Atom comes from the Greek work 'atomos,' meaning indivisible. An atom is thus the smallest possible piece of an element that retains the chemical properties of that element. For example, if a piece of aluminum was cut over and over again into smaller and smaller pieces, eventually it could be divided no further and still be aluminum. At that point, a single atom would remain. Actually, the atom actually could be split into smaller pieces, but those pieces would no longer retain the properties of aluminum.
The pieces that comprise an atom are called protons, neutrons, and electrons. The protons are tiny, positively-charged particles that give the atom its identity. That is, the type of the atom is determined by the number of protons it contains (e.g., each aluminum atom has 13 protons). The neutrons are very similar to protons, but have no charge. The protons and neutrons are located together in the tiny, dense core of the atom, called the nucleus. Orbiting the nucleus are even smaller particles called electrons, which have a negative charge.
Nuclear reactions involve changes in the nucleus (protons and neutrons) of atoms, while chemical reactions only involve interactions between the electrons of different atoms.
[Article 65 by musculus, 2001-02-20 | Review this article]