Chapter 16 begins by showing how quantum mechanics explains the structure and basic properties of multielectron atoms and their position in the periodic table of elements. It introduces the main approximation methods that have been developed to study such systems, explains the atomic shell structure, and discusses the helium atom in detail. It then looks at the different types of forces that bind atoms together to form molecules, focusing on diatomic molecules and the hydrogen molecule in particular. It discusses the long-range intermolecular forces that are responsible for many of the chemical and structural properties of matter, and identifies the zero-point or vacuum field as being responsible for the van der Waals and Casimir forces. In the final section, the properties of atomic nuclei are shown to reveal an internal structure with periodicities reminiscent of those of atoms and to disclose the effect of the nuclear spin–orbit interaction.