How did John Dalton's ideas about atoms help to explain the differences between elements? How could JJ Thomson's atomic model be used to explain...

John Dalton

John Dalton proposed the first modern atomic theory. Much of his original theory is still valid today. Prior to Dalton, many scientists speculated that matter was made of tiny unseen particles (atoms), but had failed to suggest how differences in atoms might result in different elements. 

Dalton's theory consisted of the following postulates:

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  Matter is composed of indivisible atoms. This postulate was later shown to be incorrect when Thomson discovered that atoms contain even smaller particles (electrons).

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  The atoms of a particular element have the same properties and mass. Therefore, the atoms of different elements can be distinguished from one another based on differences in their properties and masses.
  

• Chemical reactions occur when the atoms of different elements combine to form new compounds. 

J.J. Thomson

Thomson's experiments with cathode ray tubes led to the discovery of the electron. This proved that Dalton's first postulate was incorrect. Thomson suggested that the atom was composed of diffuse positive charge with small negatively charged electrons dispersed throughout. Thomson's revised model of the atom did not introduce any new factors that would enable scientists to distinguish between different types of elements. 

Ernest Rutherford

Earnest Rutherford's experiments with gold foil and alpha particles led to the discovery that the positive charge in an atom is located in a small dense area called the nucleus. The positive charge in the nucleus was later determined to be from small positively charged particles (protons). It was later determined that the number of protons in an atom is unique for each different type of element.