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Designed by Fu-Kwun Hwang http://www.phy.ntnu.edu.tw/ntnujava/  

Translations

Code	Language		Translator	Run

Credits

Fu Kwun Hwang; Loo Kang Wee; Fremont

Purpose: To provide a concise overview of the Bohr model of the hydrogen atom, as presented within the context of an interactive simulation tool developed by Open Educational Resources / Open Source Physics @ Singapore.

Main Themes & Key Ideas:

• The Bohr Model as a Stepping Stone: The resource emphasizes that the Bohr model, while historically significant, is not the final, definitive description of atomic structure. It served as an "intermediate step on the way to a precise theory of the atomic structure, which was made possible by quantum mechanics and quantum electrodynamics." This highlights the importance of understanding the model's limitations.
• Explaining the Rydberg Formula: A key achievement of the Bohr model was its ability to theoretically explain the Rydberg formula for the spectral emission lines of atomic hydrogen. As the document states, "the model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen." This signifies the model's predictive power and its grounding in experimental observations.
• Combining Particle and Wave Perspectives: The resource introduces the concept of the electron as both a particle and a de Broglie wave. Stable orbits are only possible when the electron's de Broglie wave meets the condition for a standing wave, where "the circumference must be an integer multiple of the wavelength."
• Quantized Energy Levels and Photon Emission: The model postulates that electrons can only occupy specific energy levels, and they do not radiate energy while in those levels. Energy is only emitted when an electron transitions between energy levels, releasing the energy difference as a photon. In Bohr's model, "an electron in Bohr's model emits no energy, as long as its energy has one of the above-mentioned values. However, an electron which is not in the lowest energy level (n = 1), can make a spontaneous change to a lower state and thereby emit the energy difference in the form of a photon."
• Addressing Classical Physics Limitations: The resource directly acknowledges the conflict between classical electrodynamics and the observed stability of atoms. Classical theory predicted that accelerating electrons should continuously radiate energy and spiral into the nucleus. Bohr's model overcomes this problem by stating that electron energy is quantized.
• Interactive Simulation: The resource describes a JavaScript simulation applet that allows users to explore the Bohr model. The applet enables users to visualize the hydrogen atom in both particle and wave models.
• Origin and Context: The simulation is rooted in the work of Niels Bohr, the Danish physicist who introduced it in 1913. The simulation was designed by Fu-Kwun Hwang.

Key Facts:

• The Bohr model was introduced in 1913 by Niels Bohr.
• It was an improvement over earlier atomic models like the plum-pudding model and Rutherford model.
• The model successfully explained the Rydberg formula.
• Electrons can only occupy specific, quantized energy levels.
• Electrons emit photons when transitioning between energy levels.
• The simulation allows users to visualize the model in both particle and wave representations.

Quotes:

• "the model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen."
• "the circumference must be an integer multiple of the wavelength."
• "an electron in Bohr's model emits no energy, as long as its energy has one of the above-mentioned values. However, an electron which is not in the lowest energy level (n = 1), can make a spontaneous change to a lower state and thereby emit the energy difference in the form of a photon."
• "Bohr's model of the hydrogen atom was only an intermediate step on the way to a precise theory of the atomic structure, which was made possible by quantum mechanics and quantum electrodynamics."

Conclusion:

The "Bohr's Theory of the Hydrogen Atom JavaScript Simulation Applet HTML5" provides a valuable interactive resource for understanding the historical context, key postulates, and limitations of the Bohr model. While it is essential to recognize that the Bohr model is not the complete picture of atomic structure, it serves as a crucial stepping stone in the development of quantum mechanics and our understanding of atomic phenomena. The simulation tool enhances learning through visualization and interactive exploration.

 

Bohr's Atomic Model: A Study Guide

Quiz

Answer the following questions in 2-3 sentences each.

1. What was the primary improvement of the Bohr model over previous atomic models like the Rutherford model?
2. What experimental observation did Bohr's model successfully explain?
3. According to Bohr's model, what force is responsible for the attraction between the electron and the nucleus?
4. Explain the concept of "standing waves" in the context of electron orbits in Bohr's model.
5. Why, according to classical electrodynamics, should an electron spiral into the nucleus? How does Bohr's model address this issue?
6. What happens when an electron transitions from a higher energy level to a lower energy level in Bohr's model?
7. Why is Bohr's model considered an intermediate step toward a more precise theory of atomic structure?
8. What are the two modes available in the JavaScript simulation of Bohr's model described in the source material?
9. In the simulation, what information is available about each energy level of the hydrogen atom?
10. What is the principal quantum number n and how does it relate to the energy levels in Bohr's model?

Quiz Answer Key

1. The Bohr model introduced quantized energy levels for electrons, restricting them to specific orbits, unlike previous models that allowed for any orbit. This explained the stability of atoms and their discrete emission spectra.
2. Bohr's model successfully explained the Rydberg formula for the spectral emission lines of atomic hydrogen. It provided a theoretical basis for the formula and justified its empirical results using fundamental physical constants.
3. According to Bohr's model, the attractive force between the negatively charged electron and the positively charged nucleus is the electrostatic force, governed by Coulomb's law. This force keeps the electron in its orbit.
4. In Bohr's model, electrons behave as de Broglie waves, and only orbits where the circumference is an integer multiple of the electron's wavelength are stable. This condition creates a standing wave, preventing the electron from radiating energy and spiraling into the nucleus.
5. Classical electrodynamics predicts that a charged particle undergoing acceleration (like an electron in circular motion) should continuously radiate electromagnetic waves, losing energy and spiraling into the nucleus. Bohr's model avoids this by postulating that electrons in specific orbits do not radiate energy.
6. When an electron transitions from a higher energy level to a lower energy level in Bohr's model, it emits the energy difference in the form of a photon, a particle of light. The wavelength of this photon corresponds to a specific line in the hydrogen spectrum.
7. Bohr's model, while successful in explaining the hydrogen spectrum, is limited and does not accurately describe more complex atoms. Quantum mechanics and quantum electrodynamics provide a more complete and accurate description of atomic structure.
8. The JavaScript simulation offers two modes: a particle mode where the electron is treated as a particle orbiting the nucleus and a wave mode where the electron is treated as a wave, demonstrating the wave-particle duality.
9. The simulation allows users to read off the orbital radius (r) and the total energy (E) associated with each energy level of the hydrogen atom.
10. The principal quantum number n is an integer that defines the energy level of an electron in the Bohr model. Higher values of n correspond to higher energy levels and larger orbital radii.

Essay Questions

1. Discuss the key assumptions of Bohr's model of the hydrogen atom. How did these assumptions deviate from classical physics, and what problems did they solve?
2. Explain how Bohr's model successfully predicted the spectral lines of hydrogen. What are the limitations of this model when applied to other atoms?
3. Compare and contrast the Bohr model with earlier atomic models, such as the Rutherford model. What were the strengths and weaknesses of each model?
4. Describe the role of the de Broglie hypothesis in the development of Bohr's model. How did the concept of wave-particle duality influence Bohr's postulates?
5. Assess the significance of Bohr's model in the history of atomic physics. How did it pave the way for the development of quantum mechanics?

Glossary of Key Terms

• Bohr Model: A model of the atom, proposed by Niels Bohr in 1913, depicting electrons orbiting a positively charged nucleus in specific, quantized energy levels.
• Rydberg Formula: An empirical formula used to predict the wavelengths of spectral lines of many chemical elements, including hydrogen.
• Spectral Emission Lines: Specific wavelengths of light emitted by an atom when electrons transition between energy levels.
• Quantum Physics: A branch of physics that deals with the discrete, quantized nature of energy and matter at the atomic and subatomic levels.
• Electrostatic Force: The attractive or repulsive force between charged particles, described by Coulomb's law.
• Coulomb's Law: A law stating that the electrostatic force between two charged particles is directly proportional to the product of the magnitudes of their charges and inversely proportional to the square of the distance between them.
• De Broglie Wave (Wave of Matter): The concept that all matter exhibits wave-like properties, with a wavelength inversely proportional to its momentum.
• Standing Wave: A wave that appears to be stationary, formed by the interference of two waves traveling in opposite directions.
• Photon: A particle of light, representing a quantum of electromagnetic radiation.
• Principal Quantum Number (n): An integer that defines the energy level of an electron in the Bohr model, with higher values indicating higher energy levels.
• Quantum Mechanics: A more advanced theory of physics that describes the behavior of matter and energy at the atomic and subatomic levels, incorporating wave-particle duality and uncertainty.
• Quantum Electrodynamics: The quantum field theory of electromagnetism, describing how light and matter interact and providing a more complete model for electromagnetism than classical physics.
• Energy Levels: Discrete, quantized values of energy that an electron can possess in an atom, corresponding to specific orbits around the nucleus.
• Wave-Particle Duality: The concept that all matter and energy exhibit both wave-like and particle-like properties.
• Rutherford Model: An earlier model of the atom, proposed by Ernest Rutherford, which depicted a small, positively charged nucleus surrounded by orbiting electrons.

Sample Learning Goals

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For Teachers

In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons  that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces  providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a quantum physics-based modification of the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model.
http://en.wikipedia.org/wiki/Bohr_model

Introduced by Niels Bohr in 1913, the model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, it also provided a justification for its empirical results in terms of fundamental physical constants.

In 1913, the Danish physicist Niels Bohr (1885 - 1962) managed to explain the spectrum of atomic hydrogen by an extension of Rutherford's description of the atom. In that model, the negatively charged electrons revolve about the positively charged atomic nucleus because of the attractive electrostatic force according to Coulomb's law.

But the electron can be taken not only as a particle, but also as a de Broglie wave (wave of matter) which interferes with itself. The orbit is only stable, if it meets the condition for a standing wave: The circumference must be an integer multiple of the wavelength. The consequence is that only special values of radius and energy are allowed. The mathematical appendix explains how to calculate these values.

According to classical electrodynamics, a charge, which is subject to centripetal acceleration on a circular orbit, should continuously radiate electromagnetic waves. Thus, because of the loss of energy, the electron should spiral into the nucleus very soon. By contast, an electron in Bohr's model emits no energy, as long as its energy has one of the above-mentioned values. However, an electron which is not in the lowest energy level (n = 1), can make a spontaneous change to a lower state and thereby emit the energy difference in the form of a photon (particle of light). By calculating the wavelengths of the corresponding electromagnetic waves, one will get the same results as by measuring the lines of the hydrogen spectrum.

You must not take the idea of electrons, orbiting around the atomic nucleus, for reality. Bohr's model of the hydrogen atom was only an intermediate step on the way to a precise theory of the atomic structure, which was made possible by quantum mechanics and quantum electrodynamics.

This applet illustrates a hydrogen atom according to particle or wave model. You can choose a principal quantum number n (with slider). The right part of the graphics represents the energy levels of the atom. You can read off the orbital radius r and the total energy E.

This simulation  is similar to the one at http://www.walter-fendt.de/ph11e/bohrh.htm
Default setting is the particle mode. you can switch to wave mode.

It will switch to wave mode when you click and drag within simulation region.
If you try to vary the orbit's radius with pressed on drag mouse button, this will generally lead to a non-stationary state. You can test this out by using the option "Wave model".

https://www.youtube.com/watch?v=fKYso97eJs4 Spectral Lines by cassiopeiaproject

Research

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FAQ: Bohr's Theory of the Hydrogen Atom

• What is the Bohr model of the atom, and how does it relate to earlier models?
• The Bohr model, proposed by Niels Bohr in 1913, pictures the atom as a small, positively charged nucleus surrounded by electrons orbiting in specific circular paths, much like planets around the sun. It built upon earlier models such as the cubic model, plum-pudding model, Saturnian model, and the Rutherford model. The Bohr model is a quantum physics-based modification of the Rutherford model.
• What was the key success of the Bohr model?
• The Bohr model's main achievement was its explanation of the Rydberg formula for the spectral emission lines of atomic hydrogen. This formula was known empirically, but the Bohr model provided a theoretical basis, justifying the formula's structure and constants in terms of fundamental physics.
• How does the Bohr model incorporate the concept of de Broglie waves?
• The Bohr model considers the electron not only as a particle but also as a de Broglie wave. Stable orbits are only those where the electron wave interferes constructively with itself, meaning the circumference of the orbit must be an integer multiple of the electron's wavelength. This wave nature restricts the allowed radii and energy levels of the electron.
• How does the Bohr model address the issue of electron radiation predicted by classical electrodynamics?
• Classical electrodynamics predicts that an electron accelerating in a circular orbit should continuously emit electromagnetic waves, losing energy and spiraling into the nucleus. The Bohr model avoids this issue by postulating that electrons in specific energy levels do not radiate energy. Radiation only occurs when an electron transitions between these energy levels, emitting a photon with energy equal to the energy difference.
• What are the limitations of the Bohr model?
• The Bohr model, while groundbreaking, is not a complete representation of atomic structure. It is primarily applicable to hydrogen-like atoms (atoms with only one electron). The model is an intermediate step towards a more precise theory of atomic structure based on quantum mechanics and quantum electrodynamics. The concept of electrons orbiting the nucleus should not be taken as reality.
• How does the Bohr model explain the hydrogen spectrum?
• When an electron transitions from a higher energy level to a lower energy level in the Bohr model, it emits energy in the form of a photon. The wavelength of this photon corresponds to a specific spectral line. By calculating the wavelengths of these emitted photons for different transitions, the Bohr model accurately predicts the lines observed in the hydrogen spectrum.
• What is the significance of the principal quantum number 'n' in the Bohr model?
• The principal quantum number 'n' determines the energy level of the electron in the Bohr model. Higher values of 'n' correspond to higher energy levels and larger orbital radii. Only integer values of 'n' are allowed (n = 1, 2, 3, ...), representing quantized energy levels.
• What are some interactive resources available to help visualize and understand the Bohr model?
• There are JavaScript-based simulation applets, such as the one mentioned in the resource, that allow users to explore the Bohr model interactively. These simulations allow users to adjust parameters like the principal quantum number (n) and visualize the corresponding orbital radius and energy levels. They also illustrate the wave-particle duality of the electron and the concept of non-stationary states.