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

A pure substance made of only one kind of atom is called an element. Elements are the fundamental building blocks of matter and cannot be broken down into simpler substances through chemical reactions. Each element is defined by the number of protons in its atoms, which is unique to that element. This uniqueness differentiates elements from compounds, which are made up of two or more types of atoms bonded together, and from mixtures, which contain different substances combined physically but not chemically. Molecules, on the other hand, refer to two or more atoms bonded together, which can be the same or different types of atoms. Therefore, an element represents the simplest form of a substance that retains specific properties, making it the correct answer.