Chemical classification systems attempt to classify elements or compounds according to certain chemical functional or structural properties. Whereas the structural properties are largely intrinsic, functional properties and the derived classifications depend to a certain degree on the type of chemical interaction partners on which the function is exerted. Sometimes other criteria like purely physical ones (e.g. molecular weight) or - on the other hand - functional properties above the chemical level are also used for building chemical taxonomies. The periodic table of the chemical elements (this version outdated on October 13, 2006) A chemical element, or element for short, is a pure substance that cannot be decomposed into any simpler substance. ... A chemical compound is a chemical substance consisting of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. ... Chemical structure refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. ... Intrinsic is used to describe a characteristic or property of some thing or action which is specific to that thing or action, and which is wholly independent of any other object, action or consequence. ... Interaction is a kind of action that occurs as two or more objects have an effect upon one another. ... The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). ... Taxonomy (from Greek ταξινομία (taxinomia) from the words taxis = order and nomos = law) may refer to either the classification of things, or the principles underlying the classification. ...

Some systems mix the various levels, resulting in hierarchies where the domains are slightly confused, for example having structural and functional aspects end up on the same level. Whereas chemical function is closely dependent on chemical structure, the situation becomes more involved when e.g. pharmacological function is integrated, because the QSAR can usually not be directly computed from structural qualities. Pharmacology (in Greek: pharmacon is drug, and logos is science) is the study of how chemical substances interfere with living systems. ... QSAR (Quantitative Structure-Activity Relationship, sometimes the A stands also for Affinity=reactivity) is the quantitative correlation of the biological (ecological, toxicological or pharmacological) activity to the structure of chemical compounds, which allows the prediction of the so-called drug efficacy of a structurally related compound. ...

Physico-chemical classification

• by molecular weight
• by electrical charge: uncharged, positively, negatively, partially charged
  • formal charge, oxidation state
• solubility
• pH value

The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). ... Electric charge is a fundamental property of some subatomic particles, which determines their electromagnetic interactions. ... A partial charge is a charge with an absolute value of less than one elementary charge unit. ... In chemistry, a formal charge (FC) on an atom in a molecule is defined as: FC = number of valence electrons of the atom - number of Lone pair electrons on this atom - half the total number of electrons participating in covalent bonds with this atom. ... In chemistry, the oxidation state is a measure of the degree of oxidation of an atom in a chemical compound. ... Solubility refers to the ability for a given substance, the solute, to dissolve in a solvent. ... The title given to this article is incorrect due to technical limitations. ...

Structural classification

Usually binary classifications and combinations thereof:

• radicals, non-radicals
• cyclic vs. linear
• type of major bond: ionic (salts) vs. covalent
• organic vs. inorganic compounds (depends on whether carbon is the main constituent or not)
• (in organic chemistry)
  • aliphatic vs. aromatic compounds

In chemistry, radicals (often referred to as free radicals) are atomic or molecular species with unpaired electrons on an otherwise open shell configuration. ... Electron configurations of lithium and fluorine. ... A magnified crystal of a salt (halite/sodium chloride) Salt covering the floor of Bad Water in Death Valley, CA, the lowest point in the US. A salt, in chemistry, is any ionic compound composed of cations (positively charged ions) and anions (negative ions) so that the product is neutral... Covalent bonding is a description of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. ... Benzene is the simplest of the arenes, a family of organic compounds An organic compound is any member of a large class of chemical compounds whose molecules contain carbon and hydrogen; therefore, carbides, carbonates, carbon oxides and elementary carbon are not organic (see below for more on the definition controversy... An inorganic compound is a chemical compound that is not an organic compound. ... In chemistry, non-aromatic and non-cyclic (acyclic) organic compounds are called aliphatic. ... In chemistry, an aromatic molecule is one in which electrons are free to cycle around circular arrangements of atoms, which are alternately singly and doubly bonded to one another. ...

Functional classification

Chemical function

• by functional group

In organic chemistry, functional groups are specific groups of atoms within molecules, that are responsible for the characteristic chemical reactions of those molecules. ...

Pharmacological/Biological function

Mostly appropriate only for large biological molecules (as at least one interacting partner), in particular enzymes, depends on chemical functions of their constituent amino acids. Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... Phenylalanine is one of the standard amino acids. ...

• ligand vs. receptor, coenzyme
• EC number
• TC number
• pharmacophore vs. non-drug

See also: biological activity In chemistry, a ligand is an atom, ion, or molecule (see also: functional group) that generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions (these ligands act as a... Look up Receptor in Wiktionary, the free dictionary. ... Coenzyme A Coenzymes are small organic non-protein molecules that carry chemical groups between enzymes. ... The Enzyme Commission number (EC number) is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze. ... In biology, the TC number refers to a standard used for classifying transport protein and ion channel proteins. ... A pharmacophore is a three-dimensional substructure of a molecule that carries (phoros) the essential features responsible for a drugs (pharmacon) biological activity. ... Oral medication Caffeine is one of the most widely used psychoactive substances in the world. ... It has been suggested that this article or section be merged with Bioactivity. ...

Mixed systems

Anatomical Therapeutic Chemical Classification System Gene Ontology The Anatomical Therapeutic Chemical Classification System is used for the classification of drugs. ... The Gene Ontology, or GO, project can be broadly split into two parts. ...

External links

• http://www.ebi.ac.uk/chebi/ Chemical Entities of Biological Interest