A hydrogen atom has a three-dimensional wave function that gives the probability of locating the electron in a region of space.

• Three quantum numbers are needed.

• Both energy and angular momentum are quantized.

• The probability density shows electron clouds rather than well-defined orbits.

• A fourth quantum number describes the electron’s spin, an inherent magnetic moment that can point up or down.

《 LOOKING BACK Chapter 39 Wave functions

Quantum mechanics also explains the properties of multielectron atoms, including their energy levels, ionization energies, and spectra.

• We’ll use energy-level diagrams to understand which states are occupied and how spectra are produced.

• The Pauli exclusion principle—that only one electron can occupy each quantum state—is the key to understanding the periodic table of the elements.

《 LOOKING BACK Chapter 38 The Bohr model

You’ll learn to interpret atomic spectra in terms of excitation followed by emission.

• Excitation can be by collision with a particle or the absorption of a photon.

• An excited state has a lifetime of typically a few nanoseconds.

• Emission obeys selection rules that allow some quantum jumps but not others.

Lasers work because of stimulated emission of light, where an incoming photon causes an excited state to emit an identical photon.

• A laser requires a population inversion, with more atoms in an excited state than in a lower energy level.

• Lasers can be continuous or pulsed.

Matter consists of atoms. A quantum understanding of atoms is the basis for modern chemistry and material science. Lasers depend on the quantum properties of atoms. Atomic clocks provide the precise timekeeping needed for GPS measurements and the World Wide Web. Optical techniques with atoms have revolutionized our ability to image the details of cells. And the new field of quantum computing relies on a precise manipulation of atomic energy levels.