Atomic Models [John Dalton, Thomson's Plum pudding, Rutherford & Bohr] Explained with Distinctions

Atomic models

There has been a wide range of atomic models throughout the history of atomic physics, that refers primarily to a period from the beginning of the 19th century to the very first half of the 20th century. Here are some standard atomic models.

atomic-models

Table of Contents Hide

1) John Dalton’s atomic model
2) Plum pudding model
3) Rutherford’s Atomic Model
- 3.1) Outcomes of the experiment
- 3.2) Defects in Rutherford’s Model
4) Bohr’s Atomic Theory
- 4.1) Postulates of Bohr’s atomic model
5) Distinction between Rutherford and Bohr’s atomic models
6) Frequently Asked Questions (FAQs) – Atomic Models
7) Multiple Choice Questions (MCQs) – Atomic Models
8) Conclusion
9) You may also like to learn:

John Dalton’s atomic model

John Dalton was an English scientist, who came up with an idea that all matter is made up of really small things. It was the very first total effort to explain all matter in regard to particles. He called these particles atoms and formed an atomic theory. In this theory, he declares that:

All matter is made from atoms. Atoms are indivisible and unbreakable.
All atoms of a given element are equal in mass and properties.
Substances are formed by a mix of 2 or more different sorts of atoms.
A chemical reaction is a rearrangement of atoms

Plum pudding model

After the discovery of an electron in 1897, people understood that atoms are made up of even smaller particles. Quickly after in 1904 J. J. Thomson proposed his famous “plum pudding model”. In this model, atoms were understood to include negatively charged electrons, nevertheless, the atomic nucleus had not been discovered yet.

Thomson understood that the atom had an overall neutral charge. He thought that there need to be something to counterbalance the negative charge of an electron. He came up with an idea that negative particles are floating within a soup of scattered positive charge. His model is typically called the plum pudding model, because of his similarity to a popular English dessert.

Rutherford’s Atomic Model

Rutherford carried out the ‘Gold Foil’ experiment to comprehend how negative and positive charges could exist side-by-side in an atom. He bombarded alpha particles on a 0.00004 cm thick gold foil. Alpha particles are given off by radioactive elements like radium and polonium.

These are in fact helium nuclei (He²⁺). They can penetrate through matter to some extent. He observed the impacts of- particles on a photographic plate or a screen coated with zinc sulphide. He proved that the ‘plum-pudding’ model of the atom was not accurate.

Observations made by Rutherford were as follows:

Almost all the particles passed through the foil un-deflected.
Out of 20000 particles, just a couple of were deflected at fairly large angles and very few recovered on striking the gold foil.

Outcomes of the experiment

Keeping in view the experiment, Rutherford proposed a planetary model for an atom and concluded the following outcomes:

Because most of the particles passed through the foil undeflected, for that reason most of the volume occupied by an atom is empty.
The deflection of a couple of particles showed that there is a ‘center of positive charges’ in an atom, which is called ‘nuclei’ of an atom.
The total rebounce of a few particles shows that the nucleus is very dense and hard.
Considering that a couple of particles were deflected, it shows that the size of the nucleus is really small as compared to the total volume of an atom.
The electrons move around the nucleus.
An atom as a whole is neutral, for that reason the number of electrons in an atom is equal to the number of protons.

Further Reading: Molybdenum: Occurrence, Properties, Uses, and Isotopes of Molybdenum

Other than electrons, all other fundamental particles that lie within the nucleus, are referred to as nucleons.

Defects in Rutherford’s Model

Although Rutherford’s experiment showed that the ‘plum-pudding’ model of an atom was not correct, yet it had the following problems:

According to the classical theory of radiation, electrons being the charged particles need to release or emit energy constantly and they should ultimately fall under the nucleus.
If the electrons give off energy constantly, they need to form a constant spectrum but in fact, the line spectrum was observed.

Bohr’s Atomic Theory

Keeping in view the defects in Rutherford’s Atomic Model, Neil Bohr provided another model of the atom in 1913.

The Quantum Theory of Max Planck was utilized as the foundation for this model According to Bohr’s model, revolving electron in an atom does not soak up or release energy continuously. The energy of a revolving electron is ‘quantized’ as it revolves just in orbits of fixed energy, called ‘energy levels’ by him.

Postulates of Bohr’s atomic model

The Bohr’s atomic model was based upon the following postulates:

i. The hydrogen atom includes a small nucleus and electrons are revolving in one of the circular orbits of radius ‘r’ around the nucleus.

ii. Each orbit has set energy that is quantized.

iii. As long as electron stays in a particular orbit, it does not emit or absorb energy. The energy is released or taken in just when electron leaps from one orbit to another.

iv. When an electron jumps from lower orbit to higher orbit, it absorbs energy and when it jumps from a higher orbit to lower orbit it radiates energy. This change in energy, E is given by following Planck’s formula

Where, h is Planck’s consistent equal to 6.63 10^-34 Js, and v is the frequency of light.

v. Electron can revolve just in orbits of a fixed angular moment mvr, given as:

Where ‘n’ is the quantum number or orbit number having values 1,2,3 and so on.

Distinction between Rutherford and Bohr’s atomic models

Rutherford’s Model	Bohr’s Model
It was based upon classical theory.	It was based upon quantum theory.
Electrons revolve around the nucleus.	Electrons revolve around the nucleus in orbits of fixed energy.
No concept about orbits was introduced.	Orbits had angular momentum.
Atoms ought to produce constant spectrum.	Atoms should produce a line spectrum.
Atoms should collapse.	Atoms need to exist.

Frequently Asked Questions (FAQs) – Atomic Models

1. What is the foundation of John Dalton’s atomic model?
John Dalton’s atomic model is based on the idea that all matter is composed of indivisible and unbreakable particles called atoms. Each element’s atoms are identical in mass and properties.2. Who proposed the Plum Pudding model, and what does it suggest?
J.J. Thomson proposed the Plum Pudding model in 1904. This model envisions atoms as a positively charged “pudding” with negatively charged electrons dispersed within, resembling the popular English dessert.3. How did Rutherford challenge the Plum Pudding model?
Rutherford’s Gold Foil experiment demonstrated that the Plum Pudding model was inaccurate. Most alpha particles passed through the foil un-deflected, revealing that atoms have a mostly empty space with a dense, positively charged nucleus.4. What were the outcomes of Rutherford’s Gold Foil experiment?
The experiment led to several conclusions, including the presence of a nucleus with positive charges, the small size and density of the nucleus compared to the atom, and the electrons orbiting the nucleus.5. What are the defects in Rutherford’s Atomic Model?
Rutherford’s model faced challenges as it couldn’t explain the stability of electrons in orbits and the absence of continuous spectra. According to classical theory, electrons should emit energy continuously and collapse into the nucleus.6. How did Bohr address the defects in Rutherford’s model?
Bohr’s Atomic Theory, proposed in 1913, addressed the issues in Rutherford’s model by introducing quantized energy levels for electrons. Electrons revolve in fixed orbits, absorbing or emitting energy only when transitioning between orbits.7. What are the postulates of Bohr’s atomic model?
Bohr’s model is based on the postulates that electrons revolve in quantized orbits, each with a fixed energy level. Energy is released or absorbed when electrons move between orbits, and electron orbits are restricted to specific angular momentum values.

Further Reading: Osmium: Occurrence, Properties, Uses and Isotopes of Osmium

8. How is the energy change during electron transitions in Bohr’s model calculated?
Bohr’s formula calculates energy change (ΔE) during electron transitions using Planck’s formula: ΔE = hν, where h is Planck’s constant and ν is the frequency of light associated with the transition.

9. What does the quantum number ‘n’ represent in Bohr’s model?
The quantum number ‘n’ in Bohr’s model represents the orbit number, indicating the electron’s energy level or quantized orbit. It takes values such as 1, 2, 3, and so on.

Multiple Choice Questions (MCQs) – Atomic Models

Conclusion

In the exploration of atomic models, the journey from John Dalton’s groundbreaking atomic theory to the refined models of J.J. Thomson, Ernest Rutherford, and Niels Bohr has unveiled the intricate nature of matter at its fundamental level. Dalton’s notion of indivisible and equal atoms laid the foundation for a deeper understanding.

The “plum pudding model” introduced by Thomson captured the imagination, suggesting electrons embedded in a positive charge “soup.” However, Rutherford’s ingenious Gold Foil experiment shattered this conceptual framework, revealing a nucleus at the atom’s core, challenging the prevailing ideas.

Rutherford’s outcomes prompted a paradigm shift, leading to the proposal of Bohr’s atomic model. Bohr introduced the concept of quantized energy levels, revolutionizing our perception of electron behavior. His model addressed some of the flaws in Rutherford’s theory, providing a more accurate depiction of atomic structure.

While these models represent significant strides in atomic physics, they also underscore the iterative and dynamic nature of scientific progress. The evolution from Dalton to Bohr exemplifies the relentless pursuit of understanding, with each model building upon the strengths and addressing the limitations of its predecessors.

As we delve into the microcosm of atoms, the interplay of experiments, observations, and theoretical frameworks continues to shape our comprehension of the fundamental building blocks of the universe. The quest for knowledge in atomic physics remains an ever-evolving journey, with each revelation bringing us closer to unraveling the mysteries of the subatomic realm.