4. Structure of an Atom

Description:

Struggling to revise Chapter 4 – Structure of the Atom in Class 9 Science?

This all-in-one important questions pack is your go-to resource for scoring high in exams.

Based on the latest CBSE syllabus, this guide includes MCQs, short answers, long answers, and concept explanations — all in one downloadable PDF.

Whether you're preparing for a school test or final board exams, this book simplifies everything you need to know about atoms and their structure.

Why Choose This Book?

✅ Strictly Follows CBSE Class 9 Science Syllabus

✅ All Question Types Covered – MCQs, 1-mark, 3-mark & 5-mark

✅ Answer Keys with Step-by-Step Solutions

✅ Clear, Student-Friendly Language

✅ Compiled by Expert Teachers

✅ Instant PDF Download

What You Will Learnt?

✅ Credit for the discovery of electron and proton goes to J.J. Thomson and E.Goldstein, respectively. 

✅ J.J. Thomson proposed that electrons are embedded in a positive sphere.
✅ Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus. 

✅ Rutherford’s model of the atom proposed that a very tiny nucleus is present inside the atom and electrons revolve around this nucleus. The stability of the atom could not be explained by this model. 

✅ Neils Bohr’s model of the atom was more successful. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive. 

✅ J. Chadwick discovered presence of neutrons in the nucleus of an atom. So, the three sub-atomic particles of an atom are: (i) electrons, (ii) protons and (iii) neutrons. Electrons are negatively charged, protons are positively charged and neutrons have no charges. The mass of an electron is about 12000 times the mass of an hydrogen atom. The mass of a proton and a neutron is taken as one unit each. 

✅ Shells of an atom are designated as K,L,M,N,…. 

✅ Valency is the combining capacity of an atom. 

✅ The atomic number of an element is the same as the number of protons in the nucleus of its atom. 

✅ The mass number of an atom is equal to the number of nucleons in its nucleus. 

✅ Isotopes are atoms of the same element, which have different mass numbers. 

✅ Isobars are atoms having the same mass number but different atomic numbers. 

✅ Elements are defined by the number of protons they possess.

What's Inside the Book?

✅ Discovery of Subatomic Particles

✅ Electron (J.J. Thomson), Proton, Neutron

✅ Canal rays and cathode ray experiments

✅ Atomic Models

✅ Thomson’s Model

✅ Rutherford’s Gold Foil Experiment

✅ Atomic Numbers, Mass Numbers

✅ Calculations based on atomic structure

✅ Neutrons, Isotopes, and Isobars

✅ Electronic Configuration

✅ Distribution of electrons in shells

✅ Valency and orbit rules (2n² rule)

✅ Key Definitions & Diagrams

✅ Atom structure sketches

✅ Differences between models

✅ Real exam-type explanation

Who Can Benefit from This Book?

✅ Class 9 Students preparing for school and board exams

✅ Teachers looking for ready-made classroom revision material

✅ Parents who want to help their child revise at home

✅ Self-learners who prefer guided PDF study materials

How to Use This Book Effectively?

✅ Start with Topic Summaries – Revisit key concepts like subatomic particles and atomic models.

✅ Practice Section-Wise Questions – Use the MCQs and short answers to test understanding.

✅ Revise with Diagrams – Refer to included diagrams of Rutherford and Bohr’s model.

✅ Attempt the Long Answers – Practice 5-mark questions just like in your final exams.

✅ Use the Answer Keys – Cross-check your responses with clear, step-by-step solutions.

Frequently Asked Questions (FAQs)