Chemical species are atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s, molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s, molecular fragments, ion

Ion

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s, etc., being subjected to a chemical process

Chemical process

In a "scientific" sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody. Such a chemical process commonly involves a chemical reaction of some sort...

 or to a measurement

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entities

Molecular entity

According to the IUPAC Gold Book a molecular entity is "any constitutionally or isotopically distinct atom, molecule, ion, ion pair, radical, radical ion, complex, conformer, etc., identifiable as a separately distinguishable entity"....

 that can explore the same set of molecular energy levels on a characteristic or delineated time scale. The term may be applied equally to a set of chemically identical atomic or molecular structural units in a solid array.

In supramolecular chemistry

Supramolecular chemistry

Supramolecular chemistry refers to the area of chemistry beyond the molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components...

, 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.