Structure of Atom - Notes
1. Thomson’s Atomic Model (1898)
• Also known as Plum Pudding Model.
• Thomson proposed that an atom may be regarded as a sphere of approximate
radius 10^-8 cm carrying positive charge due to protons in which the negatively
charged electrons are embedded.
• This model was called Thomson’s Plum Pudding Model.
• This model did not have experimental evidence in support and was therefore
rejected.
2. Rutherford’s Alpha Ray Scattering Experiment (1911)
• Alpha ray particles source (radium) was enclosed in a box of lead.
• After passing through a slit, these particles were made to strike against a very thin
foil of heavy metal like gold.
• The particles got scattered and were then made to fall on a circular screen coated
with zinc sulphide which produced scintillations.
Observations:
(i) Most of the alpha particles passed through the gold foil undeflected, which means
most space in an atom is empty.
(ii) Some alpha particles were deflected and very few suffered major deflection and
even came back along the same path.
(iii) From the observations, it was concluded that the nucleus of an atom is very
small.
Rutherford’s Model of Atom:
(i) An atom consists of a nucleus present at the centre of the atom.
(ii) The nucleus contains positively charged protons and neutral neutrons.
(iii) Electrons revolve around the nucleus in some orbital paths.
Drawback of Rutherford’s Theory:
• According to the theory of physics, when a charged particle moves around another
charged particle, it releases energy.
• Negatively charged electrons moving around the positively charged nucleus would
continuously lose energy.
• They would follow a spiral path and ultimately fall into the nucleus.
• Therefore, Rutherford’s theory could not explain the stability of an atom.
1. Thomson’s Atomic Model (1898)
• Also known as Plum Pudding Model.
• Thomson proposed that an atom may be regarded as a sphere of approximate
radius 10^-8 cm carrying positive charge due to protons in which the negatively
charged electrons are embedded.
• This model was called Thomson’s Plum Pudding Model.
• This model did not have experimental evidence in support and was therefore
rejected.
2. Rutherford’s Alpha Ray Scattering Experiment (1911)
• Alpha ray particles source (radium) was enclosed in a box of lead.
• After passing through a slit, these particles were made to strike against a very thin
foil of heavy metal like gold.
• The particles got scattered and were then made to fall on a circular screen coated
with zinc sulphide which produced scintillations.
Observations:
(i) Most of the alpha particles passed through the gold foil undeflected, which means
most space in an atom is empty.
(ii) Some alpha particles were deflected and very few suffered major deflection and
even came back along the same path.
(iii) From the observations, it was concluded that the nucleus of an atom is very
small.
Rutherford’s Model of Atom:
(i) An atom consists of a nucleus present at the centre of the atom.
(ii) The nucleus contains positively charged protons and neutral neutrons.
(iii) Electrons revolve around the nucleus in some orbital paths.
Drawback of Rutherford’s Theory:
• According to the theory of physics, when a charged particle moves around another
charged particle, it releases energy.
• Negatively charged electrons moving around the positively charged nucleus would
continuously lose energy.
• They would follow a spiral path and ultimately fall into the nucleus.
• Therefore, Rutherford’s theory could not explain the stability of an atom.
