Structure of Atom

Introduction
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.
2. Atomic spectrum
If a metal or one of its compounds is heated in the flame of a burner,
a color characteristic of the metal appears. This is the well-known flame
reaction. If the colored light is separated by means of a prism, a few
strong line spectra are observed, and the wavelength of each line is
characteristic of the metal involved. For instance, the yellow flame
reaction of Na corresponds to two yellow lines of its spectrum in the
visible region, and the wavelengths of these two lines are 5.890 x 10-7 m
and 5.896 x 10-7 m, respectively. If a gas is sealed in a high vacuum tube,
and a high voltage is applied, the gas discharges and emits light.
Separation of this light by means of a prism will give a series of
discontinuous line spectra. Since the wavelengths of this light are
characteristic of the atom, the spectrum is called its atomic spectrum.