The understanding of the nature of matter which is called the atomic theory of matter , first postulated by John Dalton , is the basis of all modern chemistry. The observations which Dalton's theory explains had in many cases been made long before his formulation of the theory, and it was these observations and empirical relationships which led him to it. All of the atoms of a given element are identical. The atoms of one element are different from the atoms of any other element in some fundamental way.
Atoms combine with each other to form compounds .
A given compound always has the same relative numbers of different types of atoms. The atomic theory of matter holds that atoms are the fundamental units of matter and that atoms are conserved in chemical reactions. In other words, chemical reactions consist of rearrangements of atoms to form compounds while the atoms themselves remain unchanged. The law of conservation of total mass is a consequence of the atomic theory of matter . Since the numbers of atoms are not changed in any chemical reaction, or in other words a chemical reaction is the rearrangement of atoms to form different molecules, atoms are conserved both in number and in type whenever a chemical reaction occurs.