What distinguishes an element from a compound?

An element is defined as a pure substance that consists of only one type of atom. This means that all atoms in an element have the same number of protons, which determines the element's identity. For example, oxygen (O) is an element made up solely of oxygen atoms, and all such atoms are identical in terms of atomic structure.

In contrast, a compound is formed when two or more different elements chemically bond together in fixed proportions, resulting in a substance with unique properties that differ from those of its constituent elements. Thus, the distinguishing characteristic of an element is its singular atomic composition, which sets it apart from compounds where multiple types of atoms are present. Elements cannot be broken down into simpler substances by chemical means; this definition highlights their foundational role in chemistry, making option C the correct distinction between an element and a compound.