Lesson 2.2 Characterization of the Atom
Suggested Reading:
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Zumdahl: Chapter 2 Section 2.4
Essential Questions:
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What developments led to our present day model of the atom?
A model is a simplified representation of some aspect of the
world. In what way may models help or hinder the search for
knowledge?
Learning Objective:
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Discuss the development of the atom from its earliest model to
the present day atom.
The AP Chemistry exam from time to time asks questions regarding the
early models of the atom that helped shape our modern view of the atom.
Therefore, you should be familiar with the various atomic models that have
been used over time. Examining a timeline of atomic structure will help
you in this respect (Please note that the dates of different events often
differ from source to source). In this lesson we will look at the major
developments leading to the nuclear model resulting from the work of
Faraday (1832), Thomson (1897), Milikan (1909) and Rutherford (1911).
The Bohr solar system model and Schrodinger's wave-mechanical model
will be discussed in chapter 7.
1832, Michael Faraday
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Demonstrated the electric nature of the elements in a series of
experiments that showed an electric current can cause a chemical
reaction to occur.
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Developed the laws of electrolysis (Faraday's laws). You will learn
about these when we study electrochemistry. Students often struggle
with Faraday's laws, so you may want to get a head start on this topic.
1897, J.J. Thomson
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Studied cathode rays.
Determined that they were made of
negatively charged particles he called electrons.
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Determined the charge to mass ratio of an electron (e/m = -1.76 x 10⁸
coulombs/gram)
Reasoned that since atoms were known to be electrically neutral, that
they must contain some positive charge.
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Based the plum pudding model on his observations.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=IdTxGJjA4Jw
Thomson's Plum Pudding Model
1909, Robert Milikan


Performed oil drop experiments, which allowed him to calculate the
charge of the electron (-1.60 x 10 -¹⁹ coulomb).
Used the charge the electron along with Thomson's charge to mass
ratio to determine the mass of the electron (9.11 x 10 -²⁸ g).
Watch the following YouTube Video:
https://www.youtube.com/watch?v=XMfYHag7Liw
1911, Earnest Rutherford
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Discovered alpha and beta particles.
Used alpha and beta particles in experiments with gold foil to test
Thomson's model.
If Thomson's model was correct that the particles would be expected
to pass through the foil.
However, some particles bounced back suggesting the presence of a
nucleus.
This led Rutherford to the nuclear model of the atom.
Watch the following YouTube Video:
https://www.youtube.com/watch?v=wzALbzTdnc8