http://www.ck12.org Chapter 23. The Atom
23.2 The Quantum Mechanical Model of the Atom
- Describe the nuclear model of the atom.
- Describe the relationship between atomic emission spectra and electron configuration.
- Calculate energy levels.
- Describe the shortcomings of the Bohr model of the atom.
The gold foil experiment was conducted under the supervision of Ernest Rutherford at the University of Manchester
in 1909 by Hans Geiger and Ernest Marsden. The popular theory of atomic structure at the time of Rutherford’s
experiment was the “plum pudding model.” This theory held that the negatively charged electrons in an atom were
floating in a sea of positive charge (the electrons playing the role of plums in a bowl of plum pudding). Rutherford’s
gold foil experiment demonstrated that almost all of the mass of an atom is in a tiny volume in the center of the atom
which Rutherford called the nucleus. This positively charged mass was responsible for deflecting alpha particles
propelled through the gold foil. Rutherford’s “nuclear model” of the atom replaced the plum pudding model in
1911.
The Bohr Model of the Atom
Early History of the Atom
Hypotheses concerning the existence of atoms as indivisible particles were known as early as the ancient Greek
philosophers Democritus and Epicurus. The concepts of atoms that prevailed in the 17thand 18thcenturies were
poorly defined. Some scientists supposed that atoms could combine into more complex particles but specific
chemical and physical properties were not attributed to atoms. In the period 1780-1803, Lavoisier made careful
quantitative measurements which allowed the compositions of compounds to be determined accurately, Proust es-
tablished the Law of Definite Proportions (each compound always contained the same proportions of elements), and
John Dalton had developed the Law of Simple Proportions (compounds composed of the same elements contained
simple multiples of component elements). These laws provided a solid foundation for a more thorough atomic
theory.
Dalton’s Atomic Theory
John Dalton developed his atomic theory in 1803. Dalton’s theory was different from previous discussions of
atoms because it had the weight of careful chemical measurements behind it. Dalton’s theory went beyond just