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The ordinary variety of matter that is dealt with in everyday experience consists of discrete units called atoms. The name comes from the Greek atomos, meaning indivisible, since these were thought to represent fundamental building blocks of the universe. The existance of such things was first proposed by Greek philosophers such as Democritus, Leucippus, and the Epicureans, but without any real way to be sure the concept disappeared until it was revived by Dalton. The atomic theory explains why gasses always combine in simple ratios. In modern times atoms have been observed experimentally.

As it turns out, atoms are themselves made out of smaller particles. In fact, almost all of an atom is empty space. At the center is a tiny positive nucleus composed of nucleons (protons and neutrons), and the rest of the atom contains only the fairly flexible electron shells. Usually atoms are electrically neutral with as many electrons as protons (the atomic number); the electrons determine the chemistry of the atom. The different kinds of atoms are listed on the Periodic table.

Atoms that are not electrically neutral -- that is, possessing a net charge -- are called ions. Atoms join together to form larger structures through a process called bonding. Various types of bonds exist, including ionic bonds (which form radicals), covalent bonds (which form molecules), and hydrogen bonds, which are found in solutions.

Models of the atom:

The simplest atom is the Hydrogen atom, which has been the subject of much interest in scence, particularly in the early development of quantum theory.