Democritus was a Greek philosopher that lived around the 460's. He was the first to formulate the atomic theory of the universe. His theory was that everything was made of tiny "pieces" called atoms. His believed that atoms were indestructible, and were always in motion. Yet, he thought that the shape of the atom modified the solidness of the material. He believed that the atoms interact mechanically, a theory that has been disproven today.
John Dalton was a Quaker English chemist and physicist in the mid 18 to mid 19th century. He is best known for his atomic theory. His theory includes that belief that atoms cannot be destroyed or created, that when atoms are combined they form a chemical compound, and what happens during a chemical reaction.
J.J. Thomson was an English physicist that is best known for his discovery of the electron, making him the first person to discover a subatomic particle.
Ernest Rutherford was a scientist from New Zealand sometimes known as the father of nuclear physics. He was the first person to theorize that atoms held their charge in a small particle in their center, also known as a "nucleus". His research allowed the first atom "splitting" to happen. He also discovered the proton.
Neils Bohr was a Swedish physicist and noble prize winner during the 20 century. He was the first to propose that electron circulate the nucleus, and can jump through orbits.
Originally it was believed that the nucleus was circled by the electron like the planets rotate about the sun. The electron cloud model states that the electrons are placed around in a cloud-like position. The model helps us visualize the most probable location of the electrons.