Atoms are so tiny that it is hard to imagine how tiny they are. The ink in the period at the end of this sentence has more atoms in it than there are people in the whole world. In a thimbleful of air there are more atoms than you could count if you lived to be a million years old. Of course, atoms are too small to be seen even with powerful microscopes. We know about them only from the way they act.
There can be millions of different substances because atoms of different kinds can join together in different ways. Atoms of oxygen and atoms of hydrogen, for instance, can join to form water. They can
join in different proportions to form hydrogen peroxide.
Scientists have a way of writing the names of substances so that it is easy to tell what kinds of atoms the substances have in them and how the numbers of the different kinds of atoms compare. The scientists' way of writing water is H2O. H stands for hydrogen, O for oxygen. The 2 shows that there are two atoms of hydrogen for every atom of oxygen. Hydrogen peroxide is written H2O2.
Atoms are so small that it is almost unbelievable that anything could be smaller. But atoms are made up of even smaller particles. Every atom has a center, or nucleus. The nucleus of an atom always has in it one or more particles called protons. In the case of every element except hydrogen it has particles called neutrons in it, too. Traveling around the nucleus there are one or more tiny particles called electrons.
