This beam was deflected toward the positive charge and away from the negative charge, and was produced in the same way with identical properties when different metals were used for the electrodes. When high voltage was applied across the electrodes, a visible beam called a cathode ray appeared between them. This apparatus consisted of a sealed glass tube from which almost all the air had been removed the tube contained two metal electrodes. If matter were composed of atoms, what were atoms composed of? Were they the smallest particles, or was there something smaller? In the late 1800s, a number of scientists interested in questions like these investigated the electrical discharges that could be produced in low-pressure gases, with the most significant discovery made by English physicist J. Atomic Theory after the Nineteenth Century While the historical persons and dates behind these experiments can be quite interesting, it is most important to understand the concepts resulting from their work. Here, we will discuss some of those key developments, with an emphasis on application of the scientific method, as well as understanding how the experimental evidence was analyzed. Much of this came from the results of several seminal experiments that revealed the details of the internal structure of atoms.

In the two centuries since Dalton developed his ideas, scientists have made significant progress in furthering our understanding of atomic theory.

Define isotopes and give examples for several elements.

Describe the three subatomic particles that compose atoms.

Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford.

Outline milestones in the development of modern atomic theory.

Learning Objectives By the end of this section, you will be able to:

Development of atomic theory