Substance composed of chemically identical molecular entities
Chemical species are a specific form of chemical substance or chemically identical molecular entities that have the same molecular energy level at a specified timescale. These entities are classified through bonding types and relative abundance of isotopes.[1] Types of chemical species can be classified based on the type of molecular entity and can be either an atomic, molecular, ionic or radical species.
Generally, a chemical species is defined as a chemical identity that has the same set of molecular energy levels in a defined timescale (i.e. an experiment). These energy levels determine the way the chemical species will interact with others through properties such as bonding or isotopic compositions. The chemical species can be an atom, molecule, ion, or radical, with a specific chemical name and chemical formula.[2]
Insupramolecular chemistry, chemical species are structures created by forming or breaking bonds between molecules, such as hydrogen bonding, dipole-dipole bonds, etc.[3] These types of bonds can determine the physical property of chemical species in a liquid or solid state.[4]
The term is also applied to a set of chemically identical atomic or molecular structures in a solid compound.
Table salt is an example of an ionic species that is composed of the chemical formula NaCl.
Atomic species: Specific form of an element defined by the atom's isotope, electronic or oxidation state. Argon is an atomic species of formula Ar.[5]
Molecular species: Groups of molecules that are held together by chemical bonds. An example is ozone, which has the chemical formula O3.[6]
Ionic species: Atoms or molecules that have gained or lost electrons, resulting in a net electrical charge that can be either positively (cation) or negatively charged (anion).
Species with an overall positive charge will be a cationic species. The sodium ion is an example of a cationic species and its formula is Na+.
Species with an overall negative charge will be an anionic species. Chloride is an anionic species, and its formula is Cl−.[7]
Radical species: Molecules or atoms with unpaired electrons. Triarlborane anion is a radical species and its formula is Ar3B−[8][9]
Chemicals can be two different types of species. For example, nitrate is a molecular and ionic species, with its formula being NO3−.
Note that DNA is not a species; the name is generically applied to many molecules of different formulas (each DNA molecule is unique).[10]
