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Science Class 9 - Structure Of-Atom Notes

Comprehensive study notes for Class 9 - Structure Of-Atom olympiad preparation

Structure of Atom

Welcome to the chapter on Structure of Atom for Class 9. In this chapter, you will learn about the basic building blocks of matter, the discovery of the atom, its structure, and the particles inside it. By the end of this chapter, you will understand how atoms make up everything around us and how their structure affects their properties.

Introduction

Atoms are the smallest units of matter. Everything around us is made up of atoms. The study of atoms helps us understand the properties of elements and compounds.

Key Concepts

  • Atom: The smallest unit of matter that retains the properties of an element.
  • Subatomic Particles: Particles found inside an atom—protons, neutrons, and electrons.
  • Nucleus: The central part of the atom containing protons and neutrons.
  • Electron: A negatively charged particle that moves around the nucleus.

Discovery of Atom

  • John Dalton proposed that matter is made up of tiny particles called atoms.
  • J.J. Thomson discovered electrons using the cathode ray experiment.
  • Ernest Rutherford discovered the nucleus and protons through the gold foil experiment.
  • James Chadwick discovered neutrons.

Structure of Atom

  • Atoms have a small, dense nucleus at the center.
  • The nucleus contains protons (positive charge) and neutrons (no charge).
  • Electrons (negative charge) move around the nucleus in orbits or shells.
  • Most of the atom’s volume is empty space.

Models of Atom

  • Thomson’s Model: "Plum pudding" model—electrons scattered in a positive sphere.
  • Rutherford’s Model: Atom has a nucleus with electrons moving around it.
  • Bohr’s Model: Electrons move in fixed orbits or shells around the nucleus.

Atomic Number and Mass Number

  • Atomic Number (Z): Number of protons in the nucleus.
  • Mass Number (A): Total number of protons and neutrons in the nucleus.

Example: Carbon has 6 protons and 6 neutrons.
Atomic Number = 6, Mass Number = 12

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons but the same number of protons.

  • Example: Hydrogen has three isotopes—Protium, Deuterium, and Tritium.

Practice Questions

  1. Name the three subatomic particles found in an atom.
  2. Who discovered the electron?
  3. What is the charge on a neutron?
  4. What is the atomic number and mass number of oxygen (8 protons, 8 neutrons)?
  5. What are isotopes? Give one example.

Challenge Yourself

  • Draw and label the structure of an atom showing nucleus, protons, neutrons, and electrons.
  • List the differences between Thomson’s, Rutherford’s, and Bohr’s models of atom.

Did You Know?

  • Atoms are so small that millions of them can fit on the tip of a needle!
  • The word "atom" comes from a Greek word meaning "uncuttable".

Glossary

  • Atom: The smallest unit of matter.
  • Electron: Negatively charged particle in an atom.
  • Proton: Positively charged particle in the nucleus.
  • Neutron: Particle with no charge in the nucleus.
  • Isotope: Atoms of the same element with different numbers of neutrons.

Answers to Practice Questions

  1. Proton, neutron, electron.
  2. J.J. Thomson.
  3. No charge (neutral).
  4. Atomic number = 8, Mass number = 16.
  5. Isotopes are atoms of the same element with different numbers of neutrons. Example: Hydrogen—Protium, Deuterium, Tritium.

Understanding atoms helps us unlock the secrets of chemistry and the world around us!

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