Chapter 4 Atomic Structure

Solid-state infrared laser cutting 1.6-mm steel sheet. This laser uses an yttrium-aluminum-garnet crystal doped with neodymium. The neodymium is pumped with radiation from small semiconductor lasers, a highly efficient method.

Far in the past people began to suspect that matter, despite appearing continuous, has a definite structure on a microscopic level beyond the direct reach of our senses. This suspicion did not take on a more concrete form until a little over a century and a half ago. Since then the existence of atoms and molecules, the ultimate particles of matter in its common forms, has been amply demonstrated, and their own ultimate particles, electrons, protons, and neutrons, have been identified and studied as well. In, this chapter and in others to come our chief concern will be the structure of the atom, since it is this structure that is responsible for nearly all the properties of matter that have shaped the world around us.

Every atom consists of a small nucleus of protons and neutrons with a number of electrons some distance away. It is tempting to think that the electrons circle the nucleus as planets do the sun, but classical electromagnetic theory denies the possibility of stable electron orbits. In an effort to resolve this paradox, Niels Bohr applied quantum ideas to atomic structure in 1913 to obtain a model which, despite its inadequacies and later replacement by a quantum-mechanical description of greater accuracy and usefulness, still remains a convenient mental picture of the atom. Bohr’s theory of the hydrogen atom is worth examining both for this reason and because it provides a valuable transition to the more abstract quantum theory of the atom.

4.1 THE NUCLEAR ATOM

An atom is largely empty space

4.2 ELECTRON ORBITS

The planetary model of the atom and why it fails

4.3 ATOMIC SPECTRA

Each element has a characteristic line spectrum

4.4 THE BOHR ATOM

Electron waves in the atom

4.5 ENERGY LEVELS AND SPECTRA

A photon is emitted when an electron jumps from one energy level to a lower level

4 .5 CORRESPONDENCE PRINCIPLE

The greater the quantum number; the closer quantum physics approaches classical physics

4.7 NUCLEAR MOTION

The nuclear mass affects the wavelengths of spectral lines

4.8 ATOMIC EXCITATION

How atoms absorb and emit energy

4.9 THE LASER

How to produce light waves all in step

APPENDIX: RUTHERFORD SCATTERING

EXERCISE