1. Atomic & Molecular Theory of Matter

2. History of the Atomic Theory
In the 5th century BC, the Greek philosopher Leucippus is believed to be the first to conceptualize an atomic theory. His student, Democritus is credited with working out a detailed application of the Theory.
Leucippus
Democritus

3. Democritus
“Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole. 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. He called these small pieces of matter "atomos," the Greek word for indivisible.”

4. John Dalton
Democritus's ideas about the atom did not explain why chemicals react the way they do. So his ideas were not very useful to science at the time. Chemists didn’t start describing chemical behavior in terms of the atom until the late 1700’s.
John Dalton, an English Chemist, finely put the pieces together to form a formal atomic theory in 1803.

5. Amadeo Avogadro's Theory
In 1811 Avogadro proposed a different theory from Dalton’s that could explain the combined volume of gasses.
His theory states that "equal volumes of gases at the same temperature and pressure contain the same number of molecules regardless of their chemical nature and physical properties".

6. Cannizaro's Compromise
Avogadro's Theory was not well accepted by the scientific community.
During the same time Dalton’s theory had sparked extensive experimentation particularly on the determination of combining the weights of elements. In the process many problems were found with Dalton’s theory. An acceptable modification of the theory could not be agreed upon by the scientific community.
In 1858 Stanislao Cannizaro suggested merging the theories of Avogadro and Dalton.

7. 5 Assumptions of Dalton’s Atomic Theory
All matter is made up of tiny particles called of atoms.
Atoms are Indestructible and Unchangeable. (Law of Conservation of Mass)
Elements are categorized by the weight of their atoms.
In chemical reactions atoms combine in small, whole number ratios.
When elements react, their atoms may combine in more than one whole number ratio.

8. Resources
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