dec 31, 1805 - John Dalton (1805)
Description:
All atoms of a given element are identical.The atoms of different elements vary in mass and size.
Atoms are indestructible. Chemical reactions may result in their rearrangement, but not their creation or destruction.
Dalton also outlined a law of multiple proportions, which described how reactants will combine in set ratios.
Added to timeline:
History of the Atomic Theory
Date: