Atoms are the fundamental building blocks of matter, which is defined as anything that occupies space and has mass. This broad category includes both living organisms, such as plants and animals, and nonliving entities like rocks and water. At the core of all matter are chemical elements, which are pure substances composed of only one type of atom. Therefore, an atom can be described as the smallest unit of an element, and consequently, the smallest unit of matter itself.

For example, a diamond is a form of nonliving matter made entirely of carbon atoms, represented by the symbol "C." When we examine the structure of a diamond, we see that it consists solely of carbon atoms, highlighting that it is a single chemical element. In contrast, living matter, such as a honeybee or a plant, is composed of various chemical elements. A common example is glucose, a sugar found in living organisms, which contains carbon, hydrogen, and oxygen atoms. Each of these elements is represented by its own atomic structure, with hydrogen being the simplest, consisting of a single hydrogen atom.

In summary, all matter is composed of chemical elements, and the atom serves as the smallest unit of these elements. Understanding this relationship is crucial as we delve deeper into the structures and properties of atoms in future lessons.