Chemical space is the space spanned by all possible (i.e. energetically stable) 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 and chemical compound

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...

s – that is, all stoichiometric

Stoichiometry

Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...

 combinations of electrons and atomic nuclei

Atomic nucleus

The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...

, in all possible topology isomers

Topology (chemistry)

In chemistry, topology provides a convenient way of describing and predicting the molecular structure within the constraints of three-dimensional space. Given the determinants of chemical bonding and the chemical properties of the atoms, topology provides a model for explaining how the atoms...

. Chemical reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

s allow us to move in chemical space. The mapping between chemical space and molecular properties

Molecular property

Molecular properties include the chemical properties, physical properties, and structural properties of molecules, including drugs. Molecular properties typically do not include pharmacological or biological properties of a chemical compound.- See also :...

 is often not unique, meaning that there can be multiple molecules which exhibit the same properties. Material design and drug discovery

Drug discovery

In the fields of medicine, biotechnology and pharmacology, drug discovery is the process by which drugs are discovered or designed.In the past most drugs have been discovered either by identifying the active ingredient from traditional remedies or by serendipitous discovery...

 both involve the exploration of chemical space.

As of July 2009, there were 49,037,297 organic and inorganic substances registered with the Chemical Abstracts Service

Chemical Abstracts Service

Chemical Abstracts is a periodical index that provides summaries and indexes of disclosures in recently published scientific documents. Approximately 8,000 journals, technical reports, dissertations, conference proceedings, and new books, in any of 50 languages, are monitored yearly, as are patent...

, indicating that they have been reported in the scientific literature

Scientific literature

Scientific literature comprises scientific publications that report original empirical and theoretical work in the natural and social sciences, and within a scientific field is often abbreviated as the literature. Academic publishing is the process of placing the results of one's research into the...

. Systematic exploration of chemical space is possible by creating in silico

In silico

In silico is an expression used to mean "performed on computer or via computer simulation." The phrase was coined in 1989 as an analogy to the Latin phrases in vivo and in vitro which are commonly used in biology and refer to experiments done in living organisms and outside of living organisms,...

databases of virtual molecules.