More than 1 million atoms lined up side by side would equal the thickness of a book page. The modern atomic theory was proposed in 1803 by English chemist John Dalton. His premise is based on the fact that all elements are composed of atoms. An atom is defined as the smallest part of an element. It also keeps the identity of the element. Individual atoms are very small. Most elements in their pure form exist as individual atoms. Some elements are made up of groups of atoms.