- Rutherford Atomic Model: The Rutherford atomic model describes the atom with a dense, positively charged nucleus surrounded by electrons.
- Gold Foil Experiment: The deflection of alpha particles during the gold foil experiment indicated a small, dense nucleus within the atom.
- Planetary Analogy: Electrons orbit the nucleus like planets around the sun, highlighting the structured movement in Rutherford’s atomic model.
- Model Limitations: Rutherford’s model did not account for the stability of electrons or their energy distribution.
- Scientific Progression: The limitations of the Rutherford atomic model were foundational for the development of more accurate atomic models like Bohr’s.
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 negligible effect of the atom’s tiny electric field on particle motion led to the prediction of minimal alpha particle deflection—less than 1°. Inspired to test the plums in pudding model, Ernest Rutherford directed scientists Ernest Marsden and Hans Geiger to bombard a thin gold foil with alpha particles. In a landmark experiment conducted in a dark room, they observed alpha particles passing through the foil and striking a zinc sulphide screen, creating visible bright spots.
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.
Rutherford hypothesized that alpha particles rebounding sharply must have struck a heavy, positively charged mass. Observations of alpha particles deflecting at various angles suggested they were influenced by a strong, concentrated positive charge. This led Rutherford to propose that both mass and positive charge were concentrated in a central area he termed the nucleus, with the remainder of the atom being largely empty space.
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, being small, dense, and positively charged, scatters positively charged particles. This scattering of alpha particles by the nucleus, as observed in Rutherford’s experiments, effectively debunked the Thomson’s Plum Pudding model proposed by J.J. Thomson, marking a significant advancement in atomic theory.
Rutherford’s atomic model posited that electrons orbit the nucleus or remain stationary in space. However, if stationary, they should fall into the nucleus due to attraction forces. Conversely, if orbiting, electromagnetic theory suggests they would lose energy and spiral into the nucleus. These observations revealed limitations in Rutherford’s model regarding electron stability.
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).
