What form of matter is made up of only one kind of atom?

An element is the fundamental form of matter that consists of only one type of atom. Each element is defined by the number of protons found in the nucleus of its atoms, which is known as the atomic number. For instance, iron is an element because it is made up solely of iron atoms. Elements cannot be broken down into simpler substances through chemical reactions, making them the basic building blocks of all matter.

In contrast, compounds are substances formed when two or more different types of atoms bond together in fixed proportions, resulting in a substance with unique properties (like water, which is a compound made of hydrogen and oxygen). Mixtures consist of two or more substances physically combined, where each retains its own properties and can be separated by physical means (like a salad or a sand and salt mixture). Molecules consist of two or more atoms bonded together, but they can be made up of the same type of atom (like O2, which is a molecule of oxygen) or different types (like H2O). Therefore, while elements represent the simplest form of matter with a single kind of atom, compounds, mixtures, and molecules involve combinations of different types of atoms or substances.