Atom:
An atom is the smallest unit of matter that retains the chemical properties of an element. It is the basic building block of matter. Atoms are composed of three subatomic particles: protons, neutrons, and electrons. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. The protons and neutrons are found in the nucleus at the center of the atom, while the electrons orbit around the nucleus in energy levels or shells.

Element:
An element is a pure substance made up of only one type of atom. Each element is represented by a unique symbol, such as H for hydrogen, O for oxygen, and Na for sodium. Elements are organized in the periodic table based on their atomic number, which represents the number of protons in an atom of that element. There are currently 118 known elements, and they can combine to form various compounds.

Molecule:
A molecule is a group of two or more atoms bonded together. It can be composed of atoms from the same element or different elements. The atoms in a molecule are held together by chemical bonds, which can be covalent, ionic, or metallic. Covalent bonds involve the sharing of electrons between atoms, while ionic bonds involve the transfer of electrons from one atom to another. Molecules can exist as discrete units or as part of a larger structure.

Compound:
A compound is a substance made up of two or more different elements that are chemically bonded together. It has a fixed ratio of atoms. Compounds can have properties different from the elements they are composed of. For example, water is a compound made up of two hydrogen atoms and one oxygen atom (H2O). Compounds can be represented by chemical formulas, which indicate the types and numbers of atoms present. Chemical reactions involve the breaking and forming of bonds between atoms to create new compounds.

In summary, atoms are the basic units of matter, elements are pure substances made up of one type of atom, molecules are groups of atoms bonded together, and compounds are substances made up of two or more different elements. Understanding these concepts is fundamental to the study of chemistry and helps explain the composition and behavior of matter.