external image 709px-Atom_of_Atheism-Zanaq.svg.pngDalton's Atomic Theory
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This picture is from: http://www.mayoclinic.org/tradition-heritage-artifacts/images/2-1-lg.jpg


"We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen" - John Dalton

In 400 B.C., Democritus, a Greek thinker, proposed the idea of the atom. He called these tiny, indivisible particles "atomos" which is Greek for "indivisible" or "uncuttable". Democritus had supporters but there was no solid evidence to support his idea and it was rejected by Aristotle. It wasn't until the early 1800's that the idea of the atom was finally put together into the first useful theory. An English schoolteacher named John Dalton wrote an atomic theory that was composed of four parts:
  • Every element is composed of extremely small, indivisible particles called atoms which are the smallest form of matter that still contained the identity of the element.
  • All of the atoms of an element are the same in all properties such as mass and the atoms of one element are different than atoms of other elements.
  • Chemical reactions can not change an atom of one element into an atom of another, atoms are neither created or destroyed in a chemical reaction and cannot be divided or broken down.
  • When atoms of more than one element combine, compounds are formed. A compound will always have the same number of atoms and the same kind of atoms.
Dalton worked with pressures and weights of different gases and his experiments led him to believe that the different gases' properties depended on tiny particles with varying weights. According to his theory, atoms are the smallest particles of an elements that retain the element's identity. Dalton was the first to attempt a table of atomic weights which was later modified by the Russian chemist Dmitri Mendelev to become the first version of the periodic table.

Evidence From Evaporation

John Dalton had a fascination with meterology and had successfully kept a weather diary until the day he died. He carefully studied rainfall and noticed that when the water evaporated, it occupied the same space as air. He was curious as to why gas and liquid were capable of inhabiting the same space while solid matter was not. Dalton hypothesized that the water and air were made of tiny particles that were being mixed together. He experimented, seeing what effect different properties of gas made on the whole mixture, and suggested that the particles of the different gases are different sizes. Dalton wrote, "...it became an object to determine the relative sizes and weights, together with the relative numbers of atoms entering into such combinations... Thus a train of investigation was laid for determining the number and weight of all chemical elementary particles which enter into any sort of combination one with another."


The Atomic Theory and Laws

Dalton's theory supports some simple laws that were known in his time such as the law of conservation of mass and the law of constant composition. The law of conservation of mass states that the mass of materials present at the beginning of a chemical reaction will be equal to the mass of materials present at the end. The law of constant composition states that in a compound, the relative numbers and kinds of atoms are always the same. Through his research, Dalton also came up with a new law based on his theory called the law of multiple proportions that states if two elements react and form more than one compound, the masses of one can that combine with a mass of the other are in the ratio of small whole numbers.

Changes in the Atomic Theory

Although John Dalton's theory was a significant step in science, some of his ideas were later proven incorrect. For example, subatomic particles were discovered after Dalton's time which disproved his idea that atoms were incapable of being divided. Also, we now know that different atoms of the same element do not always have the same masses. One problem with his theory that prevented it from being accepted by many scientists for several years was his suggestion that when atoms combine in just one ratio then it is binary. This idea led him to believe the formula for water was OH and ammonia was NH. He was unable to support this theory with sufficient evidence which left him with an incorrect atomic mass for both water and ammonia.

Resources:
http://antoine.frostburg.edu/chem/senese/101/atoms/dalton.shtml
http://www.learnchem.net/tutorials/atoms.shtml#Dalton
http://www.chemheritage.org/EducationalServices/webquest/dalton.htm
http://www.chemheritage.org/classroom/chemach/periodic/dalton.html
http://www.iun.edu/~cpanhd/C101webnotes/composition/dalton.html
http://www.aare.edu.au/02pap/har02049.htm