We all have seen plums in padding. Previously it was thought that electrons in an atom are distributed on positive charge just like plums in a pudding. In other words, it was thought that positive charge exists throughout the atom and negative electrons are unevenly distributed on it just like plums in the pudding. This concept of the atomic model is so known as plums in pudding model of atoms. This concept was introduced by J.J.Thomson who was also the inventor of electrons. As according to plums in the pudding model, positive and negative charges of an atom are distributed throughout the body of the atom and there must not be any concentrated mass in an atom.
In 1899, Ernest Rutherford of Manchester University had discovered alpha particles which are positively charged helium ions emitted from radioactive substance like uranium. These alpha particles create bright spots when they strike on a zinc sulphide coated screen. As there is no concentrated mass in an atom, it was predicted that if a thin metallic foil is bombarded with positively charged alpha particles, then all such alpha particles would pass the foil without much deflection in their travelling path.
The tiny electric field developed in the atoms can not affect the motion of the particle much. So it was predicted that there may be less than 1o deflection in the path of the motion of the alpha particles. This prediction inspired Ernest Rutherford to conduct the experiments to verify the plums in pudding model of atoms. He instructed his fellow scientist Ernest Marsden and Hans Geiger to bombard with alpha particles on a thin metallic foil to verify this prediction. As per instruction, Ernest Marsden and Hans Geiger conducted an experiment and made a history. They placed a very thin gold film in front of alpha ray gun. They also placed a zinc sulphide screen surrounding the gold film to observe the bright spots on it when alpha particles strike on it. They conducted the experiment in a dark room. They observed during the experiment that as predicted, the alpha particles are crossing the film and striking on the zinc sulphide screen behind the film.
But after counting the bright spots on the screen they found an unexpected result came. All the alpha particles did not cross the foil in the straight way as expected. Very little percentage of bombarded alpha particles changed their way of travel during crossing the gold foil. Not only the particles diverted from their way, but also very few of them directly bounced back towards the source or alpha gun. After detailed study of the observation, Ernest Marsden and Hans Geiger submitted a report to Ernest Rutherford. After viewing and studying their report Rutherford predicted a different model of an atom, which is known as Rutherford model of the atom.
He predicted that the alpha particles which directly bounced back must have collided with some much heavier mass and that mass should be positively charged. This was also found that some of the diverted alpha particles are not bounced back but they had a very large angle of diversion. By observing different angles of diversion and the number of particles diverted with these angles he predicted that the positive alpha particles are also influenced by a comparatively huge concentrated positive charge. He stated that the concentrations of mass and positive charge are in the same place in an atom and this is at the center of the atom and he called it as a nucleus of the atom. He also stated that except central nucleus, the entire space in the atom is vacant.
After this gold foil experiment, Rutherford gave a more realistic model of an atom. This model is also called Nuclear Atomic Model or Planetary Model of Atom. This model is given in the year of 1911. According to Rutherford’s Atomic Model, almost all the mass of an atom is concentrated in this nucleus. This nucleus is positively charged and is surrounded by tiny light negatively charged particles, which are called electrons. These electrons circulate around the nucleus in the same manner such as planets circulate around the sun in the planetary system. That is why this model is also referred as the Planetary Model of Atom.
The radius of the nucleus is about 10-13 cm. The radius of circular path travelled by electrons around the nucleus is about 10-12 cm which is greater the diameter of an electron. The radius of the atom is about 10-8 cm. Thus the, like a planetary system, the atom is also of exceedingly open nature, due to which it can be penetrated by high-speed particles of various kinds. The Rutherford’s Planetary Atomic Model is shown in figure below-
A force of attraction exists between positively charged nucleus and negatively charged electrons travelling around the nucleus. This electrostatic force between positively charged nucleus and negatively charged electrons is similar to the gravitational force of attraction between Sun and planets revolving around the sun. Most of the part of this planetary atom is open space, which does not offer any resistance for passes of positively charged tiny particles such as Alpha particles.
The nucleus of the atom is very small, dense and positively charged which results in the scattering of positively charged particles. This phenomenon for the scattering of positively charged alpha particles by positively charged nucleus, explains the scattering of positively charged alpha particles by the gold foil as observed by Ernest Rutherford. The Ernest Rutherford Atomic Model succeeded to replace the atomic model Thomson’s Plum Pudding model given by English Physicist Sir J.J. Thomson.
According to the Ernest Rutherford’s atomic model, the electrons are not attached to the mass of atom. The electrons are either stationary in space or rotate in circular paths around the nucleus. But if the electrons are stationary they must be fallen to nucleus due to attraction force between electron and nucleus. On the other hand if the electrons are moving in a circular path, then according to electromagnetic theory, the accelerated charge of electron would have continuously lost its energy and would have down into the nucleus as shown in figure below Rutherford Atomic Model fails to explain why electrons are not fallen into positively charge nucleus.
Thus, the deficiencies of Rutherford’s Atomic model can be described as below-
- The Rutherford’s atomic model does not explain the distribution of electrons in the orbits.
- The Rutherford’s atomic model does not explain the stability of the atom as a whole.
The above drawbacks of Rutherford’s atomic model were overcome by Bohr’s Atomic Model (1913).