1. Proton: The smallest fundamental particle of the group. It resides in the nucleus of an atom and carries a positive charge.
2. Electron: Also a fundamental particle, but much smaller than a proton. It carries a negative charge and orbits the nucleus of an atom.
3. Atom: The smallest unit of an element. It is composed of a nucleus (containing protons and neutrons) surrounded by electrons.
4. Molecule: Two or more atoms bonded together. Examples include water (H₂O) and oxygen (O₂).
5. Compound: A substance made up of two or more different elements chemically combined. Water (H₂O) is a compound because it contains hydrogen and oxygen.
Important Note: While a compound is technically larger than a molecule, the distinction can be subtle. Many molecules are also compounds. The key difference is that a compound *must* have multiple elements, while a molecule can have multiple atoms of the same element (like O₂).
