According to Atomic Theory postulates that mass  has neither created nor destroyed in a chemical reaction . 
According to John Dalton's Atomic theory , all matter , whether an element , a compound or a mixture is composed of small  particles called atoms . 

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Tha atomic theory of matter was first proposed on a firm scientific basis bi john dalton , a british school teacher in 1808. His theory called Dalton's atomic theory , regarded the atom as the ultimate particle of matter.Although the origin of idea that matter is composed of small indivisible particles called 'a-tomio' (meaning -indivisible) , dates back to the time of Democritus , a Greek Philosopher (460 -370 BC),  it again started emerging as a result of several experimental studies which led to many other laws. In 1808 , dalton published a new system which contained all these postulates
-matter consists of indivisible atoms
-all the atoms   of a given element have identical properties including identical mass. Atoms of different elements differ in mass
- compounds are found when atoms of different elements combine in a fixed ratio
-chemical reactions involve reorganisation of atoms. These are neither created nor destroyed in a chemical reaction

dalton's theory could explain the laws of chemical combinations 

atomic theory

matter is composed of small particles called atoms.They are the smallest units and we can't divide further.