as the number of protons in the nucleus so that the atom as a whole is electrically neutral.

The chief properties (except mass) of atoms, molecules, solids, and liquids can be traced to the behavior of atomic electrons. But the atomic nucleus is also significant in the grand scheme of things. The continuing evolution of the universe is powered by energy that comes from nuclear reactions and transformations. Like other stars, the sun obtains its energy in this way. In turn, the coal, oil, and natural gas of the earth, as well as its wind and falling water, owe their energy contents to the sun's rays. Nuclear processes are responsible for the heat of the earth's interior and for the energy produced by nuclear reactors. Thus all the energy at our command has a nuclear origin, except for the energy of the tides, which are the result of the gravitational pull of the moon and sun on the waters of the world.