Lesson 3-2 The Development of the Atomic Model

Lesson 3-2

Development of the Atomic Model

To borrow an example from Albert Einstein, imagine if you had never seen a clock or a watch before, and someone gave you an intricate Swiss timepiece. Imagine studying the motion of the hands, but never being allowed to remove the watch face and see the mechanisms which produced the sychronized movements. If you thought about it long enough, you might be able to come up with a model to explain the motion of the hands, but you could never be sure that your model was an accurate depiction of what was going behind the face of the watch. In fact, if someone was to come along with a better explanation for the motion of the hands, you would be forced to update your model.

Our atomic model has much in common with the imaginary watch from the above example. We can't base our model on actual observations of atoms, because they are too small to be seen with our most sensitive instruments. Instead, we must come up with a model of an atom that can account for and explain observations that we can actually see. As new observations are made, we are forced to update our model to accommodate them. As a result, our model of the atom has evolved over time, and we must accept the fact that it is likely to change again in the future.

The story so far . . .

Democritus c460-371 BC	Democritus may not have been the first of the ancient Greeks to suggest an atomic theory, this distinction goes to his teacher Leucippus, but his name is often associated with the first atomic theory, because of his support of it. To Democritus, atoms were completely solid, homogeneous, indestructible objects.
J.J. Thomson 1856-1940	Joseph John Thomson subjected cathode rays to magnetic and electric fields and showed that the beam was deflected as would be expected for negatively charged particles. He calculated the ratio of the electron's charge to its mass. On April 30, 1897, Thomson announced that the cathode rays consisted of negatively charged particles, which represented fundamental particles of matter. He was not the first person to suggest that these particles existed, nor did he coin the term "electron", yet he is generally credited with the discovery of the electron. He was awarded with the Nobel Prize in Physics in 1906. J.J. Thomson is also remembered for his "plum-pudding" model of the atom, which suggested a solid atom with positively and negatively charged particles evenly distributed throughout the mass of the atom.
Link One - The original paper in which J.J. Thomson announces his discovery of the electron to the world. Link Two - Thomson on the number of corpuscles (electrons) in an atom. Link Three - Thomson on the structure of the atom. Link Four excerpts from Thomson's Nobel prize address
Lord Ernest Rutherford 1871-1937	Ernest Rutherford, who was once a student of Thomson's, is credited with discovering that most of the atom is made up of "empty space." In 1909 he and his assistants conducted the "gold foil" experiment, from which he concluded that "the greater part of the mass of the atom was concentrated in a minute nucleus." In this model, the positively charged nucleus was surrounded by a great deal of "empty space" through which the electrons moved.
Link One - Geiger's paper on the gold foil Link Two - Rutherford describing the gold foil experiment Link Three - Rutherford's paper on the structure of the atom
Robert Millikan 1868-1953	In 1909, Robert Millikan conducted his "oil-drop" experiment which allowed him to measure the charge on an electron. Combining his results with those of Thomson, Millikan found the mass of the electron to be 9.11x10^-28 g. He was awarded with the Nobel Prize in physics in 1923.
Niels Bohr 1913-1963	In 1913, Niels Bohr proposed improvement to Rutherford atomic model. For this reason, the planetary model of the atom is sometimes called the Rutherford-Bohr model. Bohr added the idea of fixed orbits, or energy levels for the electron traveling around the nucleus. This model allowed for the idea that electrons can become "excited" and move to higher energy levels for brief periods of time.
Link One - Bohr's address on the spectrum of hydrogen Link Two - An article on atomic structure written by Niels Bohr
James Chadwick 1891-1974	Lord Rutherford predicted the existence of the neutron is 1920. Walter Bothe obtained evidence of the neutron in 1930. However it was James Chadwick, who repeated Bothe's work, who is known as the discoverer of the neutron. He found these uncharged particles with essentially the same mass as the proton. He was awarded the Nobel Prize in physics in 1935.
Link One - A letter on the possible existence of the neutron Link Two - Chadwick's paper on the discovery of the neutron
	Although there is something attractive about the idea of an atom being very much like a tiny solar system, the planetary model of the atom was found to be inadequate. Planck's quantum theory had illustrated the "particle-like" properties of waves. Louis de Broglie suggested that particles might have properties of waves. The result of this investigation is sometimes called the wave-particle duality of nature. This duality, which states that particles act like waves and waves like particles, applies to all waves and all particles. However, the more massive the particles, the less obvious the wave properties. Electrons, having very little mass, exhibit significant wave-like properties.

Werner Heisenberg	Heisenberg pointed out that it is impossible to know both the exact position and the exact momentum of an object at the same time. Applying this concept to the electron we realize that in order to get a fix on an electron's position at any time, we would alter its momentum. Any attempt to study the velocity of an electron will alter its position. This concept, called the Heisenberg Uncertainty principle, effectively destroys the idea of electrons traveling around in neat orbits. Any electron that is subjected to photons will have its momentum and position affected.
The Charge-Cloud Model	Experiments conducted in the 1920's, 1930's and 1940's continued to point out problems with the planetary model of the atom. These experiments, which will be discussed in next chapter, lead to the development of the charge-cloud model. The charge-cloud model, which is also called the quantum-mechanical model, does not attempt to describe the path of each electron in a fixed orbit. Scientists now describe the possible positions of electrons in terms of probability. Computers can calculate the points in space that an electron has the highest probability of occupying. These points can be connected to form a three-dimensional shape. Electrons are characterized in terms of the three-dimensional shapes that their probability fields define. The sum total of the various paths of electrons, traveling at very high speeds, is described as the electron cloud.
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