When elements react, their atoms may combine in more than one whole-number ratio.Experiments that Dalton and others performed indicated that chemical reactions proceed according to atom to atom ratios which were precise and well-defined. In chemical reactions, atoms combine in small, whole-number ratios.However, atoms of different elements, such as oxygen and mercury, are different from each other. Therefore, every single atom of an element such as oxygen is identical to every other oxygen atom. Dalton suggested that all atoms of the same element have identical weights. Elements are characterized by the weight of their atoms.

Dalton based this hypothesis on the law of conservation of mass as stated by Antoine Lavoisier and others around 1785. Atoms of an element cannot be created, destroyed, divided into smaller pieces, or transformed into atoms of another element.

Atoms are indestructible and unchangeable.

Dalton and others imagined the atoms that composed all matter as tiny, solid spheres in various stages of motion.

All matter consists of tiny particles called atoms.

It was the English chemist, John Dalton, who put the pieces of the puzzle together and developed an atomic theory in 1803.ĭalton’s atomic theory contains five basic assumptions: They demonstrated that substances could combine to form new materials. Joseph Priestly, Antoine Lavoisier, and others set the stage for the foundation of chemistry. It was not until the late 1700’s that early chemists began to explain chemical behavior in terms of the atom. Early Atomic TheoryĪlthough the idea of the atom was first suggested by Democritus in the fourth century BC, his suppositions were not useful in explaining chemical phenomena, because there was no experimental evidence to support them. Although Democritus’ theory was remarkable, it was rejected by Aristotle, one of the most influential philosophers of Ancient Greece and the atomic theory was ignored for nearly 2,000 years. Therefore, changes in matter were a result of dissociations or combinations of the atoms as they moved throughout the void. In addition, Democritus believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other and during these collisions, could rebound or stick together. Democritus, theorized that atoms were specific to the material which they composed. He called these small pieces of matter “ atomos,” the Greek word for indivisible. Therefore, he reasoned that if the stone were to be continually cut into smaller and smaller pieces then at some point, there would be a piece which would be so small as to be indivisible. Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole. One of the first atomic theorists was Democritus, a Greek philosopher who lived in In this lesson, we will review the development of the atomic theory.

Atomic Structure, Periodicity, and Matter: Development of the Atomic Theory Objective

Democritus atomic theory model