ATOMIC THEORY OF MATTER - BASIC INFORMATION AND TUTORIALS

Philosophers from the earliest times speculated about the nature of the fundamental “stuff” from which the world is made. Democritus (460–370 BC) and other early Greek philosophers described the material world as made up of tiny indivisible particles they called atomos, meaning “indivisible or uncuttable.”

Later, however, Plato and Aristotle formulated the notion that there can be no ultimately indivisible particles, and the “atomic” view of matter faded for many centuries during which Aristotelean philosophy dominated Western culture.

The notion of atoms reemerged in Europe during the seventeenth century. As chemists learned to measure the amounts of elements that reacted with one another to form new substances, the ground was laid for an atomic theory that linked the idea of elements with the idea of atoms.

That theory came from the work of John Dalton during the period from 1803 to 1807. Dalton’s atomic theory was based on the four postulates:

1. Each element is composed of extremely small particles called atoms.

2. All atoms of a given element are identical, but the atoms of one element are different from the atoms of all other elements.

3. Atoms of one element cannot be changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.

4. Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

Dalton’s theory explains several laws of chemical combination that were known during his time, including the law of constant composition, based on postulate 4:

In a given compound, the relative numbers and kinds of atoms are constant. It also explains the law of conservation of mass, based on postulate 3:

The total mass of materials present after a chemical reaction is the same as the total mass present before the reaction.

A good theory explains known facts and predicts new ones. Dalton used his theory to deduce the law of multiple proportions:

If two elements A and B combine to form more than one compound, the masses of B that can combine with a given mass of A are in the ratio of small whole numbers.

We can illustrate this law by considering water and hydrogen peroxide, both of which consist of the elements hydrogen and oxygen. In forming water, 8.0 g of oxygen combine with 1.0 g of hydrogen. In forming hydrogen peroxide, 16.0 g of oxygen combine with 1.0 g of hydrogen.

Thus, the ratio of the mass of oxygen per gram of hydrogen in the two compounds is 2:1. Using Dalton’s atomic theory, we conclude that hydrogen peroxide contains twice as many atoms of oxygen per hydrogen atom as does water.

John Dalton (1766–1844), the son of a poor English weaver, began teaching at age 12. He spent most of his years in Manchester, where he taught both grammar school and college. His lifelong interest in meteorology led him to study gases, then chemistry, and eventually atomic theory. Despite his humble beginnings, Dalton gained a strong scientific reputation during his lifetime.