CHE 101 Lecture Notes - Lecture 4: Plum Pudding Model
11 Jun 2018
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The Thomson Model of the Atom
By the end of the nineteenth century a large amount of chemical research had been carried
out and interpreted in terms of atoms and molecules, although there was little direct evidence
for atoms. Nevertheless, by the middle of the nineteenth century evidence began to emerge,
not for the direct existence of atoms but for the even smaller particles which make up atoms.
The first piece of the puzzle regarding the composition of the atom fell into place in 1897
when J.J. Thomson discovered that atoms contain negatively-charged particles which he
called corpuscles and were later named electrons. Later, he showed that positively-charged
particles could also be obtained from atoms. Because individual atoms are electrically
neutral, Thomson hypothesizes that atoms must possess both positive and negative charges.
Although he did not know why such a positive charge might exist, he proposed that the atom
consisted of a ball of positive charge with negative charges distributed throughout the ball, as
shown below.
This model was later nicknamed the pudding model because it resembled an english pudding
which has raisins spread through its interior. In a magnetic field, the oscillation of electrons
back and forth through the positive charge explained many puzzling experimental
observations. Although the Thomson model was later disproved, it was consistent with the
data available at the time and represented an elegant model having a great deal of explaining
power.
Thomson found that the electron possessed a mass as well as a charge and showed that the
charge to mass ratio was where a coulomb is a unit of electrical charge.
Later work by Robert Milliken in 1909 showed that an electron’s mass is a tiny 9.11x10(-
28)g.
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Document Summary
By the end of the nineteenth century a large amount of chemical research had been carried out and interpreted in terms of atoms and molecules, although there was little direct evidence for atoms. Nevertheless, by the middle of the nineteenth century evidence began to emerge, not for the direct existence of atoms but for the even smaller particles which make up atoms. The first piece of the puzzle regarding the composition of the atom fell into place in 1897 when j. j. thomson discovered that atoms contain negatively-charged particles which he called corpuscles and were later named electrons. Later, he showed that positively-charged particles could also be obtained from atoms. Because individual atoms are electrically neutral, thomson hypothesizes that atoms must possess both positive and negative charges. Although he did not know why such a positive charge might exist, he proposed that the atom consisted of a ball of positive charge with negative charges distributed throughout the ball, as shown below.
