What is a pure substance made of only one kind of atom called?

A pure substance made of only one kind of atom is known as an element. Elements are the basic building blocks of matter and cannot be broken down into simpler substances through chemical means. Each element is defined by the number of protons in its atomic nucleus, which identifies it in the periodic table. For example, oxygen (O) and carbon (C) are both elements, consisting solely of oxygen and carbon atoms, respectively.

In contrast, a compound consists of two or more different types of atoms that are chemically bonded together, such as water (H2O) or carbon dioxide (CO2). Mixtures are made up of two or more substances that are physically combined but not chemically bonded, allowing their individual properties to remain intact. Molecules, while they consist of atoms bonded together, can be composed of one type of atom (like O2, which is a molecule made of two oxygen atoms) or different types (like H2O). Hence, in the context of the question, the term that accurately describes a pure substance made of only one kind of atom is an element.