The original atomic theory was proposed in the 5th century BCE by the Greek philosopher Democritus, who speculated that everything in the world is ultimately composed of combinations of small, hard, and indivisible particles. He called the particles atoms, and suggested that they came in various shapes and sizes but were all made of the same basic material.The modern scientific theory of matter states that the great variety of substances we see in the Universe are made from combinations of different chemical elements. These elements do indeed consist of trillions of identical sub-units, or atoms. The internal structure of an atom is specific to each element and gives that element its particular properties and characteristics. Thus, a hydrogen atom is constructed differently from an atom of gold.
Modern atomic theory kicked off at the beginning of the 19th century with the work of the English chemist John Dalton. However, it was not until 1905 that Einstein proved the existence of atoms mathematically in his famous paper on Brownian motion. A few years later Ernest Rutherford was the first to look inside atoms in an experiment in which he bombarded a thin sheet of gold with alpha particles. He discovered that every atom consists of a tiny, positively charged nucleus surrounded by empty space in which even tinier, negative electrons orbit around the nucleus.by Jim Al-Khalili, 30 Second Theories