ABSTRACT
Until the late 1800s, atoms were thought of as indivisible balls or units, the building blocks of molecules and all matter. When investigating the phenomena in discharge tubes, J. Perrin proved in 1895 that the cathode rays were negatively charged. In ingenious experiments, J. J. Thomson was able, in 1897, to determine the speed of the charged particles and the ratio (e/m), where e is the charge and m the mass. Thomson then proposed a model of atomic structure where the positive charge was uniformly distributed throughout the atomic volume, and the negative charged electrons embedded in this claude of positive charges. Ernest Rutherford was studying the fast Alpha-particles' penetration and scattering in a very thin gold foil. Although the Rutherford model could explain most of the observations, including the scattering of Alpha-particles, it had a serious defect, because Rutherford could not explain the stability of the atom.
