Saturnian Model of the Atom – Nagaoka’s Planetary Model
It must be noted, the Rutherford model of the atom was not the first model, which proposed a nuclear structure. The idea of a nuclear structure was previously proposed in 1903 by a Japanese physicist Hantaro Nagaoka, who rejected Thomson’s model on the grounds that opposite charges are impenetrable.His model was known as the Saturnian model of the atom and according to this model the atom consists of a massive positive centre surrounded by a number of orbiting electrons, in the manner of Saturn and its rings.
Nagaoka’s planetary model had two predictions:
- a very massive atomic center (in analogy to a very massive planet)
- electrons revolving around the nucleus, bound by electrostatic forces
Both predictions were successfully confirmed by Ernest Rutherford in 1911, however, other details of the model was incorrect. Notwithstanding the Rutherford model of the atom was really very close to modern concept of the atom, it was based on classical physics. But neither classical physics nor the Rutherford model can explain the following problem. Why the negatively charged electrons are prevented from falling into the positively charged nucleus? Forces between static electrically charged particles are governed by the Coulomb’s law:
Rutherford suggested that perhaps the electrons revolve in orbits about the nucleus. In classical mechanics, a condition for the dynamic stability of such systems of rotating particles is that only forces of attraction act between its components.
Classical physics also states that any accelerating electric charge (either oscillating or revolving) emits electromagnetic radiation. As a result, an electron orbiting inside an atom should be continually losing energy and it should spiral into the nucleus within a fraction of a second. In fact, physicists calculated that the electron should lose all of its energy and spiral down into the proton in only about 0.000000000001 second, which means that every atoms should not exist longer than a mere 10-12 seconds.
The solution to the problem of the stability of the electrons in an atom was provided by young Danish physicist Niels Bohr working with Ernest Rutherford at the University of Manchester in 1913. His model adopted Planck’s quantum hypothesis and he proposed a model in which the electrons of an atom were assumed to orbit the nucleus but could only do so in a finite set of orbits. He postulated that an atom emits or absorbs energy only in discrete quanta corresponding to absorption or radiation of a photon.