The idea that everything is made of atoms was firstly introduced by John Dalton in his book which he published in the year 1808. He was an English chemist and physicist He is referred to as the "father" of atomic theory. It was speculated by him that all atoms of an element are identical in size and mass. It was reasoned by him that elements were composed of smaller atoms. He told that the mass of one atom of hydrogen (H) is the atomic mass unit. The unit became the Dalton.