After applying the Saha equation to the data at hand, Payne-Gaposchkin published a monumental paper demonstrating that stars were mostly made of hydrogen and helium, and that hydrogen was the most abundant element in the universe. But Payne-Gaposchkin knew that Indian physicist Meghnad Saha had developed a method to calculate stellar ionization to obtain an accurate measure of star temperatures. Up until that point, many researchers believed that stars - particularly the sun - were made up of similar elements to Earth, based in part on the spectral analysis. In the 1920s, Cecilia Payne-Gaposchkin undertook a thorough analysis of a collection of stellar spectra that had been organized by astronomer Annie Jump Cannon. This also means that the radiation emitted by atoms of each element has exactly the same wavelengths as the radiation they absorb.įigure 6.16 The emission spectrum of atomic iron: The spectral positions of emission lines are characteristic for iron atoms.īy observing the spectra from starlight and applying additional knowledge and calculations, astronomers can determine the makeup of objects millions of light-years away. This means that atoms of a specific element absorb radiation only at specific wavelengths and radiation that does not have these wavelengths is not absorbed by the element at all. For each element, the positions of its emission lines are exactly the same as the positions of its absorption lines. Each chemical element has its own characteristic emission spectrum. Positions of the emission lines tell us which wavelengths of the radiation are emitted by the gas. This spectrum is seen as colorful lines on the black background (see Figure 6.15 and Figure 6.16). The emission spectrum is observed when light is emitted by a gas. The missing wavelengths tell us which wavelengths of the radiation are absorbed by the gas. This spectrum appears as black lines that occur only at certain wavelengths on the background of the continuous spectrum of white light ( Figure 6.13). An absorption spectrum is observed when light passes through a gas.

The difference between the absorption spectrum and the emission spectrum is explained in Figure 6.14. During 1854–1861, Gustav Kirchhoff and Robert Bunsen discovered that for the various chemical elements, the line emission spectrum of an element exactly matches its line absorption spectrum. Solar absorption lines are called Fraunhofer lines after Joseph von Fraunhofer, who accurately measured their wavelengths. When we use a prism to analyze white light coming from the sun, several dark lines in the solar spectrum are observed ( Figure 6.13). To understand the specifics of Bohr’s model, we must first review the nineteenth-century discoveries that prompted its formulation. The model has a special place in the history of physics because it introduced an early quantum theory, which brought about new developments in scientific thought and later culminated in the development of quantum mechanics. Historically, Bohr’s model of the hydrogen atom is the very first model of atomic structure that correctly explained the radiation spectra of atomic hydrogen. Summarize how Bohr’s quantum model of the hydrogen atom explains the radiation spectrum of atomic hydrogen.Describe the postulates of the early quantum theory for the hydrogen atom.

Explain the atomic structure of hydrogen.

Describe the Rutherford gold foil experiment and the discovery of the atomic nucleus.

Explain the difference between the absorption spectrum and the emission spectrum of radiation emitted by atoms.By the end of this section, you will be able to:

Bohr model of atomic structure