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Wiki-http://en.wikipedia.org/wiki/Plum_pudding_model
The plum pudding model of the atom was
proposed by J. J. Thomson, who discovered the electron in
1897. The plum pudding model was proposed in 1904 before
the discovery of the atomic nucleus. In this model, the
atom is composed of electrons (which Thomson still called
"corpuscles," though G.J. Stoney had proposed
that atoms of electricity be called electrons in 1894),
surrounded by a soup of positive charge to balance the
electron's negative charge, like plums surrounded by
pudding. The electrons (as we know them today) were
thought to be positioned throughout the atom, but with
many structures possible for positioning multiple
electrons, particularly rotating rings of electrons (see
below). Instead of a soup, the atom was also sometimes
said to have had a cloud of positive charge.
The model was disproved by the 1909 gold foil
experiment, which was interpreted by Ernest Rutherford in
1911 to imply a very small nucleus of the atom containing
a very high positivecharge (enough to balance about 100
electrons in gold), thus leading to the Rutherford model
of the atom, and finally (after Henry Moseley's work
showed in 1913 that the nuclear charge was very close to
the atomic number) to the Antonius Van den Broek
suggestion that atomic number is nuclear charge.
Eventually, by 1913, this work had culminated in the
solar-system-like (but quantum-limited) Bohr model of the
atom, in which a nucleus containing an atomic number of
positive charge is surrounded by an equal number of
electrons in orbital shells.
Thomson's model was compared (though not by Thomson) to
a British treat called plum pudding, hence the name. It
has also been called the 'chocolate chip cookie model or
"Blueberry Muffin Model"', but only by those who
have not read Thomson's original paper (On the Structure
of the Atom: an Investigation of the Stability and Periods
of Oscillation of a number of Corpuscles arranged at equal
intervals around the Circumference of a Circle; with
Application of the Results to the Theory of Atomic
Structure), published in the Philosophical Magazine (the
leading British science journal of the day). For an
excerpt see [1].
A little-known (or now forgotten) fact about the
original Thomson "plum pudding" model is that it
was dynamic, not static. The electrons were free to rotate
within the blob or cloud of positive substance. These
orbits were stabilized in the model by the fact that when
an electron moved farther from the center of the positive
cloud, it felt a larger net positive inward force, because
there was more material of opposite charge, inside its
orbit (A particle like a small black hole would feel the
same restorative force if it penetrated the body of the
Earth; such a particle would feel only the gravity of the
Earth inside its radius). In Thomson's model, electrons
were free to rotate in rings which were further stabilized
by interactions between the electrons, and spectra were to
be accounted for by energy differences of different ring
orbits. Thomson attempted to make his model account for
some of the major spectral lines known for some elements,
but was not notably successful at this. Still, Thomson's
model (along with a similar Saturnian ring model for
atomic electrons, put forward also in 1904 by Nagaoka
after the Maxwell model of Saturn's rings), were earlier
harbingers of the later and more successful
solar-system-like Bohr model of the atom. |
