In a way, astrochemistry describes a cosmic dance of the elements in which atoms are constantly reshuffled from one species to another. This molecular rearrangement may be effected by gas phase binary collisions where atoms change partner or through recombination on grain surfaces. This “dance” is driven by the action of various energy sources, including photons and cosmic rays. In order to appreciate astrochemistry properly, we first have to get a basic understanding of the “dance” steps involved. This chapter will focus on the basic chemical processes that are of importance. In later chapters, we will then overview the resulting chemical reaction schemes that drive molecular complexity in the Universe.

There is a variety of processes that can lead to the formation of molecules in the interstellar medium, but these can be separated into two broad classes: reactions that occur in the gas phase and reactions that occur on the surfaces of small grains prevalent throughout the interstellar medium. These two classes of reactions are discussed in turn in the two sections of this chapter. The focus is on the formation of relatively simple species. The chemistry of large and complex interstellar molecules is discussed in Chapter 6.

Gas-phase chemical reactions

Gas phase reactions can be divided into different categories depending on their general effects. There are the bond-formation processes, including radiative association (cf. Section 4.2), which link atoms into simple or more complex species.