Atomic Structure
History of an atom
By Priyanka
Dalton’s Atomic Theory
Thomson’s Atomic Model
Thomson’s Model of an Atom
Rutherford’s Atomic Model
Observations
Hypothesis
Drawbacks of Rutherford’s Atomic model
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This atomic model failed to explain stability of atoms.
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According to the model, electrons revolve around the positively charged
nucleus. It's not possible for a long run as we know atoms are stable while any
particle in a circular orbit would undergo acceleration. During acceleration
charged particles would radiate energy. Revolving electron will lose energy and
finally fall into the nucleus.
•
This model of atom also failed to explain the existence of definite lines in the
hydrogen spectrum.
Chadwick’s Experiment
Discovery of Neutrons by Chadwick
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James Chadwick fired alpha radiation at beryllium sheet from a polonium source. This led to the
production of an uncharged, penetrating radiation.
•
This radiation was made incident on paraffin wax, a hydrocarbon having a relatively high
hydrogen content.
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The protons ejected from the paraffin wax (when struck by the uncharged radiation) were
observed with the help of an ionization chamber.
•
The range of the liberated protons was measured and the interaction between the uncharged
radiation and the atoms of several gases was studied by Chadwick.
•
He concluded that the unusually penetrating radiation consisted of uncharged particles having
(approximately) the same mass as a proton. These particles were later termed ‘neutrons’.
Bohr’s Atomic Model
Bohr’s Postulates
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In an atom, electrons (negatively charged) revolve around the positively charged nucleus in a
definite circular path called orbits or shells.
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Each orbit or shell has a fixed energy and these circular orbits are known as orbital shells.
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The energy levels are represented by an integer (n=1, 2, 3…) known as the quantum number.
This range of quantum number starts from nucleus side with n=1 having the lowest energy level.
The orbits n=1, 2, 3, 4… are assigned as K, L, M, N…. shells and when an electron attains the
lowest energy level, it is said to be in the ground state.
•
The electrons in an atom move from a lower energy level to a higher energy level by gaining the
required energy and an electron moves from a higher energy level to lower energy level by losing
energy.
Limitation of Bohr’s Model

Limitations of Bohr’s Model of an Atom
•
Bohr’s model of an atom failed to explain the Zeeman Effect (effect of magnetic field on the
spectra of atoms).
•
It also failed to explain the Stark effect (effect of electric field on the spectra of atoms).
•
It violates the Heisenberg Uncertainty Principle.
•
It could not explain the spectra obtained from larger atoms.