• Dalton's atomic theory suggested that atoms are indivisible.

  • Atoms can not be divided further.

  • However, later it was found that atoms contained even smaller particles like electrons and protons.

  • Different scientists proposed different models of an atom to explain the structure of an atom.

1. structure of an atom-Teachoo.png




  • Before the discovery of any model of an atom

  • E.Goldstein discovered the presence of a new radiation.

  • Electricity was passed through the discharge tube at high voltage through a gas at low pressure taken in a tube.

  • These rays were positively charged raditions called canal rays .

  • It was only because of these rays that charged subatomic particles (particles of an atom) were discovered .

2. anode-Teachoo.png


The sub-particles discovered were: -


  1. Protons (denoted by 'p+')


  1. Electrons (denoted by ‘e-’)




Protons are positively charged particles.

Electrons are negatively charged particles.

As estimated by the scientists, a proton is about 2000 times heavier than an electron and weighed about 1 atomic mass unit

Mass of an electron was very less as compared to a proton.

Charge on a proton was +1

Charge on an electron was -1

It is denoted by " p+ "

It is denoted by " e- "

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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo