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| Thomson's plum-pudding model of atom |
Thomson’s Plum Pudding
Model of Atom
In
the early days when there was no concept of atomic model, the emission spectrum
of many elements, especially that of hydrogen was extensively studied with the
mathematical formulations. But the fundamental question regarding the origin of
spectral lines was in darkness. Actually the discovery of the electron as a
fundamental particle provided the first important insight into the atomic
structure. The electrons were proved to bear negative charge. Hence to explain
the electroneutrality, the existence of enough number of positively charged
particles just to balance the negative charge was required to be considered.
Based on this feature, J. J. Thomson in 1898 proposed that the atoms are of
uniform spheres of positively charged matter in which the electrons are embedded
in such a way that the electrostatic repulsion among the electrons and also
within the positively charged matter is overcome due to the electrostatic
attraction between the electrons and positively charged matter. This is known
as Plum-Pudding Model. This model at that time was applied to explain some
observations:
Origin of emission
spectrum: To
explain the emission spectrum from the excited atoms, it was proposed that when
the atoms are excited either by electric discharge or by heating, the normal
situation is disturbed and the electrons start to vibrate around the
equilibrium position emitting electromagnetic radiation corresponding to their
vibrational frequencies. Thus the origin of the emission spectral lines was
explained.
Formation of ions: The gain of additional electrons
in the matrix formed by the positive charge leads to the formation of anions
and similarly, the loss of electrons convert the neutral atoms into the
positive ions.
Thermionic
emission: If the
element is highly heated then the vibrational energy of the electrons will be
so increased that they can get no more bound by the electrostatic attractive
force and they will come out.
Kinetic theory of
gas: The model
provided an elastic hard model for the atoms which was required in the
development of kinetic theory of gas.
Drawbacks of the model
Contradiction to
the classical theorem in electromagnetism: In the light of classical theory of
electromagnetism, the stability of Thomson's model cannot be explained.
According to Earnshaw's classical theorem, 'no system of charged particles of
opposite nature can remain in a static equilibrium under the action of
electrostatic forces alone'. Thus no stability exists in the plum-pudding model
where the negative charge particles are supposed to remain in an equilibrium
with the positively charged matrix in a static condition.
No explanation of
atomic spectra in a quantitative way:
Though the origin of the emission spectral lines was explained by considering
the vibrating frequencies of the excited electrons, the calculated frequency is different from those observed. Moreover, in the case of hydrogen atom (one
electron system) only one spectral line was predicted, but in practice it
leads to several series in which is series consists of several discrete
spectral lines.
No explanation of the observation of scattering of α-particles: In 1911, a definite experimental test was carried out by Geiger and Marsden under the suggestion of Ernest Rutherford to verify the validity of the model. This was the experiment of scattering of α-particle by a gold foil. No explanation of scattering for a very few α-particles by a large angle was provided by the Thomson's model, then the scientist were left without any alternative one but to reject the model and replace it by a new one which is Rutherford's Model.
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| Hydrogen atom spectrum |
1) Concise inorganic chemistry by J. D. Lee.
2) Inorganic Chemistry by James E. Huheey, Ellen A Keither, Richard L. Keither, Okhil K. Medhi.
3) Shriver and Atkins Inorganic Chemistry.
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