1|Page
Science Class 9 Notes – Structure of Atom
1. Sub-Atomic Particles
Electrons: Electron was discovered in cath¬ode ray experiment.
The term electron was coined by GJ Stoney
Protons were discovered in anode ray ex-periment. Anode rays are also called posi¬tive
rays or canal rays. Protons was discovered by Wilhelm Wien in 1902. It was identified
by J.J. Thomson.
Neutron was discovered by James Chadwick in 1932.
2. Atomic Models :
Models are the ideas about how things behave the way they do.
Various Models of atom
(a) Thomson Model or Water Melon Model or Plum pudding Model
According to this model electrons are embedded in the positively charged mass distributed
uniformly throughout the atomic sphere.
This model was proposed by Joseph James Thomson in 1897. This model is also know as apple
pie model.
(b) Rutherford’s Model or Planetary Model:
This model is based on experiments conducted by Rutherford. This model was given by Ernest
Rutherford in 1911.
According to this model all the positively charged particles are present in a small space
in the centre of the atom. This small space is called nucleus.
The electrons revolve around the nucleus just as the planets revolve around the sun. Due
to this similarity, Rutherford’s model is called the planetary model or Solar model.
Electrons (negatively charged) revolve around the nucleus in orbits with a high speed to
overcome the elecro-static force of attraction between positively charged particles
(protons) present in the nucleus.
Drawbacks of Rutherford’s Model
The two deficiencies of this model are:
www.ncerthelp.com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)
Science Class 9 Notes – Structure of Atom
1. Sub-Atomic Particles
Electrons: Electron was discovered in cath¬ode ray experiment.
The term electron was coined by GJ Stoney
Protons were discovered in anode ray ex-periment. Anode rays are also called posi¬tive
rays or canal rays. Protons was discovered by Wilhelm Wien in 1902. It was identified
by J.J. Thomson.
Neutron was discovered by James Chadwick in 1932.
2. Atomic Models :
Models are the ideas about how things behave the way they do.
Various Models of atom
(a) Thomson Model or Water Melon Model or Plum pudding Model
According to this model electrons are embedded in the positively charged mass distributed
uniformly throughout the atomic sphere.
This model was proposed by Joseph James Thomson in 1897. This model is also know as apple
pie model.
(b) Rutherford’s Model or Planetary Model:
This model is based on experiments conducted by Rutherford. This model was given by Ernest
Rutherford in 1911.
According to this model all the positively charged particles are present in a small space
in the centre of the atom. This small space is called nucleus.
The electrons revolve around the nucleus just as the planets revolve around the sun. Due
to this similarity, Rutherford’s model is called the planetary model or Solar model.
Electrons (negatively charged) revolve around the nucleus in orbits with a high speed to
overcome the elecro-static force of attraction between positively charged particles
(protons) present in the nucleus.
Drawbacks of Rutherford’s Model
The two deficiencies of this model are:
www.ncerthelp.com (Visit for all ncert solutions in text and videos, CBSE syllabus, note and many more)
