About 2,400 years ago, the Greek philosopher Plato accepted the notion that all matter is made up of four primary substances: earth, air, fire, and water.
A near-contemporary of his, Democritus, believed that these substances were in turn made up of smaller units that were both invisible and indivisible, they could not be broken down further. He called these units atoms.
Centuries of painstaking work lying between his time and ours has confirmed that matter is indeed composed of tiny pieces of matter, which we still call atoms. But these atoms are not indivisible, as Democritus thought. Rather, they are themselves composed of constituents parts.
There are three important constituent parts of an atom: protons, neutrons, and electrons. Protons and neutrons are packed tightly together in a core (the atom's nucleus), and electrons move around this core some distance away.
Protons are positively charged and electrons are negatively charged. Neutrons have no charge, but are electrically neutral.
Then, what is matter? Matter is any substance that exists in our everyday experience. For example, the iron that goes into cars is matter. An iron atom has 26 protons in its nucleus, while a gold atom has 79.
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