Atoms, elements, molecules, compounds, and mixtures are all forms of matter. Matter is defined as anything that has mass and volume.
Atoms, elements, molecules, and compounds are all pure substances. A pure substance is made of only one type of particle. This contrasts mixtures, which are made of more than one pure substance that do not chemically combine.
An atom is the smallest unit of matter. An element is simply a type of atom that has a specific number of protons. The periodic table is a chart that displays all of the different known elements.
Compounds and molecules are composed of more than one atom. The atoms of compounds or molecules are chemically bound via either ionic, covalent, or metallic bonds. Molecules are composed of two or more atoms. Compounds consist of two or more atoms of different elements. Therefore, compounds are molecules. However, not all molecules are compounds.