Structure of Atom Class 9 Notes

Structure of the Atom Class 9 Notes – Here We have provided summary and revision notes for Class 9 Science Chapter 4. This CBSE notes contains CBSE Key Notes, CBSE Revision Notes, Short Key Notes, images, diagrams of the complete Chapter 4 titled Structure of the Atom of Science taught in class 9. If you are a student of class 9 who is using the NCERT Textbook to study Science, then you must come across Chapter 4 Structure of the Atom. After you have studied the lesson, you must be looking for notes to memorize. Here you can get complete Chapter 4 Structure of the Atom class 9 notes in one place. For a better understanding of this chapter, you should also see NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom.
CBSE Class 9 Science Notes Chapter 4 Structure of the Atom Pdf free download is part of Class 9 Science Notes for Quick Revision. Here we have given NCERT Class 9 Science Notes Chapter 4 Structure of the Atom.

1. Board CBSE
2. Textbook NCERT
3. Class Class 9
4. Subject  Notes
5. Chapter  Chapter 4
6. Chapter Name Structure of Atom
7. Category CBSE Revision Notes

Structure of Atom Class 9 Notes PDF

Introduction to Structure of an Atom
Atoms are the building blocks of matter. It is the smallest unit of matter that is composed of three sub-atomic particles: the proton, the neutron, and the electron.
Cathode ray experiment

  • J. J. Thomson discovered the existence of electrons.
  • He did this using a cathode ray tube, which is a vacuum-sealed tube with a cathode and anode on one end that created a beam of electrons traveling towards the other end of the tube.
  • The air inside the chamber is subjected to high voltage and electricity flows through the air from the negative electrode to the positive electrode.
  • The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube.
  • The experiment showed that the atom was not a simple, indivisible particle and contained at least one subatomic particle – the electron.

Apparatus of the experiment

  • Electrons are the negatively charged sub-atomic particles of an atom.
  • The mass of an electron is considered to be negligible, and its charge is -1.
  • The symbol for an electron is e
  • Electrons are extremely small.
  • They are found outside the nucleus.

Thomson’s model of an atom

  • According to Thomson,(i) An atom consists of a positively charged sphere and the electrons are embedded in it. (ii) The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral
  • The first model of an atom is to be put forward and taken into consideration.
  • He proposed a model of the atom be similar to that of a Christmas pudding/watermelon.
  • The red edible part of the watermelon is compared with the positive charge in the atom.
  • The black seeds in the watermelon are compared with the electrons which are embedded in it.


  • Radioactivity is the term for the process by which an unstable nucleus of an atom loses energy by giving out particles.
  • It does so by giving out particles such as alpha and beta particles.
  • This process is spontaneous.
  • An atom is unstable if the nucleus has an imbalance, meaning a difference in the protons and neutrons.

Rutherford Model
Rutherford’s experiment and observations
In this experiment, fast-moving alpha (α)-particles were made to fall on a thin gold foil. His observations were:

  • A major fraction of the α-particles bombarded towards the gold sheet passed through it without any deflection, and hence most of the space in an atom is empty.
  • Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed.
  • The positive charge in an atom is concentrated in a very small volume.
  • Very few of the α-particles were deflected back, that is only a few α-particles had nearly 180o angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.

Rutherford’s model of an atom
Rutherford concluded the model of the atom from the α-particle scattering experiment as:
(i) There is a positively charged center in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
(ii) The electrons revolve around the nucleus in well-defined orbits.
(iii) The size of the nucleus is very small as compared to the size of the atom.
Rutherford’s Model
Drawbacks of Rutherford’s model

  • He explained that the electrons in an atom revolve around the nucleus in well-defined orbits. Particles in a circular orbit would experience acceleration.
  • Thus, the revolving electron would lose energy and finally fall into the nucleus.
  • But this cannot take place as the atom would be unstable and the matter would not exist in the form we know.

Be More Curious!!!

  • The Millikan’s Oil Drop Experiment was an experiment performed by Robert A. Millikan and Harvey Fletcher in 1909 to measure the charge of an electron.
  • In the experiment, Millikan allowed charged tiny oil droplets to pass through a hole into an electric field.
  • By varying the strength of the electric field, the charge over an oil droplet was calculated, which always came as an integral value of ‘e.’
  • The conclusion of this is that the charge is said to be quantized, i.e. the charge on any particle will always be an integral multiple of e which is 1.6*10-19

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