Chapter – 4

Structure of The Atom

In this post we have given the detailed notes of class 9 Science Chapter 4 (Structure of The Atom) in English. These notes are useful for the students who are going to appear in class 9 board exams.

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Chapter 4: Structure of The Atom

Introduction 🎬

• Atoms and molecules are the basic building blocks of all matter. 🧱
• Different kinds of matter exist because they are made up of different atoms. ⚛️
• This chapter explores the structure of atoms and their sub-atomic particles. 🔬

Charged Particles in Matter 🧲

• Activities like combing dry hair 빗 or rubbing a glass rod with silk 🧪 demonstrate static electricity. ⚡
• These activities show that objects become electrically charged when rubbed together. 🧲
• This charge comes from the charged particles within atoms. ⚛️

Sub-atomic Particles 🔬

• Electrons: elektron Discovered by J.J. Thomson, electrons are negatively charged sub-atomic particles. ➖
• Protons: Discovered by E. Goldstein through canal rays, protons are positively charged sub-atomic particles. ➕
• Neutrons: Discovered by J. Chadwick, neutrons have no charge and a mass similar to protons. 🥚

Models of the Atom ⚛️

• Thomson’s Model:
  • Proposed that atoms are like plum pudding, with electrons embedded in a positively charged sphere. 🍇
  • This model explained electrical neutrality but couldn’t explain other experimental results. 🧪
• Rutherford’s Model:
  • Based on the alpha-particle scattering experiment, Rutherford proposed that atoms have a small, dense, positively charged nucleus at the center. 🎯
  • Electrons revolve around the nucleus in circular paths. 💫
  • This model couldn’t explain the stability of atoms. 🚫
• Bohr’s Model:
  • Proposed that electrons revolve around the nucleus in specific orbits or shells with fixed energy levels.
  • Electrons don’t radiate energy while in these orbits. 🚫
  • This model explained the stability of atoms. ✅

Distribution of Electrons in Different Orbits

• The distribution of electrons in different shells follows the Bohr-Bury scheme. ⚛️
• The maximum number of electrons in a shell is given by the formula 2n², where n is the shell number (1, 2, 3…). 🔢
• The outermost shell can hold a maximum of 8 electrons. 🎱
• Shells are filled in a step-wise manner, from inner to outer shells. ➡️

Valency ⚛️

• Valency is the combining capacity of an atom. 🤝
• It is determined by the number of electrons an atom needs to gain, lose, or share to achieve a stable outermost shell with 8 electrons (octet). 🔄
• For example, sodium has a valency of 1 because it can lose 1 electron to achieve an octet. 🅰️

Atomic Number and Mass Number ⚛️

• Atomic Number (Z): The number of protons in the nucleus of an atom. ⚛️
• Mass Number (A): The sum of protons and neutrons in the nucleus of an atom. ⚛️

Isotopes and Isobars ⚛️

• Isotopes: Atoms of the same element with the same atomic number but different mass numbers. ⚛️
  • Example: Hydrogen isotopes – protium (¹H), deuterium (²H), and tritium (³H). 🇭
• Isobars: Atoms of different elements with different atomic numbers but the same mass number. ⚛️
  • Example: Calcium (²⁰Ca) and Argon (¹⁸Ar) both have a mass number of 40. 🇨🇦

Conclusion 🎬

• Atoms are made up of sub-atomic particles: electrons, protons, and neutrons. ⚛️
• The distribution of electrons in different shells determines the chemical properties of an element.
• Isotopes have similar chemical properties but different physical properties. ⚛️
• The concept of valency explains how atoms combine to form molecules. 🤝

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