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Rutherford Model of the Atom


The nuclear model of the atom was first suggested by Rutherford in 1910. The first exploration of the structure of atom was made by Rutherford by making use of $\alpha$ particles. On the basis of the results obtained from the scattering experiments, Rutherford suggested an atomic model, according to which the entire positive charge in the atom is concentrated to a very small region called nucleus.

On the basis of his analysis, Rutherford introduced his model of the planetary atom or solar atom according to which the positive charge and the majority of the atomic mass is concentrated in a tiny nucleus at the center of the atom. The electrons on the other hand, are distributed throughout a sphere of atomic dimensions.

Thomson Model

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Based on the various results obtained experimentally J.J.Thomson in 1898 suggested a model of atom. His model is sometimes called "plum-pudding" model, because the atom to be full of some positive fluid like a pudding in which electrons were embedded like "plums" in that pudding. The radius of the spherical model was about 10-8cm. The total positive charge in the positive fluid was evenly and symmetrically balanced by the negative charges of the electrons.

Thomson Model

This model satisfactorily explained as to how on heating a substance, it starts radiating light. Though this was the first attempt at understanding what an atom could be, it failed to satisfy the results of the experiments performed later by Rutherford and others. Thomson suggested in 1911 that his model just cannot answer how and why the $\alpha$-ray scattering should take place, on the basis of his atomic model. So this model is discarded.

Ernest Rutherford's Discovery

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Ernest Rutherford explanation of radioactivity that earned him a Nobel Prize his most significant contribution to science was made later, when he established that the atom consisted of a dense nucleus orbited by electrons.

Rutherford discovered that most of the mass of the atom is concentrated at its center in a very tiny nucleus. The electrons move around the nucleus but most of the atoms is in empty space.

Ernest Rutherford

At the beginning of the 20th century scientists still thought that atoms were the smallest particles and that they could not be divided. This changed in 1911, when New Zealander Ernest Rutherford discovered that the nucleus was surrounded by particles called electrons traveling at incredible speeds. Rutherford work won him a Noble Prize and he is often called the father of nuclear science.

Rutherford Atomic Theory

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The atom is mostly composed of empty space. The entire positive charge and mass of the atom is concentrated in small central part known as nucleus. The size of the nucleus is so small that its diameter is 103 times less than of an atom. The diameter of the nucleus has been estimated by Rutherford as 10-13cm in contrast to that of an atom to be 10-8cm. The electrons present outside the nucleus revolve round the nucleus with high velocities.

Planetary Model

The electrons revolve round the nucleus with high velocities to counterbalance the electrostatic forces of attraction between protons and electrons. Rutherford's atomic model resembles the planetary motion in solar system. Therefore, Rutherford's model of an atom called planetary model.

Gold Foil Experiment

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In 1911, Ernest Rutherford performed his now famous gold foil experiment. This experiment allowed him found out some fundamental information about the composition of the atom.

In his gold foil experiment, Rutherford bombarded a piece of thin gold foil with alpha particles. These particles would do one of two things: they would either hit the foil and continue moving in the same direction or they would bounce off of the foil and be reflected at acute angles.

Gold Foil Experiment

Rutherford found that alpha particles travel right through the foil, while only a few were deflected. He realized that these deflections were the result of electrical repulsions between the positive alpha rays and the positively charged components of the atom. Rutherford was able to conclude that the positive charges inside the atoms of the foil had to be concentrated otherwise the alpha particles would would not have been deflected backwards as he had observed. He call this small positively charged region the nucleus.

The graph between the angle of scattering and the number of $\alpha$-particles scattering in the corresponding direction is shown below.

Gold Foil Experiment Graph

The Rutherford Formula for the number N of particles scattered at an angle $\theta$ is such that

$N \propto \frac{1}{sin^{4}(\frac{\theta}{2})}$

The Rutherford formula used for scattering experiments is given below.

$b = \frac{Ze^{2}cot\frac{\theta }{2}}{4\pi \epsilon_{o}[\frac{1}{2}mv_{j}^{2}]}$

The deflection of the positively charged alpha particles in passing through the atom can be due to two causes.
  1. The attraction of electrons
  2. The repulsion by positive charge in the atom

Ernest Rutherford Atomic Model

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The Ernest Rutherford model of the atom shows
  1. The atom consists mainly of space
  2. The mass of the atom is concentrated in the nucleus which is a small core at the center of the atom.
  3. The nucleus has positive charges.
  4. Electrons move around the nucleus like planets orbiting the sun.
  5. The atom is neutral as its has the same number of positive charges and negatively charged electrons.

The Rutherford atomic model features are as follows.

  1. Mass and positive charge of the atom are concentrated in the nucleus the size of which is of the order of 10-15m = 1 Fm
  2. Negatively charged electrons revolve about the nucleus in a cloud, the radius of which is of the order of 10-10m = 1A

The Rutherford atomic model picture is shown below.

Ernest Rutherford Atomic Model

Rutherford Nuclear Model

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This model was proposed by Rutherford in 1911. According to this an atom of a matter consists of a central positively charged nucleus of radius 10-12cm. It practically carries the whole mass of the atom. It is surrounded by "planetary" electrons of radius 10-13cm at distances relatively greater as compared to the diameter of the electrons. The radius of the atom is 10-8cm. Thus the atom like the solar system is an exceedingly open structure and this is why it can be presented by high speed particles of various kinds.

Rutherford Nuclear Model

Rutherford nuclear model assumed the electrons outside the nucleus, either to be stationary or moving in a circle. Thus if electrons were stationary they would have been attracted towards nucleus and would have fallen into the nucleus.

The drawbacks of this model is

  1. The theory did not explain the distribution of electrons in the orbit.
  2. The theory did not explain the stability of the atom as a whole.

Rutherford Contribution to the Atomic Theory

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The atom consists of a positively charged heavy nucleus surrounded by revolving negatively charged extremely light particles called electrons, like the planets round the sun at such a speed that the mechanical centrifugal force would just balance the net excess of electrostatic attraction and in consequence stability of the atom could be secured.

After doing many calculations Rutherford concluded that the radiation could only come back if that atom had a hard  positively charged core at the center of the atom.