Structure of the Atom CLICK HERE
Matter is made up of tiny particles called atoms. Atoms are further made of three
fundamental particles or sub – atomic particles called electron, proton and neutron.
Earlier Dalton postulated that atom is indivisible i.e. cannot be further divided which
proved to be wrong by discovery of sub – atomic particles.
Types of Sub - atomic Particles
Note -
• Protons and neutrons are almost of same mass.
• The mass of proton is approximately 2000 times as that of the electron.
Discovery of Sub- atomic Particles
(a) Electron (e-):
These negatively charged particles were discovered by J. J. Thomson in 1897. In the
experiment, a gas at low pressure was taken in a discharge tube made up of glass. At
the ends of the discharge tube two electrodes were placed, connected to a battery
for high voltage supply. The electrode connected to the negative end was known as
cathode and that to the positive as anode. He then observed a stream of negatively
charged particles coming out of cathode towards the anode. These particles were
called electrons.
The collection of negatively charged particles emitted from cathode of discharge tube
is called cathode rays.
, (b) Proton (p+):
In 1886, Goldstein observed in the same experiment, with different situations that the
anode emitted positive particles which were called
Canal rays: The positively charged radiations produced in the discharge tube from
anode are called Canal rays.
(c) Neutron (n):
These neutrally charged particles were discovered by James Chadwick in 1932.
Neutrons are present in atoms of all elements except Hydrogen.
(Activities related to charged particles
Activity 1 - Comb rubbed with dry hair attracts paper pieces.)
The Structure of an Atom
Following are the various models explaining structure of an atom:
1. Thomson’s Model of an atom
It is also known as Thomson’s plum pudding model. In 1903 J. J. Thomson proposed
the structure of an atom similar to that of a Christmas pudding.
Matter is made up of tiny particles called atoms. Atoms are further made of three
fundamental particles or sub – atomic particles called electron, proton and neutron.
Earlier Dalton postulated that atom is indivisible i.e. cannot be further divided which
proved to be wrong by discovery of sub – atomic particles.
Types of Sub - atomic Particles
Note -
• Protons and neutrons are almost of same mass.
• The mass of proton is approximately 2000 times as that of the electron.
Discovery of Sub- atomic Particles
(a) Electron (e-):
These negatively charged particles were discovered by J. J. Thomson in 1897. In the
experiment, a gas at low pressure was taken in a discharge tube made up of glass. At
the ends of the discharge tube two electrodes were placed, connected to a battery
for high voltage supply. The electrode connected to the negative end was known as
cathode and that to the positive as anode. He then observed a stream of negatively
charged particles coming out of cathode towards the anode. These particles were
called electrons.
The collection of negatively charged particles emitted from cathode of discharge tube
is called cathode rays.
, (b) Proton (p+):
In 1886, Goldstein observed in the same experiment, with different situations that the
anode emitted positive particles which were called
Canal rays: The positively charged radiations produced in the discharge tube from
anode are called Canal rays.
(c) Neutron (n):
These neutrally charged particles were discovered by James Chadwick in 1932.
Neutrons are present in atoms of all elements except Hydrogen.
(Activities related to charged particles
Activity 1 - Comb rubbed with dry hair attracts paper pieces.)
The Structure of an Atom
Following are the various models explaining structure of an atom:
1. Thomson’s Model of an atom
It is also known as Thomson’s plum pudding model. In 1903 J. J. Thomson proposed
the structure of an atom similar to that of a Christmas pudding.
