The document traces the development of atomic theory over time from ancient Greek philosophers to modern models. It describes Democritus' idea that matter is made of indivisible particles called "atomos", Dalton's atomic theory of elements composed of atoms, J.J. Thomson's "plum pudding" model showing atoms contain smaller particles, Rutherford's gold foil experiment proving atoms have a small, dense nucleus, Bohr's model of electrons in specific energy levels orbiting the nucleus, and the modern wave model showing electrons as probability clouds rather than definite orbits.