John Dalton first proposed his atomic theory in a lecture he gave in 1803. His theory was based on the idea that the atoms of elements could be identified by the differences in the weight of the atoms. While there were errors in some of the ideas he proposed, his theory did provide an explanation of concepts and led the way for experimentation in new fields of study.
Dalton’s theory made use of several basic ideas:
1) All matter is composed of atoms. This was first suggested 2000 years before Dalton’s birth by Democritus and Leucippus.
2) All atoms of the same element are identical. It is now known that atoms of the same element may have different weights but have identical nuclear charges.
3) Different elements have atoms of different types. Compounds are formed from atoms of the constituent elements.
4) Atoms cannot destroyed or made. It was after Dalton’s time that the discovery of sub-atomic particles was made and it became known that atoms could be broken down into smaller parts.
5) When atoms are rearranged, chemical reactions occur
What makes Dalton’s theory and the ideas he proposed unique was that he was involved in meteorology studies at the time. Dalton questioned how water and air could occupy the same space at the same time. His end conclusion was that evaporated water is an independent gas in air. Dalton felt that evaporation could be thought of as the mixing of air particles with water particles.
Dalton performed experiments on different mixtures of gases in an effort to find what type of effect the properties of each individual gas had on the properties of the combined mixture of the gasses. His hypothesis was that the sizes of the particles that made up each different gas were different themselves.
Dalton was the first to associate atoms with stoichiometry. He was the first to recognize that the sum of the partial pressures of different components of a gas mixture would equal the total pressure of the gas mixture. He derived his Law of Partial Pressures from his work on the amount of water vapor that air, at different temperatures, could absorb.
The basic concepts of Dalton’s theory, that include the explanation of chemical reactions and the characteristic properties of atoms, are part of the foundation of physical science today. While his system of finding atomic weights was not as accurate as what we know today his theories have proven significant in the field of science.