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atom /'ætem/ n. 1 a the smallest particle of a chemical element that can exist and still retain its characteristic chemical properties. ...

atomic theory 1 the theory that all matter is made up of fundamental particles called atoms. 2 the concept of an atom as being composed of subatomic particles.

The above definitions come from the Oxford Encyclopedic English Dictionary. Quite accurate statements. But there is much more to it than that. The Atomic Theory is the overall definition of "atom" within the field of science. It has been built upon from the time of ancient Greeks and is still being modified to this very minute. The information supplied by this page has been provided by the authors' learning over the years as students. Some of the information was referenced from chemistry and physics text books that can be found in the Individual Exploration section. The following covers the history of the atomic theory and many of its contributors.

By the year 400 B.C., the Greeks were convinced that matter consisted of four fundamental things: Earth, Fire, Water, and Air. This led them to ask the question, "Is matter infinite? Or is there a size and quantity that truly defines it?" Democritus of Abdera was one such philosopher. His theory was that matter was composed of little pieces. Leucippos, another Greek philosopher, later used the term "atomos" to describe this small piece of matter. The Greeks could not test these theories, so for the next 2000 years, no one had a clue as to which side of the question was correct.

By the 16th century, most alchemists became known as chemists. In the early 1600's, a person by the name of Robert Boyle wrote a book named The Skeptical Chymist. The book illustrated his idea of "elements". He simply defined them as being something that couldn't be broken down further, like what the Greeks had postulated thousands of years ago as "atomos". The Greek philosophy of the four main "elements" was set aside as the new set of elements was made.

The biggest breakthrough came from John Dalton in the 18th century. Dalton was the person to actually link the two theories of "atomos" (now "atoms"), and of elements into one. He reasoned that atoms might be what make up elements. And, that if atoms made elements, then compounds should always have the same number and configuration of atoms. Dalton published a paper known as A New System of Chemical Philosophy, which defined his view of the atom:

1. Each element is made up of atoms.
2. Atoms of an element are always the same.
3. Compounds are made when atoms combine with each other. Different samples of the same compound have the same percentage of each element.
4. Reactions involve the reorganization of atoms in space; no changes are made to the atoms themselves.

Dalton's experiments created a set of known properties of elements, including atomic masses. Later, more experiments were done by others and an idea that was born thousands of years ago came into its own.

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