Old Atomic Theory

In 1808 Dalton presented his atomic theory: Using this basis Dalton was able to develop the first table of atomic masses which chemists call atomic weights since the mass is usually determined by comparison to a standard mass -- a process called weighing. Many of his masses were incorrect due to wrong assumptions, but it was a start toward a correct table of masses.

Breakthroughs were also made by Gay-Lussac and Avogadro. Gay-Lussac found that 2 volumes of hydrogen react with 1 volume of oxygen to form 2 volumes of gaseous water and that 1 volume of hydrogen reacts with 1 volume of chlorine to form 2 volumes of hydrogen chloride. Avogadro (1811) interpreted the results of Gay-Lussac to mean that at the same temperature and pressure, equal volumes of different gases contain the same number of particles.. For example, the first experiment of Gay-Lussac with the Avogadro hypothesis says that water has the formula of 2 particles of hydrogen and 1 particle of oxygen (what we will later see can be written as H2O). However these breakthroughs were not appreciated to their fullest for about fifty years. But in the later part of the 1800's some experiments and theories finally started pulling it all together. These important advances are briefly noted in the following:

From 1898-1903 J.J. Thomson studied electrical discharges in partially evacuated tubes called cathode-ray tubes (so called because the "rays" emanated from the negative electrode which was called the cathode). And since these "rays" came from the negative electrode he deduced that the ray was a stream of negatively charged particles. Coupling this discharge with a magnetic field he was able to determine the charge-to-mass ratio of the negatively charged particles: e/m = -1.76 x 108C/g where e represents the charge of the electron in Coulombs (C) and m represents the mass of the electron in grams. Based upon his experiments Thomson developed the "plum pudding" model of the atom: An atom consists of a spherical cloud of positive charge with electrons embedded randomly within it.

In 1909, Robert Millikan performed his famous oil-drop experiments (a required experiment in my optics lab in an undergraduate physics class in 1959 -- very tiring on the eyes and requiring a deft touch with electronics). From these experiments and the Thomson ratio, Millikan was able to determine the mass of the electron to be 9.11 x 10-31.

In the late nineteenth century Marie Curie and coworkers discovered radioactivity(the spontaneous emission of radiation from certain elements). By the early 20th century, three types of radioactive emission had been characterized:

In 1911 Rutherford performed some experiments to test the Thomson plum-pudding model of the atom. His experiments showed that this model could not be correct and led to his formulation of a model of the atom in which there is a nucleus (a dense center of positive charge) with electrons somehow moving about the nucleus at a distance that is very large compared to the radius of the nucleus. You can get an idea of the relative sizes of the radius of the atom (distance from the center of the atom to the distance of the outer electron from the center) and the radius of the nucleus by imagining the Georgia Dome with a marble at the center of the dome. The relative size of the radius of the atom with the radius of the nucleus is of the same magnitude as the ratio of the radius of the Dome to the radius of the marble. Fix that picture in your mind.

Now take a practice quiz to help you understand if you understand the basic concepts.
You must use your real name when it asks for a name.
The test will only submit when you have answers all of the questions correctly.
If you are not taking this course for credit please do not answer all the questions correctly for I don't want to be flooded with email answers to the tests.

Web Author: Dr. Leon L. Combs
Copyright 2000 by Dr. Leon L. Combs - ALL RIGHTS RESERVED