Structure of Atom

After having made remarkable achievements in the study of radioactivity, Rutherford (1871-1937) became interested in the structure of atoms from which radioactivity radiated. He bombarded a thin metallic foil (thickness of 104 atoms) with parallel streams of ? particles (later it was found that the ? particle is the atomic nucleus of He). He conceived that a particle must exist in the atom that has a mass large enough to repel an ? particle possessing the mass of a helium atom, and which has an extremely small radius. According to his idea, the positive charge of an atom is concentrated in a small central part (with a radius calculated to be ca. 10-12 cm) while the negative charge could be dispersed through the whole atom. The small particle at the center of the atom he named nucleus. All previous models of atoms as uniform spheres were thus denied. However, the atomic model of Rutherford which consisted of a small nucleus with electrons dispersed around it could not explain all known phenomena. If the electrons did not move, they would join the nucleus by electrostatic attraction (Coulomb force). That was impossible because an atom is a stable entity. If the electrons circle the nucleus like planets under the influence of gravity, the electrons will undergo acceleration and lose energy through electromagnetic radiation. As a result, their orbits will diminish so that the electrons eventually fall into the nucleus. Meanwhile, the atom should emit a continuous spectrum. But again, the fact is that atoms are stable, and it was already known that atoms emit a group of line spectra rather than a continuous spectrum. It was clear that a fundamental change in thought was necessary to explain all these experimental facts.