LONG TIME ago it was thought that matter is made up of simple, indivisible particles.Atom was declared as a basic unit of matter.
LATE 17 CENTURY :-
In the late 17 centre,tge quantitative study of the composition of the pure substance disclosed that a few elements were the components of many different substances. It was also investigated that how elements combine to form compounds and how compounds could be broken down into their constitutent elements.
Jhon Dalton atomic theory:-
In 1808 an English school teacher, jhon dalton recognised the law of conservation of matter and law of definite proportions could be explained by existence of atoms. He developed a atomic theory.
All matter is composed of atoms of different elements, which differ in their properties.
HE AND NE have atoms which have independent existence while atoms of hydrogen and nitrogen do not exists independently.