Rutherford Atomic Model

Rutherford Atomic Model is also called the Rutherford model of an atom or the Planetary Model of the Atom and was proposed by Ernest Rutherford in 1911. Rutherford provided the model of the atom in which he described that an atom consists of a small, dense, positively charged center called a Nucleus around which the negatively charged Electrons revolve like planets revolving around the Sun. When J.J. Thomson’s Plum Pudding model failed, Rutherford overturned it by giving its well-known Gold Foil Experiment, the experiment was designed to explore the invisible world of Atomic Structure using alpha particles released by radioactive elements.

In this article, we will learn about Rutherford’s Atomic Model, Rutherfords’ Alpha Scattering Model, its observation, limitations, and others in detail.

What is the Rutherford Atomic Model?

According to Rutherford’s Atomic Model, the positively charged particles and the majority of the mass of an atom were said to be concentrated in a small volume. He referred to this area of the atoms as the nucleus. 

Another idea put forward by Rutherford’s model of an atom was that an atom’s nucleus should be surrounded by negatively charged electrons. Rutherford also suggested that the electrons circle the nucleus at the speed of light. He called these elliptical paths orbits.

History

The idea of the atom was first proposed by the Greek philosopher Democritus in 400 BCE. The concept of atoms and other small particles is also known to early Indians. But the modern study of atoms starts with John Dalton in 1803 when he first proposed the idea of the atom with a modern understanding of science. He proposed that atoms are indivisible particles until in 1897 negatively charged particle electrons were first discovered by British physicist J.J. Thomson.

J.J. Thomson proposed a plum pudding model of the atom also called Thomson’s Atomic Model but it had some limitations. It was unable to explain certain experimental results relating to the atomic structure of various elements.

After that, a new model of atoms was proposed by “Ernest Rutherford,” a British physicist, and the model is known as Rutherford’s Model of Atoms. He conducted an experiment in which he bombarded -particles in a thin gold sheet and concluded that atoms also had positively charged particles at the centre of the atom where all its mass is concentrated.

Rutherford Gold Foil Experiment

To determine how electrons are arranged in an atom, the Alpha (α) Particle Scattering Experiment was organized by Rutherford. Rapidly moving α-particles were directed to bombard a thin sheet of gold.

• The gold foil was selected so as to obtain an extremely thin layer. The thickness of the gold foil was about 1000 atoms.
• Doubly-charged helium ions are known as α-particles. Rapidly moving α-particles possess a great deal of energy, as they have a mass of about 4 amu.

The hypothesis was that α-particles would be deflected by the sub-atomic particles in the gold atoms. Rutherford didn’t expect to witness significant deflections as the α-particles were considerably heavier than the protons. However, the experiment produced entirely unanticipated results.

Observations of Rutherford’s Gold Foil Experiment

Rutherford observed the following from his α-particle scattering experiment:

1. A large percentage of alpha particles travelled through the gold film without being deflected, indicating that the majority of space in an atom is empty. As a result, an atom’s main portion must be empty.
2. The positive charge in an atom is concentrated in a relatively small volume and is not dispersed evenly. When bombarded, the gold foil only deflected a small number of alpha particles. They experienced extremely slight angles of deflection. So he arrived at the stated conclusion.
3. Very few alpha particles had deflected back or at large angles. In addition, relatively few particles had 180o deflected. As a result, he came to the conclusion that the positively charged particles only occupied a small portion of an atom’s overall volume.

Conclusion of Rutherford Gold Foil Experiment

Rutherford concluded the following from his observations:

• Because a large proportion of the α-particles directed toward the gold sheet went through it without any deflection, so, the majority of the space in an atom is vacant.
• Only a few α-particles were diverted off their route, suggesting that the atom’s positive charge takes up relatively little space.
• Since a very tiny percentage of α-particles completely rebounded, this implied that the atom’s mass and positive charge are concentrated in a small volume and not uniformly distributed.

Postulates of Rutherford Atomic Model 

Here are the major postulates of Rutherford’s atomic model based on observations and conclusions of the gold foil experiment:

• Positively charged particles make up an atom. The majority of an atom’s mass was contained in a very small area. The nucleus of an atom was the term used to describe this area of the atom. Later it was discovered that neutrons and protons make up the atom’s extremely tiny and dense nucleus.
• The electrons that surround an atom’s nucleus are negatively charged particles. The electrons rotate faster in a fixed circular path around the nucleus. Such a fixed circular path is called the orbit.
• Since electrons are negatively charged and the tightly packed nucleus is positively charged, an atom either has no net charge or is electrically neutral. The nucleus and electrons are held together by a strong electric force of attraction.

Drawbacks of Rutherford’s Model of Atom

There are several limitations or drawbacks of Rutherford’s atomic model, which are as follows:

• Rutherford’s Model predicts that electrons will orbit around the positively charged nucleus, which is not anticipated to be stable. A charged particle in rapid motion along a circular route, would lose energy continually and eventually collapse into the nucleus. This causes an atom to be unstable, whereas we know that atoms are extremely stable.
• Because it merely postulated the existence of protons in the nucleus, the Rutherford Model could not resolve the problem of atomic mass.
• Rutherford’s Atomic Model doesn’t explain the arrangement of electrons in the atom, which makes this model incomplete in this regard.

FAQs on Rutherford’s Atomic Model

Q1: Why was J.J. Thomson’s atomic model flawed?

Answer:

Thomson’s atomic model does not explain how the positive charge on the electrons inside the atom is maintained. It also fails to explain the stability of an atom. The nucleus of an atom is not mentioned in the hypothesis. It could not explain Rutherford’s scattering experiment.

Q2: What is  Rutherford’s scattering experiment?

Answer:

To determine how electrons are arranged in an atom, the Alpha (α) Particle Scattering Experiment was organized by Rutherford. Rapidly-moving α-particles were directed to bombard a thin sheet of gold. The gold foil was selected so as to obtain an extremely thin layer.

The hypothesis was that α-particles would be deflected by the sub-atomic particles in the gold atoms. Rutherford didn’t expect to witness significant deflections as the α-particles were considerably heavier than the protons. However, the experiment produced entirely unanticipated results.

Q3: Which sub-atomic particle was discovered by Rutherford through his Alpha (α) Particle Scattering Experiment?

Answer:

The discovery of the proton was made by Rutherford in 1917, using his alpha particle scattering experiment as a base. Also, he discovered the nucleus of the atom with his experiment in 1911.

Q4: Why was a gold foil used in the Alpha (α) Particle Scattering Experiment?

Answer:

A gold foil was selected so as to obtain an extremely thin layer and as it is the most malleable metal. However, if any other metal foil was used, the results obtained would be inconsistent.

Q5: What was the main drawback of Rutherford’s Model of the Atom?

Answer:

Rutherford’s Model predicts that electrons will orbit around the positively charged nucleus, which is not anticipated to be stable. A charged particle in rapid motion along a circular route, would lose energy continually and eventually collapse into the nucleus. This causes an atom to be unstable, whereas we know that atoms are extremely stable.

Q6: By which angles did the α-particles get deflected in the scattering experiment?

Answer:

The majority of the fast-moving α-particles went directly through the gold foil. The foil deflected some of the α-particles by fairly small angles. Only a few α-particles were completely deflected back (by 180 degrees).

Q7: How did Rutherford define an Orbit?

Answer:

According to Rutherford, negatively charged electrons encircle the nucleus of an atom. He believed that the electrons encircling the nucleus travel around it in circular routes at great speeds. He referred to these circular routes as well-defined orbits.

Q8: Who discovered Atomic Nucleus?

Answer:

Ernest Rutherford concluded that the majority of the mass of the atom is condensed at the center of the atom, which was named the nucleus.