Around 400 BC, Democritus, a Greek philosopher, suggested the first recorded theory on the atom. He thought that all things were “composed of minute, invisible, indestructible particles of pure matter which move about eternally in infinite empty.” From Democritus’ time to the early 1800s, scientists were unable to explain basic chemical reactions or understand the elements. Then, came John Dalton, a schoolteacher and chemist, to bring clarity to the science of chemistry.
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. These units he called atomos, which meant uncuttable in Greek. This was not a unique theory in that other world cultures, like those in Asia believed the same, such as India’s Kanada. It is also an idea with which Aristotle disagreed. Aristotle’s view, which prevailed through the Renaissance, was that all matter was made up of four basic elements ( fire, air, earth and water.)
Centuries past before any new thoughts, regarding atoms and the make up of matter, took place. But, in the 1700s, the law of Conservation of Mass was first presented by Antoine Lavoisier. He revealed that relative to 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 law, Proust found proportions of the masses of elements composing this substance will always remain equal.
John Dalton was intrigued by the work of Lavoiser and Proust. He often experimented with known elements oxygen, cooper, and carbon. While working through chemical reactions, he confirmed Lavoisier and Proust’s findings, but also attempted to explain the why for the what.
Dalton’s experiments with cooper 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 Law of Definite Proportions which states that when two elements combine in more than one way.
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 Theories
Dalton published his theories in the early 1800s. He subsequently published a list of known elements and assigned atomic masses. Modern atomic theories evolved from Dalton’s basic theories.
Scientists such as Chadwick, Mendeleev, Goldstein, Rutherford and Thomson discovered the subatomic particles and helped unraveled the mysteries of the atomic model. His theories helped clarify unexplainable chemical phenomena. While the knowledge of atomic particles has grown to include a model of the atom and exploration of subatomic forces, Dalton’s premises remain substantively valid. For his contributions to the field of modern chemistry, the basic unit for measuring atomic mass is named for him.
For more information on the subject of John Dalton or the history of atomic theory, check out these resources: