How many types of atoms do elements contain?

Elements are defined as pure substances consisting of only one type of atom. This uniqueness is crucial because each element possesses a specific number of protons in its atomic nucleus, which determines its identity and properties. For example, hydrogen consists solely of hydrogen atoms, while carbon is made up exclusively of carbon atoms.

The notion that an element can contain multiple types of atoms or even two or three types contradicts the foundational principles of chemistry. This is because compounds, formed by the chemical combination of different elements, contain different types of atoms, but an element itself remains singular in atomic composition. Therefore, the correct understanding is that elements are characterized by containing only one type of atom.