1. John Dalton Brendan Kilgore , Mike Quinn, Jeff Adamek

2. The First Atomic Theory
One of the first atomic theorists was Democritus.
He knew that if a stone was divided in half, the two halves would have the same properties as the original stone.
He then reasoned that if the stone were to be continually cut into smaller pieces, at some point, there would be a piece which would be so small as to be indivisible.
He called these small pieces of matter “atomos”.
He later theorized, that 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.
His theory was remarkable, but was rejected by one of the most influential philosophers of Ancient Greece, Aristotle, and was ignored for nearly 2,000 years.

3. Dalton’s Theory
John Dalton was who put the pieces of the puzzle together and developed an atomic theory in 1803.
Dalton’s atomic theory contains five basic assumptions:
All matter consists of tiny particles called atoms.
Atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, divided into smaller pieces, or transformed into atoms of another.
Elements are characterized by the weight of their atoms. Dalton suggested that all atoms of the same element have identical weights.
In chemical reactions, atoms combine in small, whole-number ratios.
When elements react, their atoms may combine in more than one whole-number ratio.

4. Evidence that Developed his Theory
In the late 18th century, Joseph Priestly and Antoine Lavoisier performed experiments that later helped develop Dalton’s theory.
Joseph Priestly
Antoine Lavoisier

5. Priestly and Lavoisier’s Experiments
In 1773, Priestly heated the mineral red calx in his laboratory. Others had shown that calx, a red mineral, appeared to “turn into” the metal mercury. Priestly was the first to observe and collect a strange gas that was released from this process.
From Priestly’s experiment, Dalton theorized that some substances are composed of discrete amounts of two or more other substances.
In 1778, Lavoisier renamed Priestley’s gas “oxygen” and conducted a series of experiments in which he burned phlogiston in the presence of oxygen.
From Lavoisier’s phlogiston experiment, Dalton theorized the total mass of the products of a chemical reaction is exactly equal to the mass of reactants.
Priestly and Lavoisier, along with several other chemists, conducted experiments that would later develop Dalton’s theory.

6. Lavoisier’s New Atomic Model
The law of conservation of mass was formulated by Lavoisier as a result of his combustion experiment. This concluded that in any closed system subjected to no external forces, the mass is constant irrespective of its changes in form; the principle that matter cannot be created or destroyed.
Lavoiser’s Atomic Model

7. Interesting Facts
From his understanding of atoms, Dalton also published a listing of atomic weights for six different elements: hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorous.
In 1803, he theorized what has now become known as Dalton's Law: essentially it states that the total pressure of combined gases is equal to the partial pressures of each of those gases separately.
Dalton's interpretation of atomic theory maintained that atoms are combined in chemical reactions, but also that they can be separated and rearranged.
However, Dalton concluded that the atoms themselves could not be separated or created.

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