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Science Class 11 - Structure Of-Atom Notes

Comprehensive study notes for Class 11 - Structure Of-Atom olympiad preparation

Structure of Atom

Welcome to the chapter on Structure of Atom for Class 11. In this chapter, you will learn about the discovery of subatomic particles, atomic models, and how the structure of the atom explains the properties of elements. By the end of this chapter, you will understand the basic building blocks of matter and the theories that describe atomic structure.

Introduction

Atoms are the smallest units of matter that retain the properties of an element. The study of atomic structure helps us understand the nature of matter and chemical reactions.

Discovery of Subatomic Particles

  • Electron: Discovered by J.J. Thomson in 1897 using cathode ray experiments. Electrons are negatively charged particles.
  • Proton: Discovered by E. Goldstein and later named by Rutherford. Protons are positively charged particles found in the nucleus.
  • Neutron: Discovered by James Chadwick in 1932. Neutrons have no charge and are also found in the nucleus.

Atomic Models

  • Thomson's Model (Plum Pudding Model): Atom is a sphere of positive charge with electrons embedded in it.
  • Rutherford's Model: Atom has a small, dense, positively charged nucleus with electrons revolving around it.
  • Bohr's Model: Electrons revolve in fixed orbits (shells) around the nucleus without radiating energy. Each orbit has a fixed energy.

Atomic Number, Mass Number, and Isotopes

  • Atomic Number (Z): Number of protons in the nucleus of an atom.
  • Mass Number (A): Total number of protons and neutrons in the nucleus.
  • Isotopes: Atoms of the same element with the same atomic number but different mass numbers (different number of neutrons).

Electronic Configuration

The arrangement of electrons in different shells (energy levels) around the nucleus is called electronic configuration. The maximum number of electrons in a shell is given by 2n², where n is the shell number.

  • K shell (n=1): 2 electrons
  • L shell (n=2): 8 electrons
  • M shell (n=3): 18 electrons

Practice Questions

  1. Who discovered the electron?
  2. What is the atomic number of an atom with 11 protons?
  3. Write the electronic configuration of sodium (atomic number 11).
  4. What are isotopes? Give one example.
  5. State two differences between Rutherford's and Bohr's atomic models.

Challenge Yourself

  • Draw and label the Bohr model of a carbon atom (atomic number 6).
  • Explain why atoms are electrically neutral.

Did You Know?

  • Hydrogen has three isotopes: protium, deuterium, and tritium.
  • The nucleus of an atom is about 100,000 times smaller than the atom itself!

Glossary

  • Atom: The smallest unit of an element that retains its properties.
  • Electron: Negatively charged subatomic particle.
  • Proton: Positively charged subatomic particle in the nucleus.
  • Neutron: Neutral subatomic particle in the nucleus.
  • Isotope: Atoms of the same element with different numbers of neutrons.

Answers to Practice Questions

  1. J.J. Thomson
  2. 11
  3. 2, 8, 1
  4. Atoms of the same element with different mass numbers; e.g., Carbon-12 and Carbon-14.
  5. Rutherford's model has electrons revolving in any orbit, Bohr's model has fixed orbits; Bohr's model explains stability, Rutherford's does not.

Understanding the structure of atom is the foundation of modern chemistry!

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