Skill 7.2 Comparing historic models of the atom

The ideas that pure materials called elements existed and that these elements were composed of fundamentally indivisible units called atoms were proposed by ancient philosophers even though they had little evidence. 

Modern atomic theory is credited to the work of John Dalton published in 1803-1807. Observations made by him and others about the composition, properties, and reactions of many compounds led him to develop the following postulates  

1) Each element is composed of small particles called atoms. 

2) All atoms of a given element are identical in mass and other properties. 

3) Atoms of different elements have different masses and differ in other properties. 

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