Chemical classification systems attempt to classify as elements

Chemical element

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...

 or compounds

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 according to certain chemical functional or structural

Chemical structure

A chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...

 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

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

.

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.

Physico-chemical classification

• by molecular weight
• by electrical charge: uncharged, positively, negatively, partially charged
  Partial charge
  A partial charge is a charge with an absolute value of less than one elementary charge unit .-Partial atomic charges:...
  
  
  • formal charge
    Formal charge
    In chemistry, a formal charge is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity....
    
    , oxidation state
    Oxidation state
    In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
    
    
• solubility
  Solubility
  Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
  
  
• pH value

Structural classification

Usually binary classifications and combinations thereof:

• radical
  Radical (chemistry)
  Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
  
  s, non-radicals
• cyclic vs. linear
• type of major bond: ionic
  Ionic bond
  An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
  
   (salt
  Salt
  In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
  
  s) vs. covalent
  Covalent bond
  A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
  
  , metallic
  Metallic bond
  Metallic bonding is the electrostatic attractive forces between the delocalized electrons, called conduction electrons, gathered in an "electron sea", and the positively charged metal ions...
  
  
• organic
  Organic compound
  An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
  
   vs. inorganic compound
  Inorganic compound
  Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
  
  s (depends on whether carbon is the main constituent or not)
• (in organic chemistry)
  • aliphatic vs. aromatic compounds

Pharmacological/biological function

Mostly appropriate only for large biological molecules (as at least one interacting partner), in particular enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s, depends on chemical functions of their constituent amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s.

• ligand
  Ligand
  In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
  
   vs. receptor
  Receptor (biochemistry)
  In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
  
  , coenzyme
• EC number
  EC number
  The Enzyme Commission number is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze....
  
  
• TC number
  TC number
  Transporter Classification database is an International Union of Biochemistry and Molecular Biology approved classification system for membrane transport proteins including ion channels...
  
  
• pharmacophore
  Pharmacophore
  thumb|right|300px|An example of a pharmacophore model.A pharmacophore is an abstract description of molecular features which are necessary for molecular recognition of a ligand by a biological macromolecule....
  
   vs. non-drug
  Drug
  A drug, broadly speaking, is any substance that, when absorbed into the body of a living organism, alters normal bodily function. There is no single, precise definition, as there are different meanings in drug control law, government regulations, medicine, and colloquial usage.In pharmacology, a...
  
  

See also: biological activity

Biological activity

In pharmacology, biological activity or pharmacological activity describes the beneficial or adverse effects of a drug on living matter. When a drug is a complex chemical mixture, this activity is exerted by the substance's active ingredient or pharmacophore but can be modified by the other...

See also

• Chemical Safety: Globally Harmonized Classification System
  Globally Harmonized System
  The Globally Harmonized System of Classification and Labeling of Chemicals or GHS is an internationally agreed upon system, created by the United Nations. It is designed to replace the various classification and labeling standards used in different countries by using consistent criteria for...
  
  

External links

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