Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 8. Journey Inside the Atom

CBSE Notes New Syllabus: Class 9

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Chapter 8. Journey Inside the Atom

This section explains how Niels Bohr improved Rutherford's atomic model by introducing the concept of fixed energy levels. It also describes how electrons are arranged around the nucleus, why atoms remain stable, and how electrons absorb or release energy while moving between different energy levels. 

Bohr's Atomic Model

Although Rutherford's model successfully explained the presence of the nucleus, it could not explain why electrons do not lose energy and fall into the nucleus. To solve this problem, Niels Bohr proposed a new atomic model in 1913.

Definition

Bohr's atomic model states that electrons revolve around the nucleus only in certain fixed circular paths called shells or energy levels, without losing energy.

Main Postulates of Bohr's Atomic Model

• Fixed Energy Levels – Electrons move only in definite circular paths called shells or energy levels.
• Stationary Orbits – While moving in a fixed shell, an electron neither gains nor loses energy.
• Energy Increases Outward – The energy of shells increases with distance from the nucleus.
• Energy Transfer – An electron absorbs or releases a fixed amount of energy while moving from one shell to another.
• Limited Capacity – Each shell can accommodate only a fixed maximum number of electrons.

Energy Levels or Shells

The shells around the nucleus are represented by the letters K, L, M, N or by the principal quantum numbers n = 1, 2, 3, 4.

Movement of Electrons

• Absorption of Energy – An electron moves to a higher energy level after absorbing a definite amount of energy.
• Release of Energy – An electron returns to a lower energy level by releasing the same amount of energy.
• Stable State – Electrons remain stable as long as they stay in their fixed shells.

Why Are Atoms Stable?

According to Bohr, electrons do not continuously lose energy while revolving around the nucleus. Since they move only in fixed energy levels, they remain stable and do not fall into the nucleus. This successfully explained the stability of atoms. 

Achievements of Bohr's Model

• Explained the stability of atoms.
• Introduced the concept of energy levels.
• Explained the movement of electrons between shells.
• Successfully explained many experimental observations related to hydrogen.

Limitations of Bohr's Model

• It explained the structure of only simple atoms successfully.
• It could not explain the behaviour of atoms containing many electrons.
• Later discoveries showed that electrons do not move in perfectly circular paths.
• It was later replaced by the quantum mechanical model.

Difference Between Rutherford's and Bohr's Models

ATP Education Concept Builder

The most important contribution of Bohr's model is the idea of fixed energy levels. Electrons remain stable because they revolve only in these allowed shells. They gain energy to move outward and release energy to move inward. This concept explains why atoms do not collapse.

ATP Education Exam Booster

• Remember all the postulates of Bohr's atomic model.
• Learn the order of shells: K → L → M → N.
• Understand why atoms remain stable according to Bohr.
• Know how electrons absorb and release energy.
• Differentiate clearly between Rutherford's and Bohr's models.
• CBSE competency-based questions often ask why Rutherford's model failed and how Bohr solved the problem.