What describes a chemical element?

A chemical element is defined as the basic building block of matter. Elements are pure substances consisting of only one type of atom and cannot be broken down into simpler substances by chemical means. Each element is uniquely defined by its atomic number, which is the number of protons in its nucleus. This fundamental property of elements serves as the foundation for all matter in the universe, as they combine in various ways to form compounds and other substances.

Molecules, compounds, and mixtures, while related concepts, do not accurately describe what a chemical element is. Molecules consist of two or more atoms bonded together, which may include the same or different elements. Compounds are substances that consist of two or more different elements chemically bonded in fixed proportions. Mixtures refer to combinations of different substances that retain their individual properties and can vary in composition. Understanding these distinctions clarifies why the basic building block concept accurately characterizes chemical elements.