Merlin's Help in  Chemistry

Dalton's Atomic Theory

Democritus was a Greek philospher and in 400 B.C. he proposed the first atomic theory. In this he stated that all matter is made up of empty space and tiny particles called atoms. He believed that atoms are the smallest bit of matter and there are different types. From that time on scientist tried to figure out this tiny thing called the atom. One of the most-well known is John Dalton.

John Dalton(1766-1844) was an English schoolteacher who studied and taught science beginning at the age of twelve. Dalton intensely studied compounds and elements that made them up all his life. During his research Dalton proposed several principles and theory about atoms and the properties of compounds. The one that Dalton is best known for is his atomic theory. Dalton first published his atomic theory in 1809.

 Here is the four parts to Dalton's atomic theory:

  1. Each element is made up of small particles called atoms.
  2. All atoms of the same element are identical; All atoms of different elements are different.
  3. Compounds are formed when atoms combine with each other. A given compound will always have the same number and type of element.
  4. Chemical reactions involve the reorganization of atoms not changes to the atoms themselves.

Dalton is also credited for preparing the first table of atomic masses. While working with the masses of elements Dalton also discovered that if two or more different compounds are composed of the same two elements, the mass of the second element combine with a certain mass of the first element can be expressed as ratios of small whole numbers. This became known as the law of multiple proportions.

Even though many of Dalton's masses for the atoms on his first table were wrong it was definitely a step in the right direction for the science world. In 1809 Dalton's work was expanded upon by a French chemist by the name of Joseph Gay-Lussac(1778-1850) and then later by an Italian chemist named Amadeo Avogadro(1776-1856).

In 1809 Gay-Lussac performed experiments in which he measured the volumes of gases that reacted with each other. He found that 2 volumes of hydrogen react with 1 volume of oxygen to form 2 volumes of water and that 1 volume of hydrogen reacts with 1 volume of chlorine to form 2 volumes of hydrogen chloride. Here is a schematic that represents these results.

 

In 1811 Avogadro summarized Gay-Lussac's experiments by proposing that at the same temperature and pressure, equal volumes of different gases contain the same number of particles.

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This web page was written by Merlin A.W. Abbott 1998.