In this chapter we will introduce you to the fundamental concepts of chemistry that you will need in your study of earth science or environmental science. We therefore start at the very beginning by considering what atoms are, their structure, how they were formed, and how some atoms are changed by radioactive decay. We then consider how atoms combine to form molecules, the different ways in which atoms bond to one another, and the structures of some inorganic and organic compounds, with particular reference to those of environmental interest, such as pollutants. We then discuss how molecules react with one another in chemical reactions. This chapter also includes a section on the scientific units that you will encounter in chemistry.