Explain Rutherford's Modle of an atom?

Rutherford's Model Of Atom: In 1911, Rutherford performed scattering experiment through which he was able to propose the model of atom which is known as Rutherford's nuclear model of atom.

Experiment: First of all Rutherford bombarded a thin foil of metal as Gold nearly 100mm thick, with a beam of fast moving alfa α-particles. The source of alfa particle was Radium, which is a radioactive substance and it was kept in a lead block in which slits were used to get fine beam of α-particles.

The gold foil was plated with Zinc sulphide screen so that when the α-particles strike over it at any place a light flash is given out.

Observation: On the basis of above experiments he found the following observtion:

Most of the α-particles passed foil without deflecting
A few were deflected through small angles. and,
very few wer revert back through an angle greater than 90 degree.

Statement: On the basis of above observation he got the following conclusions which were called as model of atom.

Since most of α-particles passed without deflection hence, the atom must have empty space inside.
as some α-particles were deflected throgu small angles, so there must be some positive charge inside the atom which deflected the positively charged α-particles.
since some α-particles were deflected back it means they must got striked to some heavy particles. these heavy particles were called as NUCLEUS.

Thus according to Rutherford,s Model the atom consists of two parts, first is the Nucleus, and second is the extra nuclear part.

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Rutherford, while performing experiments on radioactivity, bombarded fast moving alpha particles on a thin gold foil (about 1000 atoms thick). He selected the gold foil because of its high ductility. He expected to see small deflections of alpha particles by the sub-atomic particles present in gold atoms.

From the experiment, he made the following observations:

1. Most of the fast moving α-particles passed straight through the gold foil.

2. Some α-particles were deflected through the foil by small angles.

3. Surprisingly, one out of every 12,000 particles rebounded i.e., they got deflected by an angle of 180°

Deflection pattern of alpha rays as observed by Rutherford

Rutherford derived the following conclusions from the gold foil experiment:

1. Since most α-particles passed through the gold foil without any deflection, most of the space inside an atom is empty.

2. Very few particles suffered a deflection from their path. This means that positive charge occupies very little space inside an atom.

3. As a small fraction of particles got deflected completely by the angle of 180°, all positive charge and mass of gold atoms are present within a very small volume inside the atom.

Based on the above conclusions, Rutherford gave a new atomic model known as the Rutherford atomic model or nuclear model of the atom. The major features of the model are as follows.

1 All protons are present inside the nucleus, which is situated at the center of the atom.

2. Electrons reside outside the nucleus and revolve around the nucleus in well-defined orbits.

3. The size of the nucleus is very small in comparison to the size of an atom. As per Rutherford’s calculations, the size of the nucleus is 10⁵ times smaller than an atom.

4. As the mass of the electron is negligible in comparison to the mass of the proton, almost all the mass of the atom is concentrated in the nucleus.

Rutherford Atomic Model

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The radioactive particles discovery led Rutherford to perform an experiment where he bombarded a thin gold sheet with particles obtained from a radioactive substance. After striking the gold foil, some particles scattered and this produced flashes on the ZnS (zinc sulphide) screen which was placed at the back of the gold foil.
The tiny flashes were monitored by a movable microscope and the scattering experiment observations made were as follows :

Almost all the particles pass through the metal and are un deflected.
Some of these particles are deflected through small angles.
Very few of these particles are deflected through as much as 90^o or even larger angles.

The scattering experiment results could not explain the Thomson atomic model. Moreover Rutherford concluded that:

Almost all of the particles passed through undeflected, so the space inside the atom is empty or hollow.
Particles deflected with large angles show that there exists a heavy positively charged body inside the body of the atom, which helped in repelling the like charge of the α (alpha) particle. The heavy, positively charged center was termed as the nucleus.
The heavy positively charged particles number that undergoes deflection is found to be very small. The nucleus occupied a very small volume in comparison to the volume of the atom.
When heavy α (alpha) particles get deflected, the nucleus of the atom is assumed to have an appreciable mass.

Rutherford Nuclear Model

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On the basis of the scattering experiment, Rutherford described the structure of the atom as:

An atom consists of positively charged nucleus and is surrounded by electrons that revolve around it. The nucleus ' positive charge is due to the presence of protons.
Electrons and neutrons are held together by coulombic force of attraction.
The nucleus effective volume is found to be extremely small as compared to the effective volume of the atom. The nucleus occupied a volume of about 10 ^- ¹ ² times the volume of the atom.
Atoms entire mass is concentrated only at the nucleus.
As each atom is found to be electrically neutral, the number of protons in the nucleus of an atom is exactly equal to that of the electrons in it.

Rutherford model suffered some drawback, as it could not explain the stability of an atom in-spite of the revolving electrons around the nucleus. The electrons revolving around should emit radiations and subsequently lose energy. The loss of energy should slow down the electrons, and gradually move towards the nucleus in a spiral path and then fall into the nucleus. This should result in the the collapse of atom and make it unstable, which is found to be untrue.

Gold Foil Experiment

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Rutherford bombarded a narrow beam of α (alpha particles) at an extremely thin sheet (about 5�…�10^-7 meter thick) of metal like silver and gold. The alpha particle was obtained after passing it through a slit in a lead screen (obtained from polonium ²¹⁴Po)

After passing through the metal sheet, the alpha particles were made to strike a fluorescent screen coated with zinc sulphide. When alpha particles struck the screen, they produced a burst of light called scintillation, which was visible by a microscope attached to the back of the screen.

He observed the deflection of alpha particles and proposed his model.

According to Thomson model, electrons and protons are equally distributed throughout atom. So, if high velocity alpha particles bombard on atom, there would be very little deflection in particles.

Hence Rutherford expected, most of the alpha particles must be moved right through the foil. But observation was quite different from expectation. He observed that,

Most of the alpha particles passed through the metallic sheet without suffering and change in their path. It proved that the maximum space was empty in atom.
Very small number of alpha particles got deflected through a large angle while a very few even bounced back toward the source. It proved the presence of heavy positively charge body at center of atom, so, alpha particle bounced back towards source.

Conclusion of Rutherford Model

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Based on gold foil experiment and discovery of neutron by Chadwick in 1932, Rutherford purposed his atomic model. According to this model :

An atom consists a very small positively charge body located at center, known as "nucleus". This nucleus contains protons and neutrons. Since electrons have negligible mass, the entire mass of nucleus is due to presence of protons and neutrons and resides in the nucleus.
Since a very less number of alpha particles deflected, it proves that the volume of nucleus is very less compare to whole atom. The radius of nucleus is 10^-13- 10^-12 m and for atom, it is 10^-8 cm.
Electrons revolving around the nucleus in a certain path known as orbits.
The extremely small, negatively charged electrons distributed around the nucleus and balance the charge of it.

Objections of Rutherford Model

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According to classic electromagnetic science which was known that time, Rutherford atomic model has an extremely short lifetime.

J.C. Maxwell had shown that whenever an electric charge is subjected to acceleration, it emits radiation and loses energy.
It means, when an electron (negative charged particle) moves around the nucleus, it should be subjected to acceleration and should emit radiation with loss of energy.
As a result, its orbit should become smaller and smaller and finally it should drop in to the nucleus in a helical path instead of circular path. In other words, an atom is not a stable species.

On the contrary atom is stable and electrons and their energy in one of these orbits stay same.

The other discordance regarded the radiation. If an electron continually emits radiation, the atomic spectrum must be continuous. It means that there must be no line for fixed frequency. But the atomic spectrum is not continue spectrum. There are many lines with fix frequency. Hence the Rutherford atomic model failed to explain the line spectrum of atoms.

In order to explain, the first objection that why the electrons do not fall into the nucleus on account of mutual electrostatic attraction, Rutherford proposed that the electrons are revolving with extremely high speed and at great distance from center. So, centrifugal force arising from this motion balances the force of electrostatic attraction.

The electrons, therefore, do not fall in nucleus and atom will be stable. But there was no clarification for atomic spectrum pattern. Bohr supplied the solution of this difficulty on the basis of quantum theory of radiation.