1. The ancient Greek philosophers Empedocles and Democritus proposed early atomic theories, believing that all matter was made up of indivisible particles called atoms. 2. In the early 1900s, scientists such as Rutherford, Thomson, and Chadwick discovered the internal structure of atoms through experiments, finding that atoms consist of a small, dense nucleus surrounded by electrons. 3. Niels Bohr contributed to the modern atomic model in 1913 by proposing that electrons orbit the nucleus in fixed shells or energy levels.