Structure of Atom Lesson For Class 9th Chemistry


  • We have read that atoms and molecules are the fundamental building blocks of matter.
  • The existence of different kinds of matter is due to different atoms inside them.
  • Atoms are made up of 3 sub atomic particles:Electron,Neutron,Proton
  • electron give negative charge,proton give positive charge and neutron give neutral charge
  • Protons and Neutron are present in a small nucleus at the center of atom
  • Electron is outside the nucleus
  • hydrogen has only one electron and proton but no neutron

Things Comes into Our Mind that

  • What makes the atom of one element different from the atom of another element?
  • Are atoms really indivisible, as proposed by Dalton, or are there smaller constituents inside the atom?

Charged Particles in Matter

  • It was found by 1900 that the atom was not a simple, indivisible particle but contained at least one sub-atomic particle – the electron.
  • The electron was identified by J.J. Thomson.
  • Even before the electron was identified, E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays.
  • These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle and were called as protons.

Let us write the Difference between Electron and Proton here

Electron Proton
Its mass is Less compared to Proton

The absolute mass of an electron is 9.1*10-31 Kg and relative mass is 1/1840 u.

Its mass was approximately 2000 times as that of the electron.

The absolute mass of a proton is 1.6*10-27 Kg

It has Negative Charge  It is Positively charged
The mass of an electron is considered to be negligible and its charge is minus one. The mass of a proton is taken as one unit and its charge as plus one.
Protons were in the interior of the atom, Electrons are at outside
Not possible to remove Protons

an elctron has a charge of 1.6*10-19

Electrons could easily be peeled off


Discovery of Neutron

  • After the discovery of proton and electron it was noticed that the mass of an atom is not from the electron and proton
  • There is something else giving mass to the atom
  • James Chadwick in 1932 named it Neutron.
  • The absolute mass of a Neutron is 1.6*10-27 Kg
  • Neutron Has No charge

The Structure of an Atom

  • Dalton had said that the atom was indivisible and indestructible.
  • But the discovery of two fundamental particles (electrons and protons) inside the atom, led
    to the failure of this aspect of Dalton’s atomic theory.
  • J.J. Thomson was the first one to propose a model for the structure of an atom
J J Thomson


  1. Thomson proposed that: An atom consists of a positively charged sphere and the electrons are embedded in it.
  2. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral

Although Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.


Before we learn about Rutherford Model of atom we must know about α Particles.

  • Alpha Particle is a positively charged particle having 2 units of positive charge and 4 units of mass.
  • It is actually a helium Ion He2+.
  • Alpha particles are emitted by radio active elements like Radium and Polonium.
  • The fast moving particles have very high energy.
  • they can penetrate through matter to some extent
Ernest Rutherford


  • Ernest Rutherford was interested in knowing how the electrons are arranged within an atom
  • He designed an experiment for this.
  • In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil
  • He selected a gold foil because he wanted as thin a layer as possible.
  • This gold foil was about 1000 atoms thick.
  • α-particles are doubly-charged helium ions.
  • Since they have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy.
  • It was expected that α-particles would be deflected by the sub-atomic particles in the gold atoms.
  • Since the α-particles were much heavier than the protons, he did not expect to see large deflections


  • Most of the fast moving α-particles passed straight through the gold foil.
  •  Some of the α-particles were deflected by the foil by small angles.
  • Surprisingly one out of every 12000 particles appeared to rebound


Gold Foil Experiment


  1. Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
  2. Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
  3. A very small fraction of α-particles were deflected by 1800,indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.
  4. He also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

Nuclear Model of atom As given by Rutherford

  • There is a positively charged center in an atom called the nucleus.
  • Nearly all the mass of an atom resides in the nucleus.
  • The electrons revolve around the nucleus in circular paths.
  • The size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford model of the Atom

  • The revolution of the electron in a circular orbit is not expected to be stable.
  • Any particle in a circular orbit would undergo acceleration.
  • During acceleration, charged particles would radiate energy.
  • Thus, the revolving electron would lose energy and finally fall into the nucleus.
  • If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.


Niels Bohr

Neils Bohr put forward the following postulates about the model of an atom:

  1. 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
  2. These orbits or shells are called energy levels These orbits or shells are represented by the letters K,L,M,N … or the numbers, n=1,2,3,4,…

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