On the basis of the observations and conclusions of alpha particles scattering experiment, Rutherford proposed his nuclear model of atom in 1912. This atomic model is based on alpha particles scattering experiment. It is also called planetary model of atom. It gives the structure of the atom in 1912.
According to the Rutherford atomic model:
- An atom consists of a positively charged nucleus, which is concentrated in a very small volume located the center of an atom.
- The entire positive charge and most of the mass of an atom is densely concentrated in an extremely small region. Rutherford named this tiny part of the atom the nucleus. The positive charge of the nucleus is because of protons found in it.
- The nucleus is surrounded by electrons that revolve around the nucleus with a very high speed in circular paths called orbits.
- Electrons and nucleus are held together by the electrostatic force of attraction that balanced the centrifugal force acting on the revolving electrons.
- The effective size of the nucleus is extremely small as compared to the effective size of the atom. From the experiments, it was found that the approximate radius of the nucleus of the atom is of the order of 10-14 m to 10-15 m. While, the approximate radius of the atom (or the radius of sphere of the electrons) is of the order of 10-10 m (10-8 cm).
- The magnitude of the charge on the nucleus differs for atoms for distinct elements.
- The total amount of negative charge on the electrons is exactly equal to the total amount of positive charge on the nucleus so that an atom is electrically neutral.
- Most of the space between the nucleus and electron inside the atom is vacant.
Compare of Rutherford Atomic Model with Solar System
Rutherford’s atomic model or, say, a nuclear model of atom can be compared with the structure of the solar system. As in the solar system, the sun is at the center (having the maximum mass) and planets revolving around it.
Just like the nucleus is at the center of an atom and the electrons revolved around it in orbits. Thus, Rutherford’s atomic model resembles the solar system in which the nucleus plays the role of the sun and electrons that of revolving planets.
Because of this comparison, revolving electrons around the nucleus are sometimes called planetary electrons and Rutherford’s atomic model is called a planetary model of atom.
Drawback of Rutherford’s Atomic Model
As we have discussed that Rutherford, in 1912, put forward the nuclear model of atom, which was based on experimental observations. Later, this model was failed to explain certain things. They are:
(1) According to Maxwell’s electromagnetic theory, when a charged particle is accelerated, it emits energy in the form of electromagnetic radiations. As per Rutherford’s atomic model, electrons revolve around the nucleus in fixed circular paths called orbits. This means electrons are continuously accelerating due to change in the direction of the motion. Since electrons are accelerating charged particles, therefore, electron revolving in an orbit should be continuously emit electromagnetic radiations and lose energy.
As the electron loses energy in the form of radiation, it would slow down and could no longer withstand the attractive force of the nucleus. Hence, it would gradually move to the nucleus and would ultimately fall in the nucleus by following a spiral path. But, this does not happen because atoms are stable. Thus, there must be something drawback in the Rutherford’s nuclear model of the atom. In this way, Rutherford’s model was not in accordance with Maxwell’s electromagnetic theory and he failed to explain the stability of an atom. This is because the third fundamental particle named neutron was unknown at this time.
(2) Another drawback of the Rutherford atomic model is that he did not tell anything about the distribution of electrons around the nucleus and about their energies.
(3) His model also failed to explain the existence of certain definite lines in the hydrogen spectrum. If an electron continuously loses energy while spiraling toward the nucleus, it should emit radiation across a continuous range of frequencies, resulting in a continuous spectrum.
However, experimental observations showed that the hydrogen atom emits light only at certain specific wavelengths, leading to a discrete line spectrum rather than a continuous one. Rutherford’s model couldn’t account for this, which was another major drawback.
FAQ on Rutherford’s Atomic Model
1. What is Rutherford’s Atomic Model?
In 1912, Rutherford’s atomic model suggests that the atom consists of a small, dense, positively charged nucleus at the center with electrons revolving around it in fixed circular orbits, much like planets around the sun. The nucleus at the central part of an atom contains protons (and later discovered, neutrons), while the rest of the atom is mostly empty space.
2. Who was Rutherford?
Ernest Rutherford was a British chemist and physicist in the year 1911 who discovered the nucleus present in the atom. He was born on 30 August 1871 at the place of Brightwater, New Zealand and died on 19 October 1937 at the place of Cambridge, England.
3. Who was known to be the father of nuclear physics?
Ernest Rutherford is known as the “Father of Nuclear Physics.” He made groundbreaking contributions to the understanding of the structure of the atom, especially through his gold foil experiment, which led to the discovery of the atomic nucleus.
4. What are the main features of Rutherford’s atomic model?
1. Nucleus: It is a small, dense, positively charged core at the center of the atom.
2. Electrons: Negatively charged electrons revolve around the nucleus in circular orbits.
3. Empty Space: Most of the atom is empty space.
4. Mass Concentration: Almost all the mass of the atom is concentrated in the nucleus.
5. What were the main drawbacks of Rutherford’s nuclear model of atom?
1. It could not explain the stability of an atom.
2. It could not explain line spectrum of hydrogen.
6. How is Rutherford’s atomic model different from the earlier Thomson model?
Thomson’s model (also known as “plum pudding” model) proposed that the atom was a uniform sphere of positively charged material with negatively charged electrons embedded within it. In contrast, Rutherford’s model introduced the idea of a central nucleus with electrons orbiting around it, and most of the atom is vacant space.
7. Why is Rutherford’s nuclear model of atom also called the “planetary model”?
Rutherford’s nuclear model of atom is often compared with the planetary model because it resembles the solar system, where electrons revolve around the nucleus much like planets orbit the sun.
8. Did Rutherford’s model include neutrons?
No, Rutherford’s atomic model did not include neutrons. James Chadwick discovered later neutrons in 1932. Rutherford’s model focused on the atomic nucleus being made of protons (positive charge) and electrons revolving around it.
9. Why did Rutherford choose gold foil for his experiment instead of other metals?
Rutherford chose gold foil for his famous experiment because gold is highly malleable and stable, which can be made very thin without breaking. This thinness was important for the experiment so that alpha particles (positively charged helium nuclei) pass through the foil with minimal obstruction, making it easier to observe their interactions with individual atoms. Other metals were less suitable because they couldn’t be made into such thin sheets.