Many people helped develop the theory of atoms. The first was a Greek named Democritus in the 4thcentury BC. He proposed that all matter was made of tiny particles. Democritus thought that empty space filled the area between the solid atoms. That concept of the atom was unchanged for centuries. Then an English scientist, John Dalton, proposed an atomic theory in 1803. Atomic theory says that atoms are solid with an atomic mass. They can react because of the forces of attraction and repulsion. In 1873, James Clerk Maxwell proposed that electric and magnetic fields filled the empty space in atoms. In paragraph 2, attraction and repulsion refer to . electric and magnetic fields. atomic theories. tiny particles. solid atoms