9th Class Chemistry Notes of Chapter 2

Cathode rays were discovered in last decade of nineteen century. The properties of cathode rays were determined and they led to the discovery of electron.

Canal rays were discovered in 1886 by Goldstein . The properties of canal rays resulted in the discovery of proton in the atom.

Neutron in the atom was discovered in 1932 by Chadwick.

First of all structure of an atom was presented by Rutherford in 1911. He proposed that an atom contains nucleus at the center and electrons revolve around this nucleus.

Bohr presented an improved model of an atom in 1913 based upon four postulates. He introduced the concept of circular orbit, in which electrons revolve. As long as electron remains in a particular orbit, it does not radiate energy. Release and gain of energy is because of change of orbit.

The concept of shells and subshells is explained.

A shell consists of subshells.

Isotopes are defined as the atoms of elements that have the same atomic number but different atomic mass.

Hydrogen, carbon and uranium have three isotopes each, whereas chlorine has two isotopes

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