A chemical species is a chemical substance or ensemble composed of chemically identical molecular entities that can explore the same set of molecular energy levels on a characteristic or delineated time scale. These energy levels determine the way the chemical species will interact with others (engaging in chemical bonds, etc.). The species can be atom, molecule, ion, radical, and it has a chemical name and chemical formula. The term is also applied to a set of chemically identical atomic or molecular structural units in a solid array.
In supramolecular chemistry, chemical species are those supramolecular structures whose interactions and associations are brought about via intermolecular bonding and debonding actions, and function to form the basis of this branch of chemistry.