Class 9 Science | Structure of Atom Explained | Bohr's Atomic Model | NCERT CBSE | 2Nsquare Rule

Welcome to Simma Academy! Class 9 Science | Structure of Atom Explained | Bohr's Atomic Model | NCERT CBSE | 2Nsquare Rule In this video, we learn two of the most important concepts from Class 9 Science Chapter 4 – Structure of the Atom: 🌟 Bohr’s Atomic Model 🌟 Bohr–Bury Rule / 2n² Rule for Electron Distribution These concepts help students understand how electrons are arranged inside an atom, how different shells get filled, and how this structure explains the stability of atoms. This lesson is based on NCERT Class 9 and is perfect for revision, exam preparation, and conceptual clarity. 🔶 WHAT YOU WILL LEARN IN THIS VIDEO ✔ Why Bohr proposed a new atomic model Rutherford’s model could not explain why electrons do not fall into the nucleus. Bohr solved this by introducing the concept of fixed orbits/energy levels. ✔ Bohr’s Postulates Electrons revolve around the nucleus in fixed circular orbits. Each orbit has a fixed energy, hence called an energy level. As long as an electron stays in its orbit, it does not lose energy. Electrons jump to a higher energy level by absorbing energy. Electrons fall to a lower energy level by releasing energy. ✔ Names of Orbits / Shells The energy levels are denoted as: K, L, M, N… Corresponding numbers: n = 1, 2, 3, 4 … 🔶 BOHR–BURY RULE / 2n² RULE (Electron Distribution) The Bohr–Bury rules describe how many electrons can be filled in each shell. ✔ Rule 1: Maximum number of electrons in a shell = 2n² Where n = shell number. Examples: K shell (n = 1) → 2 × 1² = 2 electrons L shell (n = 2) → 2 × 2² = 8 electrons M shell (n = 3) → 2 × 3² = 18 electrons N shell (n = 4) → 2 × 4² = 32 electrons ✔ Rule 2: The outermost shell can hold a maximum of 8 electrons (Even if 2n² gives a higher number) ✔ Rule 3: The next shell begins to fill only after the inner shell is full 🔶 WHY IS THIS TOPIC IMPORTANT? ✔ Helps in writing electronic configuration ✔ Needed to understand valency of atoms ✔ Foundation for Class 10 Chemical Bonding ✔ Commonly asked in CBSE exams ✔ Essential for understanding the Modern Periodic Table This video explains everything with diagrams, examples, and simple tricks. 🔶 EXAMPLES COVERED ✔ Atomic structure of Hydrogen (1) ✔ Electronic configuration of Carbon (6) ✔ Sodium (11) = 2, 8, 1 ✔ Chlorine (17) = 2, 8, 7 ✔ Calcium (20) = 2, 8, 8, 2 00:00 – Introduction & Greeting 00:12 – Start of Chapter: Structure of Atom 00:18 – Why Students Find This Chapter Difficult 00:35 – Instructions for Note-Taking 01:10 – Start of Bohr’s Atomic Model 01:18 – Drawbacks of Rutherford’s Model 01:34 – How Bohr Solved the Stability Problem 01:47 – Bohr’s Postulate 1: Discrete Orbits / Energy Levels 02:28 – Explanation of K, L, M, N Shells 03:20 – Bohr’s Postulate 2: No Energy Radiation 04:07 – Stability of the Atom Explained 04:28 – Comparison: Thomson, Rutherford & Bohr Models 06:36 – Structure of Nucleus (Protons & Neutrons) 06:53 – Introduction to Electronic Configuration 07:01 – Bohr’s 2n² Rule 07:13 – Maximum Capacity of K Shell (2 electrons) 07:28 – Maximum Capacity of L Shell (8 electrons) 08:08 – Maximum Capacity of M & N Shells 09:22 – Summary of Electron Distribution 09:50 – Rule: Maximum 8 Electrons in Outermost Shell 10:03 – Rule: Inner Shells Filled First 10:38 – Conclusion of 2n² Rule