10.2 The Bohr Atom
Learning Objectives
By the end of this section, you will be able to:
- Explain the difference between the absorption spectrum and the emission spectrum of radiation emitted by atoms
- Describe the Bohr model of the hydrogen atom
- Summarize how Bohr’s quantum model of the hydrogen atom explains the radiation spectrum of atomic hydrogen
- Review key equations used to calculate energies of light emitted or absorbed by hydrogen atoms
Our view of objects in the sky at night, the warm radiance of sunshine, the sting of sunburn, neon signs around us, our cell phone conversations, and the X-rays revealing a broken bone—all are brought to us by electromagnetic waves. It would be hard to overstate the practical importance of electromagnetic waves, through their role in vision, through countless technological applications, and through their ability to transport the energy from the Sun through space to sustain life and almost all of its activities on Earth.
Theory predicted the general phenomenon of electromagnetic waves before anyone realized that light is a form of an electromagnetic wave. In the mid-nineteenth century, James Clerk Maxwell formulated a single theory combining all the electric and magnetic effects known at that time. Maxwell’s equations, summarizing this theory, predicted the existence of electromagnetic waves that travel at the speed of light. His theory also predicted how these waves behave, and how they carry both energy and momentum. These discoveries contributed to the emergence of the modern atomic theory.
Scientists continued to perform experiments to investigate the properties of electromagnetic radiation. Evidence supported that atoms could give off light. Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded by lighter and even tinier electrons continually moving about the nucleus was well established. This theory was called the planetary model since it pictured the atom as a miniature “solar system” with the electrons orbiting the nucleus like planets orbiting the sun. Rutherford’s model for atoms had one serious problem. Maxwell’s theory of electromagnetic radiation says that when electrons change either speed or the direction of motion, they must emit energy. Science had proved that orbiting electrons constantly change their direction of motion, so they should, in theory, emit a constant stream of energy. Applying Maxwell’s theory to Rutherford’s model, all electrons should spiral into the nucleus of the atom as they lose energy, and this collapse should happen very quickly—in about 10–16 seconds. However, experimental results were not supporting such findings. Scientists concluded the atom’s structure and how electrons move around the nucleus must be different than originally theorized. It was Danish physicist, Niels Bohr (1885–1962), who proposed an updated theory of how electrons remain in orbit.
Emission