ABSTRACT
This chapter shows the way in which atoms are arranged in a solid and what holds them together. It makes serious acquaintance with observations and examines the interstellar extinction curve. In view of the overall cosmic abundance of elements, the extinction curve rules out major dust constituents other than silicate and carbon grains. The chapter discusses the atomic structure and bonding and sketches the kind of distribution and how it can be achieved in grain-grain collisions. Equipped with a quantitative idea about size and chemical composition of interstellar dust, it presents typical and likely grain cross sections based on a reasonable set of optical constants. In a crystal, the atoms are regularly arranged in a lattice which means that the crystal can be built up by periodic repetition of identical cells.
