Electrons and nuclei

 The familiar planetary model of the atom was proposed by Rutherford in 1912 following experiments by Geiger and Marsden showing that all the mass of an atom was concentrated in a positively charged nucleus. Negatively charged electrons are attracted to the nucleus by the electrostatic force and were considered by Rutherford to 'orbit' it in a same way to the planets round the Sun. It was realized that a proper description of atoms required the quantum theory; although the planetary model remains a useful analogy from the macroscopic world, many of the physical ideas that work for familiar objects must be abandoned or modified at the microscopic atomic level.