Bohr’s Model of the Atom: Complete Guide, Postulates & Electronic Configuration | Class 9 Science Chapter 4
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Welcome to the ultimate guide on Bohr’s Atomic Model. If you are a Class 9 student, you know that the “Structure of the Atom” is one of the most conceptually rich chapters in your Science syllabus. While Rutherford gave us the nucleus, it was Neils Bohr who gave the atom its stability and structure.
In this comprehensive guide, we won’t just “describe” the model. We will dive deep into how electrons are arranged, the Bohr-Bury scheme, and how this model helps us understand the chemical behavior of the first 18 elements. Whether you are answering a 3-mark question or preparing for a 5-mark long answer, this guide covers it all.
Why is this Topic Important?
Bohr’s model isn’t just history; it is the foundation for writing Electronic Configurations and finding Valency. Without understanding shells (K, L, M, N), you cannot master Chemistry in Class 10!
📝 Jump to Exam-Ready Answer Section →1. The Need for Bohr’s Model: Why Rutherford Failed
Before we look at what Bohr said, we must understand what was wrong with the previous model. Ernest Rutherford had proposed that electrons revolve around the nucleus in circular orbits. However, there was a major flaw in this theory according to classical physics:
Any charged particle (like an electron) moving in a circular orbit undergoes acceleration. According to electromagnetic theory, an accelerating charged particle must radiate energy. If an electron radiates energy, it would lose speed and spiral into the nucleus. This would make the atom unstable. But we know matter is stable!
To save the atom from collapsing, Neils Bohr (1913) introduced a revolutionary concept: Quantization of Energy.
2. Postulates of Bohr’s Atomic Model
[Image of Bohr atomic model structure]Neils Bohr proposed the following postulates to explain the stability of the atom:
Postulate 1: Discrete Orbits
Inside the atom, electrons cannot revolve just anywhere. Only certain special orbits known as discrete orbits of electrons are allowed inside the atom. These are “permitted” paths.
Postulate 2: Stationary States (No Energy Loss)
This is the most important point. While revolving in these discrete orbits, the electrons do not radiate energy.
- As long as an electron stays in a specific orbit, its energy is fixed.
- Because it doesn’t lose energy, it doesn’t fall into the nucleus.
- This explains the stability of the atom.
Postulate 3: Energy Shells
These orbits or shells are called energy levels. They are arranged concentrically around the nucleus.
visual Representation: The Energy Shells
Orbit Numbers (n) Shell Letters
( N Shell ) n = 4
( M Shell ) n = 3
( L Shell ) n = 2
( K Shell ) n = 1
[ NUCLEUS ]
These shells are represented by the letters K, L, M, N… or the numbers n = 1, 2, 3, 4…
- K Shell (n=1): The innermost shell, closest to the nucleus. Has the least energy.
- L Shell (n=2): The second shell.
- M Shell (n=3): The third shell.
- N Shell (n=4): The fourth shell. Has higher energy than K, L, M.
3. How are Electrons Distributed? (Bohr-Bury Scheme)
Knowing that shells exist is one thing, but knowing how many electrons go into each shell is another. Bohr and Bury suggested a set of rules for the distribution of electrons in different orbits.
4. Atomic Structure of the First 18 Elements
Using the rules above, let’s look at the composition of the first 18 elements of the periodic table. This table is crucial for solving numerical problems in exams.
[Image of electron distribution in first 18 elements]| Element | Symbol | Atomic No. (Z) | Electrons | K | L | M | N | Valency |
|---|---|---|---|---|---|---|---|---|
| Hydrogen | H | 1 | 1 | 1 | – | – | – | 1 |
| Helium | He | 2 | 2 | 2 | – | – | – | 0 |
| Lithium | Li | 3 | 3 | 2 | 1 | – | – | 1 |
| Carbon | C | 6 | 6 | 2 | 4 | – | – | 4 |
| Oxygen | O | 8 | 8 | 2 | 6 | – | – | 2 |
| Neon | Ne | 10 | 10 | 2 | 8 | – | – | 0 |
| Sodium | Na | 11 | 11 | 2 | 8 | 1 | – | 1 |
| Argon | Ar | 18 | 18 | 2 | 8 | 8 | – | 0 |
*Selected elements shown for clarity. Full table available in NCERT Chapter 4.
Why is Argon’s Configuration 2, 8, 8?
Wait, didn’t we say the M shell can hold 18 electrons? Why does Argon stop at 8 in the M shell?
This is where Rule #2 (Octet Rule) comes in. The outer shell cannot hold more than 8 electrons. Since Argon has 18 electrons (2 in K, 8 in L), the remaining 8 go to M. Since M becomes the outer shell, it is stable with 8. If we added one more electron (Potassium, Z=19), it would go to the N shell (2, 8, 8, 1), not the M shell!
📝 Exam-Ready Answers (Score Full Marks)
Q1 (3 Marks)
Describe Bohr’s model of the atom.
To explain the stability of the atom, Neils Bohr proposed the following postulates:
1. Discrete Orbits: Electrons revolve around the nucleus only in certain special orbits called “discrete orbits.”
2. No Energy Radiation: While revolving in these discrete orbits, electrons do not radiate energy. This explains why the electron does not spiral into the nucleus.
3. Energy Levels: These orbits are also known as energy shells, represented by K, L, M, N (or n=1, 2, 3, 4).
Q2 (2 Marks)
Write the rules given by Bohr and Bury for distribution of electrons.
The Bohr-Bury scheme states:
1. The maximum number of electrons in a shell is given by \(2n^2\) (e.g., K=2, L=8, M=18).
2. The outermost orbit can accommodate a maximum of 8 electrons.
3. Shells are filled in a step-wise manner; electrons are not accommodated in a given shell unless inner shells are filled.
❓ Frequently Asked Questions
1. Does Bohr’s model apply to all elements?
Bohr’s model works perfectly for hydrogen and hydrogen-like species (with 1 electron). For complex atoms with many electrons, the model needed further refinements (leading to the Quantum Mechanical Model), but for Class 9 and 10, Bohr’s model is the standard we follow.
2. What is the difference between an orbit and a shell?
In Bohr’s model, they are essentially the same. An orbit is the circular path, and a shell implies the energy level associated with that path.
3. Why is the valency of Helium zero?
Helium has 2 electrons in its K shell. The maximum capacity of the K shell is 2 (\(2n^2\)). Since its outermost shell is completely filled, it has no tendency to react or combine with others. Hence, its valency is 0.
✍️ Written by Dr. Irfan Mansuri
Senior Science Educator & Researcher
Independent Educational Consultant • India
Dr. Irfan Mansuri brings 25 years of extensive experience in science education. His method of explaining atomic structure using practical examples and clear visualizations has helped thousands of students master difficult Chemistry concepts.