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Rutherford Atomic Model

Rutherford Atomic Model was proposed by Ernest Rutherford in 1911. It is also called the Planetary Model of the Atom. It introduced the concept of a dense, positively charged nucleus at the center of an atom, with electrons orbiting around it, forming the basis for modern atomic theory.

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

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.

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:

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.
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.
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.
Alpha (α) Particle Scattering Experiment

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.

Also, Read :

Atomic Structure
Thomson’s Atomic Model
Bohr’s Model of an Atom

FAQs on Rutherford’s Atomic Model

Why was j.j. thomson’s atomic model flawed.

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.

What is Rutherford’s Scattering 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 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.

Which sub-atomic particle was discovered by Rutherford through his Alpha Particle Scattering Experiment?

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.

Why was a gold foil used in the Alpha Particle Scattering Experiment?

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.

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

By which angles did the α -particles get deflected in the scattering experiment.

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).

How did Rutherford define an Orbit?

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.

Who discovered Atomic Nucleus?

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.

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Bohr Model of the Atom Explained

Planetary Model of the Hydrogen Atom

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The Bohr Model has an atom consisting of a small, positively charged nucleus orbited by negatively charged electrons. Here's a closer look at this planetary model.

Overview of the Bohr Model

Niels Bohr proposed the Bohr Model of the Atom in 1915. Because the Bohr Model is a modification of the earlier Rutherford Model, some people call Bohr's Model the Rutherford-Bohr Model. The modern model of the atom is based on quantum mechanics. The Bohr Model contains some errors, but it is important because it describes most of the accepted features of atomic theory without all of the high-level math of the modern version. Unlike earlier models, the Bohr Model explains the Rydberg formula for the spectral emission lines of atomic hydrogen .

The Bohr Model is a planetary model in which the negatively charged electrons orbit a small, positively charged nucleus similar to the planets orbiting the sun (except that the orbits are not planar). The gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons.

Main Points of the Bohr Model

Electrons orbit the nucleus in orbits that have a set size and energy.
The energy of the orbit is related to its size. The lowest energy is found in the smallest orbit.
Radiation is absorbed or emitted when an electron moves from one orbit to another.

Bohr Model of Hydrogen

The simplest example of the Bohr Model is for the hydrogen atom (Z = 1) or for a hydrogen-like ion (Z > 1), in which a negatively charged electron orbits a small positively charged nucleus. Electromagnetic energy will be absorbed or emitted if an electron moves from one orbit to another. Only certain electron orbits are permitted. The radius of the possible orbits increases as n 2 , where n is the principal quantum number . The 3 → 2 transition produces the first line of the Balmer series . For hydrogen (Z = 1) this produces a photon having wavelength 656 nm (red light).

Bohr Model for Heavier Atoms

Heavier atoms contain more protons in the nucleus than the hydrogen atom. More electrons were required to cancel out the positive charge of all of the protons. Bohr believed each electron orbit could only hold a set number of electrons. Once the level was full, additional electrons would be bumped up to the next level. Thus, the Bohr model for heavier atoms described electron shells. The model explained some of the atomic properties of heavier atoms, which had never been reproduced before. For example, the shell model explained why atoms got smaller moving across a period (row) of the periodic table, even though they had more protons and electrons. It also explained why the noble gases were inert and why atoms on the left side of the periodic table attract electrons, while those on the right side lose them. However, the model assumed electrons in the shells didn't interact with each other and couldn't explain why electrons seemed to stack irregularly.

Problems With the Bohr Model

It violates the Heisenberg Uncertainty Principle because it considers electrons to have both a known radius and orbit.
The Bohr Model provides an incorrect value for the ground state orbital angular momentum .
It makes poor predictions regarding the spectra of larger atoms.
The Bohr Model does not predict the relative intensities of spectral lines.
It does not explain fine structure and hyperfine structure in spectral lines.
The Bohr Model does not explain the Zeeman Effect.

Refinements and Improvements to the Bohr Model

The most prominent refinement to the Bohr model was the Sommerfeld model, which is sometimes called the Bohr-Sommerfeld model. In this model, electrons travel in elliptical orbits around the nucleus rather than in circular orbits. The Sommerfeld model was better at explaining atomic spectral effects, such the Stark effect in spectral line splitting. However, the model couldn't accommodate the magnetic quantum number.

Ultimately, the Bohr model and models based upon it were replaced Wolfgang Pauli's model based on quantum mechanics in 1925. That model was improved to produce the modern model, introduced by Erwin Schrodinger in 1926. Today, the behavior of the hydrogen atom is explained using wave mechanics to describe atomic orbitals.

Lakhtakia, Akhlesh; Salpeter, Edwin E. (1996). "Models and Modelers of Hydrogen". American Journal of Physics . 65 (9): 933. Bibcode:1997AmJPh..65..933L. doi: 10.1119/1.18691
Linus Carl Pauling (1970). "Chapter 5-1". General Chemistry (3rd ed.). San Francisco: W.H. Freeman & Co. ISBN 0-486-65622-5.
Niels Bohr (1913). "On the Constitution of Atoms and Molecules, Part I" (PDF). Philosophical Magazine . 26 (151): 1–24. doi: 10.1080/14786441308634955
Niels Bohr (1914). "The spectra of helium and hydrogen". Nature . 92 (2295): 231–232. doi:10.1038/092231d0
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