Rutherford's Atom Model
In 1912, Rutherford proposed his nuclear model of the atom. It is also known as Rutherford's planetary model of atom. Salient features of Rutherford's atom model are as follows :
Every atom consists of a tiny central core, named nucleus, in which the entire positive charge and almost whole mass of the atom are concentrated. The size of nucleus is typically 10-4 times the size of an atom.
Most of an atom is empty space.
In free space around the nucleus, electrons would be moving in orbits just as the planets do around the sun. The centripetal force needed for orbital motion of electrons is provided by electrostatic attractive forced experience by electron due to positively charged nucleus.
An atom as a whole is electrically neutral. Thus, total positive charge of nucleus is exactly equal to total negative charge of all the electrons orbiting in an atom.
 |
| Rutherford's Atom Model |
Limitations of Rutherford's Atom Model
Rutherford's nuclear model of atom was a major step towards the modern picture of the atom. However, it suffered with the following limitations :
In Rutherford's Atom Model, an electron is considered moving in a circular orbit around the nucleus. Thus, the electron is continuously experiencing a centripetal acceleration. According to classical electromagnetic theory, an accelerating electron must emit radiation in the form of electromagnetic waves. Therefore, the energy of an accelerating electron should continuously decrease. So, the electron would spiral inward and eventually fall into the nucleus as shown in Fig. Thus, such an atom cannot be stable.
As the electron spirals inward, it's angular velocity and hence the frequency would change continuously, and so will the frequency of the light emitted. Thus, the atom should emit a continuous spectrum. However, experimentally, spectrum emitted by excited atoms is found to be line spectrum.
From the above points, it is clear that Rutherford's model tells only a part of the story and classical ideas are not sufficient to explain the atomic structure.