In ancient Greek the word atom meant the smallest indivisible particle that could be conceived. The atom was thought of as indestructible; in fact, the Greek word for atom means "not divisible." Knowledge about the size and make up of the atom grew very slowly as scientific theory progressed. What we know/theorize about the atom now began with a core theory devised by Democrotus, a Greek philosopher who proposed that matter consisted of various types of tiny discrete particles and that the properties of matter were determined by the properties of these particles. This core theory was then modified and altered over years by Dalton, Thompson, Rutherford, Bhor, and Chadwick. The atoms original structure was simple, but as more and more research was done the atom became more complex and puzzling
             The five atomic theories of the past two centuries represent the sudden advancement of science in modern times. Beginning with a basic theory on the behavior of atoms to the current model, some changes have been made, and some ideas are still the same. Ancient Greek philosophers believed that everything was made up of invisible particles called atoms. Since then the theory of atoms did not progress until 1803.
             John Dalton was the first scientist to compose a theory of matter based on atoms. Dalton's atomic theory is based on four concepts. He stated:
             "1. All elements are composed of atoms, which are indivisible and
             2. All atoms of the same element are exactly alike; in particular, they
             3. Atoms of different elements are different; in particular, they have
             4. Compounds are formed by the joining of atoms of two or more
             All of Dalton's ideas account for the laws of definite and multiple proportions and the law of conservation of mass. Some of Dalton's points are still thought to be true, but over time this original theory has been modified.
             The first of these modifications came in 1897 when J.J. Thomson discovered ...