What are elements, compounds, and molecules?

These are three fundamental building blocks of matter, each more complex than the last.

• Element: A pure substance made of only one type of atom. Cannot be broken down further by chemical means. Examples: oxygen (O), iron (Fe), gold (Au).
• Compound: A pure substance formed when two or more elements combine chemically in a fixed ratio. Properties entirely different from constituent elements. Examples: water (H₂O), salt (NaCl).
• Molecule: The smallest particle of a substance that exists independently and retains all its chemical properties. Can be made of the same element (O₂) or different elements (CO₂).

Elements are pure single-atom substances, compounds are chemically combined elements, and molecules are the smallest independent units of any substance.