* [ChemicalEntity](ChemicalEntity.md) - An entity that can be described using using chemical properties.
    * [Atom](Atom.md) - A material entity consisting of exactly one atomic nucleus and the electron(s) orbiting it.
       * [AtomIonicForm](AtomIonicForm.md) - an atom type in which charge state is specified. Also known as (atom) ionic species
          * [MonoatomicIon](MonoatomicIon.md) - An atom that has a charge
             * [AtomAnion](AtomAnion.md) - A monoatomic ion that is negative
             * [AtomCation](AtomCation.md) - A monoatomic ion that is positive
          * [UnchargedAtom](UnchargedAtom.md) - An atom that has no charge
       * [ChemicalElement](ChemicalElement.md) - generic form of an atom, with unspecified neutron or charge
       * [FullySpecifiedAtom](FullySpecifiedAtom.md) - An atom (class) that has subatomic particle counts specified
       * [Isotope](Isotope.md) - A specific subtype of an atom with a specified neutron number
       * [Nuclide](Nuclide.md) - an atomic species characterized by the specific constitution of its nucleus
          * [Radionuclide](Radionuclide.md) - an atom that has excess nuclear energy, making it unstable