Electrons and nuclei

The well known planetary model of the atom was proposed by Rutherford in the year 1912 following experiments by Geiger and Marsden demonstrating that almost all the mass of an atom was concentrated in a positively charged nucleus. Electrons that are Negatively charged attracted to the nucleus by the electrostatic force and were referred by Rutherford to 'orbit' it in a identical way to the planets round the Sun. It was soon realized that a appropriate description of atoms needed the quantum theory; even though the planetary model remains a useful analogy from the macroscopic world, several physical ideas that work for familiar objects must be abandoned or altered at the microscopic atomic level.

The lightest atomic nucleus (that of hydrogen) than an electron, is 1830 times more massive. The size of a nucleus is approximately 10^-15 m (1 fm), a factor of 105 smaller than the apparent size of an atom, as measured through the distances between atoms in solids and molecules. Atomic sizes are determined through the radii of the electronic orbits, the electron itself having actually no size at all. Chemical bonding between atoms changes the motion of electrons, the nuclei remaining unaffected. Nuclei retain the element's  chemical identity and the occurrence of chemical elements depends on the presence of stable nuclei.