In ancient times, philosophers and scientists did not have a clear view of the atom. From Democritus’s time to the 1800s, scientists were unable to explain basic chemical reactions or understand the elements. Then, came John Dalton, a schoolteacher and chemist, who sought to bring clarity to the science of compounds and chemical reactions. In so doing, Dalton became known as the father of modern atomic theory.
Pre-Dalton Atomic Views
Democritus of Abdera stated that all matter is made up of units that move around in a void. Democritus believed that these units are indivisible and unchangeable. He called these units atomos, which meant uncuttable in Greek. Aristotle disagreed with Democritus’ ideas about atoms. As a result, Democritus’ views were rejected and Aristotle’s view accepted. And, it was Aristotle’s idea that all matter was made up of four basic elements ( fire, air, earth and water) that prevailed through the Renaissance.
Centuries past before any new thoughts, regarding atoms and the make up of matter, took place. Iin the 1700s, the Law of Conservation of Mass was first presented by Antoine Lavoisier. Under this law, Lavoiser proved that in chemical reactions the mass of the combined agents was the same as the mass of the result. Then, came the work of Joseph Louis Proust, who divined the Law of Definite Proportions. Under this scientific law, Proust found proportions of the masses of elements composing a substance will always be equal.
Dalton’s Theory
John Dalton was intrigued by the work of Lavoiser and Proust. He often experimented with known elements oxygen, copper, and carbon. While working through chemical reactions, he confirmed Lavoisier and Proust’s findings, but also attempted to explain why their laws were correct.
Dalton’s experiments with copper oxides (red and black) and water brought him to some interesting conclusions about the nature of matter and elements. His experiments lead him to expand the thoughts on the Law of Definite Proportions. From his findings, Dalton came to the following conclusions:
1) All matter is made up of atoms, which are indivisible;
2) All atoms of a given element are identical with respect to weight (mass);
3) Atoms of different elements can be identified by their relative weights;
4) Atoms cannot be divided into smaller particles nor destroyed;
5) Atoms combine in ratios such as 1:1, 1:2, 1:3 and so on;
6) Compounds are formed by a combination of two or more different types of atoms;
7) A chemical reaction is a rearrangement of atoms
Significance of Dalton’s Theory
Dalton published his theory in the early 1800s. He subsequently published a list of known elements and assigned atomic masses. Modern atomic theories evolved from Dalton’s basic conclusions.
His atomic theory helped clarify unexplainable chemical phenomena and inspired scientists that came after him. And as such, noted genuises such as Chadwick, Mendeleev, Goldstein, Rutherford, Thomson, and Bohr made remarkable discoveries about the structure of atoms and helped develop a model of the atom that included subatomic particles. Dalton’s work laid the foundation for explaining unique chemical reactions such as radiation and the existence of isotopes. For Dalton’s contributions, he is heralded as the father of modern atomic theory and in his honor the unit of measure for atomic mass bears his name.
For more information on the subject of John Dalton or the history of atomic theory, check out these resources:
http://www.timelineindex.com/content/view/1228
http://www.compsoc.man.ac.uk/~lucky/Democritus/Basic/Atheory.html
http://www.universetoday.com/guide-to-space/physics/john-daltons-atomic-theory/
http://atomic-molecular-optical-physics.suite101.com/article.cfm/john_dalton_and_atomic_theory
http://www.visionlearning.com/library/module_viewer.php?mid=49
