Chapter 4

Facts that matter

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• John Dalton assumed that atom is indivisible

• In 1866 E. Goldstein discovered the pressure of new radiations in a gas discharge tube and called them canal rays .These rays were positively charged radiations which led to the discovery of sub atomic particle called proton.

In 1900 J.J. Thomson discovered the sub-atomic particle - the electron with a negative charge.

Difference in electron and proton :

(1) Negatively charged (e–) = its mass is negligible.

Charge is -1

J.J. Thomson is discovered it.

(2) Positively charged (p+) = its mass is 1 unit.

Charge is+1

E.goldsmith was discovered it.

• The Neutrons was discovered by chadwic. Neutrons have no charge.


Alpha (α) Particles : (+2 Charge and 4 mass ) when fast moving α particles are bombarded on very thin gold foil, following observation were made:

  • Most of the α particles passed straight through the gold foil.

  • Some of the α Particles were deflected by the foil by small angles.

  • One out of 12000 particles appeared to rebound.

Conclusions made by Rutherford based on his Observations :

  • Most of the space inside the atom is empty because α particles passed through the gold foil.

  • very few particles were deflected from their path because +ve charge of the atom Occupies a very little space.

  • A very small fraction of particles were rebound back , shows all +ve charge and mass of the gold atom is concentrated in avery small volume within an atom.

  • The radius of the nucleus calculated was 105 times less than radius of the atom.

Nuclear model of an atom-

•Centre →+ve change →called nucleus. All mass resides in nucleus.

• Electrons →revolve around the nucleus in orbits.

• Size of the nucleus is very small as compared to size of the atoms.

Drawbacks of Rutherford's model of the atom : when in electron undergoes acceleration,it radiates energy.Thus revolving electron would lose energy and finally fall into the nucleus.Due to this atom should be highly unstable and hence matter would not exist in the from that we know.

But we know that atoms are quite stable.

Bohr's model of Atom

Postulates of Nell Bohr

• only special orbits know as discrete orbits of electrons are allowed inside the atom.

• while revolving in discrete orbits the electrons do not radiate energy . These orbits are called energy levels.

Orbits or shell are represented by K,L,M,N, or the numbers , n = 1,2,3,4

Neutrons -Distribution of electrons in different orbits (shells) given by Bohr and Bury: Rules;

  • Maximum number of electrons present in a shell is given by 2n² (n=shell number)

E.g.,n=1 (K shell) 2(1)² = 2 electron

  • The maximum number of electrons that can be accommodated in the outermost orbit is 8 .

  • Electrons are not accomodated in a given shell unless the inner shells are completely filled.

Definitions :

•Valency: The combining capacity of an atom is called its valency,

• Atomic number: it is equal to a number of protons.

• Mass number: it is equal to the sum of protons and neutrons.


Isotopes : Atoms of the same elements with same atomic number but a different mass number,are called isotopes.


Chemical properties→same but physical properties→different

Applications of isotopes:

• Anisotope of uranium used as fuel .

• Anisotope of cobalt is used in the treatment of cancer

• Anisotope of lodine is used in the treatment of goitre.

Isobars : Atoms of different elements with same mass number but different atomic numbers are called isobars.

We hope the given CBSE/ MP Board Class 9th Science Chapter 4th "Structure of Atom" will help you.

Written by - Divya Bhadouriya