is everything else around you. An atom is an exceedingly tiny thing: 200 million atoms lying side by side would span a distance of only one inch.

    As late as the 19th century, many leading scientists of the day thought that atoms were indivisible blobs of matter, sort of like tiny solid balls. Now we know that atoms are far more complex. We also know that under the right conditions certain atoms can be split into smaller atoms.

    An easy way to picture an atom is to think of it as a miniature solar system. In the center (somewhat like our sun) is a relatively large structure called the nucleus. Whirling around the nucleus (somewhat like the planets) are tiny particles called electrons. Each electron carries a negative electric charge.

    Atoms of one material differ from atoms of another material because of the make-up of their nuclei and the number of electrons they have.

   The nucleus of most atoms consists of two kinds of particles: protons and neutrons. Both the proton and neutron have about the same size and weight. However the proton carries a positive electric charge, while the neutron has no charge at all.