Rutherford Atomic Model

Rutherford’s Atomic model is significant, as the discovery of dense positively charged particles in the center of an atom had unfolded many ways for the scientists to discover more about the structure of an atom.

Rutherford’s atomic model, commonly known as the Rutherford model explains the structure of an atom that atom consists of a central thick nucleus around which negatively charged particles mainly electrons are revolving. These electrons revolve around the nucleus at a very high speed in circular paths called orbits.

This model is also called as “planetary Model” because the model of an atom resembles a solar system in which the sun is like a nucleus and the planets revolving around it are electrons. Before Rutherford, In 1904 Nagaoka named the model of an atom “planetary” he proposed that atomic structure is like Saturn and its rings (Saturnian model of an atom).

Background information

During 400BC, the Greek philosophers thought that the atom could not be divided further into smaller substances. It is “indivisible”. so it was named “Atom” from the Greek language “atomos” meaning “uncuttable”. The philosophers also believed that atoms are eternal, uncreated, and the composition of the substance depends upon the kind of atoms present in it.

In 1803, John Dalton proposed that ” All matter is composed of atoms, tiny indivisible particles. These are heavy, hard, and movable particles”.

But this idea of Dalton’s was changed when J.J Thomson discovered electrons during experimentation with cathode rays in 1897. He noticed that if an atom contains electrons which are negatively charged particles there must be some other entities whose presence makes an atom neutral.

then Dalton proposed his Plum Pudding Theory. According to this theory,” the electrons are embedded in a uniform sphere of positively charged particles like plums in the pudding”. The Plum pudding model suggests negatively charged particles like the blueberry in muffins.

picture of muffin and model of plum pudding

The Plum pudding model of the structure of an atom was accepted until Ernest Rutherford rejected it in 1909 when he discovered a nucleus( dense mass of protons) inside an atom. Neutrons were not discovered at that time so he thought about only protons (positively charged particles) as the nucleus. This model of rutherford formed the basis of the structure of an atom which was nearly the same as today, some other changes were made by Neil Bohr later.

Rutherford Atomic Model

The thought of this experiment started when Rutherford was working on the calculation of the charge to mass ratio of $\alpha$ -particles. He encountered the deflection of $\alpha$ -particles by air particles. Thomson also came across this kind of problem during working on cathode rays but he achieved his results by producing a nearly perfect vacuum chamber. But Rutherford believed that the atoms in the air should not scatter $\alpha$ -particles because Thomson’s atomic model suggests that positive charges are evenly distributed throughout an atom so they cannot produce an electric field strong enough to deflect an $\alpha$ -particle. And electrons being very small in size their presence cannot hinder massive $\alpha$ -particles. Then Rutherford performed a series of experiments in which he bombarded $\alpha$ -particles on metal surfaces and every time deflections were there so he concluded that Thomson’s idea regarding the structure of an atom is not true.

In 1909 Newzealand-born British Physicist, Ernest Rutherford with his colleagues Hans Gieger and Ernest Marsden performed an experiment known as the “Gold Foil Experiment“.

He took 0.00004cm thick gold foil, on which alpha particles were bombarded. if the model of the atom was plum pudding then all the alpha-particles should have passed undeflected. But the results of this experiment surprised them as some of the alpha particles were deflected at greater than 90^o angle. When rutherford administered these incredible results, he exclaimed

“It was almost as incredible as if you fired 15-inch shell at a piece of tissue paper and it came back and hit you”

These results showed that the plum pudding model is not correct. The deflection of an alpha particle at a high angle reveals that positively charged particles reside in the center of an atom. Most of the alpha particles were passed through gold foil undeflected showing that nearly all the space of an atom is empty. The size of the mass of positively charged particles in an atom is like grain on a football field and the electrons revolve around the nucleus, their size is much smaller than that of a nucleus.

Gold foil experiment

The experiment was done on extremely thin gold foil approximately 0.00004cm. A radiation source(polonium) was used through which alpha particles were bombarded on gold foil. Slits were used to direct alpha particles on foil. And the foil was surrounded by photographic film coated with zinc sulfide. Zinc sulfide is a scintillating compound, it emits light when it is struck by $\alpha$ -particles. So when any particle hit the photographic film then it produces certain pinpoints which indicate the presence of the $\alpha$ -particles.

picture

Observations of Experiment

Observations made by Rutherford were as follows:

Almost all of the particles passed through the atoms of gold undeflected.
Some of the particles deflected slightly having a small angle.
Out of 20,000 particles, few were deflected at a very high angle nearly 180^o.

Conclusions

The following conclusions were made by this experiment:

As most of the particles were passed undeflected it showed that most of the space of an atom is empty.
The positive charge of an atom is not uniformly distributed as was suggested in plum pudding theory rather all of the positive charges are gathered at the center of the atom.
The total volume of positively charged particles in an atom is very small as compared to the total volume of an atom he called this dense mass a “Nucleus”.
The strong repulsion between $\alpha$ -particles and the nucleus shows that the nucleus is dense, hard, and thick.
The electrons revolve around the nucleus in circular paths called orbits.
Massive $\alpha$ -particles do not distract by the presence of electrons.
As protons are present inside the nucleus Rutherford named them “Nucleons”.
The electrostatic force between protons and electrons provides a centripetal force for electrons to move around the nucleus.
Atom is a neutral particle, so the number of protons is equal to the number of electrons.

Defects of the Rutherford model

Even with this great discovery, there was some confusion regarding Rutherford’s atomic model. This model was not convincing with Maxwell’s theory and did not clearly mention the stability of an atom. This leads to Rutherford’s Model not being completely correct.

There were some defects in Rutherford’s atomic model which are presented here:

Maxwell’s theory says that an accelerated charged particle should emit electromagnetic radiation. As electrons are charged moving particles So they should continuously release energy and ultimately their orbit would shrink and the electron would fall into the nucleus. This would result in collapsing of an atom. That means every substance made up of an atom should get destroyed in less than 10^-11seconds. But actually, it is not true.

keeping this concept in view, an Electron which is releasing energy continuously should produce a continuous spectrum. But actually, atoms produce a line spectrum.

Continuous spectrum- a drawback of Rutherford atomic model

Rutherford Model also didn’t explain “How electrons are arranged in an atom?”.

Rutherford’s model had arisen many questions regarding the structure of an atom. This model forms the basis of quantum mechanics which would lead to the formulation of an acceptable atomic structure. The following questions were left unanswered in this model

How are atoms stable or How do they not collapse?
Why does an atom make a line spectrum?

These Questions were answered by Neil Bohr in 1913 in his Model named “Bohr Atomic Theory”.

Comparison to others atomic models

Concepts Berg

What kind of experiment did rutherford perform?

Rutherford bombarded $\alpha$ -particles on gold foil to inspect the deflection of $\alpha$ -particles.

What was the primary observation of rutherford’s atomic model?

Almost all of the particles passed through the atoms of gold undeflected.

Out of 20,000 particles, few were deflected at a very high angle nearly 180^o.

What are the conclusions of Rutherford’s experiment?

Most of the space inside an atom is empty. Positive charges are concentrated in the center of an atom called a nucleus. The size of the nucleus is very small as compared to the size of an atom. Electrons revolve around the nucleus in circular paths.

What did Rutherford help discover?

Rutherford’s model helped to discover the quantized nature of orbit, and it also helped in such a way that it produced a question regarding the stability of an atom.

Who discovered the proton?

in 1886 Goldstein performed an experiment and he noticed positively charged rays inside the discharge tube. He called them canal rays which were later named protons.

What is Rutherford’s atomic model, Is it really important in science?

Rutherford’s model is significant as it has given the concept which provided the basis of the atomic model accepted today. He discovered the presence of the Nucleus inside an atom and rejected the plum pudding theory.

What are some atomic models?

Plum Pudding model, Rutherford’s atomic model, Nuclear model, Niels Bohr’s model, Planetary model, Erwin Schrödinger’s model, Electron Cloud Model/Quantum Model.

What was the weakness of Rutherford’s model?

It could not explain the stability of an atom. It tells us that the atom forms a continuous spectrum but actually atom forms a line spectrum.

Reference Books