CBSE – 9th Standard Science

Class 9 Chapter 4 – Structure of the Atom


  • An atom consists of a positively charged sphere and the electrons are embedded in it.



  • The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.
  • Its observation could be called as aplum pudding model or a watermelon

Rutherford’s Model Of an Atom

A stream of high energy alpha particles from a radioactive source was directed at thin foil of gold metal. The thin foil had a circular fluorescent zinc sulphide screen around it. Whenever alpha particles struck the screen , a tiny flash of light was produced at a point


  • Most of the space inside the atom is empty because most of the particles passed through the gold foil without getting deflected.
  • Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.



  • Neil’s Bohr put forward the following postulates about the model of an atom:
  • Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
  • While revolving in discrete orbits the electrons do not radiate energy.
  • These orbits or shells are called energy levels.
  • The sub-atomic particle equal in magnitude but opposite in sign to that of the electron is called
  • Neutrons are present in the nucleus of all atoms. A neutron is represented as ‘n’.


  • Valency is the combining capacity of an atom.
  • The electrons present in the outermost shell of an atom are known as the valence electrons.
  • Atomic Number is the number of protons of an atom, which determines its atomic number. It is denoted by Z’.
  • The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom


  • Isotopes are the number of atoms of some elements, which have the same atomic number but different mass n
  • Atoms of different elements with different atomic numbers, which have the same mass number, are known as

Elements are defined by the number of protons they possess


CBSE 9th Standard Science

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CBSE 10th Standard Science Syllabus and Revision Notes

  • Chapter 1 – Matter in our surroundings
  • Chapter 2 – Is matter around us pure
  • Class 9 chapter 3 – Atoms & Molecules
  • Class 9 Chapter 4 – Structure of the Atom
  • Class 9 Chapter 5 – The fundamental unit of life
  • Class 9 Chapter 6 – Tissues
  • Class 9 Chapter 7 – Diversity in living organisms
  • Class 9 Chapter 8 – Motion
  • Class 9 Chapter 9 – Force and laws of motion
  • Class 9 Chapter 10 – Gravitation
  • Class 9 Chapter 11 – Work & Energy
  • Class 9 Chapter 12 – Sound
  • Class 9 Chapter 13 – Why do we fall ill
  • Class 9 Chapter 14 – Natural Resources