The law of conservation of matter takes center stage in Dalton's Atomic Theory and says that when a chemical reaction takes place, matter is neither created nor destroyed. It implies that when a chemical reaction takes place, atoms are simply rearranged.
The law of constant composition, Dalton's second law, notes that multiple samples of any pure chemical compound always contain the same percent by mass of each element making up the compound. Dalton also viewed this law as proof that elements existed in the form of atoms.
Using the basis of the laws of conservation of matter and constant composition, Dalton developed an atomic theory delineating five basic laws, of which the remaining three decry that atoms of a particular size are alike and atoms of different elements are different from one another. Finally, his theory indicates that a chemical reaction involves neither the union nor the separation of individual atoms.