ABSTRACT
Foundations for the modern understanding of the structure of matter were laid around the beginning of the 20th century. In 1897, J.J. Thomson discovered the electron. He concluded that the atom consists of negative and positive charge, and suggested the "plum pudding" model of the atom, in which the negatively charged electrons are embedded in a cloud of positive charge. Less than a quarter of a century later, in 1911, Ernest Rutherford discovered in an alpha particle scattering experiment that the positive charge of the atom is concentrated in a very small nucleus, whose size he found to be 10 000 times smaller than that of the atom. He correctly concluded that the electrons orbit the nucleus. However, he believed that they can assume any arbitrary energy. In 1913, Niels Bohr found that this was not correct, and suggested that the electrons in an atom can only take on discrete, well defined energy levels, which he called shells. Bohr's shell model enabled the understanding of the chemical, physical, and electrical properties of matter.
