Dalton's+Atomic+Theory

Early Atomic Theories
One of the earliest uses of the word "atom" was by Democritus, an ancient Greek philosopher, in 400 B.C. He used the greek word //atomos// to describe an unchangeable element.

The Hindu sage Kanada(circa 500 B.C.) also wrote an early attempt of atomic theory by describing the //paramanu//, an 'absolutely' small spherical particle that can group together to form matter.

The concept of the atom was not a popular theory; Aristotle's Four Elements theory was predominant. In fact, the next person to include the concept of the atom in a theory was John Dalton, who wrote his Atomic Theory in 1808.

Dalton's Atomic Theory
There were four major postulates to [|John Dalton's Atomic Theory]:
 * 1) Atoms are the building blocks of matter; they are extremely small particles, and the simplest forms of matter.
 * 2) All atoms of a given element are identical in chemical and physical properties.
 * 3) Atoms that chemically combine to form compounds will do so in fixed ratios (a.k.a. Law of Constant Composition)
 * 4) Atoms are neither created nor destroyed in chemical reactions (a.k.a. Law of Conservation of Matter)
 * Therefore, Dalton's model of an atom was essentially that of a small, simple sphere that contained no other parts.

Of course, this theory would be tested and there would be some modifications. Two postulates in particular would later be disproven, thanks to discoveries made in new experiments:


 * The first postulate states that atoms are the simplest forms of matter. This was later shown to be false, thanks to the discovery of radiation in the late 1890's, and by the studies done on electricity.


 * The second postulate would be disproven a few decades later than the first, when the discovery of neutrons and isotopes was made.

Go here for more on these two discoveries.