Illustrated above is a spectrum tube containing helium. The light stripes to the right are the individual frequencies of light emitted by helium atoms. The color of the tube is the result of the eye mixing all the individual frequencies together. When electric current is passed through a tube filled with the gaseous atoms of an individual element, light is emitted. In the illustration above, the pinkish-colored tube on the left is a tube filled with gaseous helium, which has electric current passing through. When the light from this tube is passed through a prism, the light is separated into the individual frequencies that compose it. The spectrum of light from helium atoms contains around 9 separate frequencies. All helium samples emit exactly the same frequencies when excited. The frequencies emitted by the various elements are so specific that the spectra can be used to identify the element involved. It was these emissions of specific frequencies from atoms that led to the Bohr model of the atom and its successor, the quantum mechanical model. This chapter will investigate the historical development of the model of the atom.
Human understanding of atomic structure and behavior has changed over time, but the current quantum mechanical model of the atom still bears many similarities to the initial concept proposed by the early Greeks. Much of our current understanding is the result of detailed study of atomic spectra, the unique series of colored lines associated with each element.