thumb|A scanning tunneling microscopy
image of [[pentacene molecules, which consist of linear chains of five carbon rings.]]
A molecule is an electrically
neutral group of two or more atom
s held together by chemical bond
Molecules are distinguished from ion
s by their lack of electrical charge
In quantum physics
, organic chemistry
, and biochemistry
, the distinction from ions is dropped and ''molecule'' is often used when referring to polyatomic ion
In the kinetic theory of gases
, the term ''molecule'' is often used for any gaseous particle
regardless of its composition. This violates the definition that a molecule contain ''two or more'' atoms, since the noble gases
are individual atoms.
A molecule may be homonuclear
, that is, it consists of atoms of one chemical element
, as with two atoms in the oxygen
); or it may be heteronuclear
, a chemical compound
composed of more than one element, as with water
(two hydrogen atoms and one oxygen atom; H2
Atoms and complexes connected by non-covalent interactions
, such as hydrogen bond
s or ionic bond
s, are typically not considered single molecules.
Molecules as components of matter are common. They also make up most of the oceans and atmosphere. Most organic
substances are molecules. The substances of life are molecules, e.g. proteins, the amino acids they are made of, the nucleic acids (DNA & RNA), sugars, carbohydrates, fats, and vitamins. The nutrient minerals ordinarily are not molecules, e.g. iron sulfate.
However, the majority of familiar solid substances on Earth are not made of molecules. These include all of the minerals that make up the substance of the Earth, soil, dirt, sand, clay, pebbles, rocks, boulders, bedrock
, the molten interior
, and the core of the Earth
. All of these contain many chemical bonds, but are ''not'' made of identifiable molecules.
No typical molecule can be defined for salts
nor for covalent crystals
, although these are often composed of repeating unit cell
s that extend either in a plane
, e.g. graphene
; or three-dimensionally e.g. diamond
, sodium chloride
. The theme of repeated unit-cellular-structure also holds for most metals which are condensed phases with metallic bond
ing. Thus solid metals are not made of molecules.
es, which are solids that exist in a vitreous disordered state, the atoms are held together by chemical bonds with no presence of any definable molecule, nor any of the regularity of repeating unit-cellular-structure that characterizes salts, covalent crystals, and metals.
The science of molecules is called ''molecular chemistry'' or ''molecular physics
'', depending on whether the focus is on chemistry or physics. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of chemical bond
s, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. In molecular sciences, a molecule consists of a stable system (bound state
) composed of two or more atom
s. Polyatomic ion
s may sometimes be usefully thought of as electrically charged molecules. The term ''unstable molecule'' is used for very reactive
species, i.e., short-lived assemblies (resonances
) of electrons and nuclei
, such as radicals
, molecular ion
s, Rydberg molecule
s, transition state
s, van der Waals complexes
, or systems of colliding atoms as in Bose–Einstein condensate
History and etymology
According to Merriam-Webster
and the Online Etymology Dictionary
, the word "molecule" derives from the Latin
" or small unit of mass.
* Molecule (1794) – "extremely minute particle", from French ' (1678), from New Latin
', diminutive of Latin ' "mass, barrier". A vague meaning at first; the vogue for the word (used until the late 18th century only in Latin form) can be traced to the philosophy of Descartes
The definition of the molecule has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise, defining molecules as the smallest particles
of pure chemical substance
s that still retain their composition
and chemical properties. This definition often breaks down since many substances in ordinary experience, such as rocks
, and metal
s, are composed of large crystalline networks of chemically bonded
atoms or ion
s, but are not made of discrete molecules.
Molecules are held together by either covalent bonding
or ionic bonding
. Several types of non-metal elements exist only as molecules in the environment. For example, hydrogen only exists as hydrogen molecule. A molecule of a compound is made out of two or more elements.
A homonuclear molecule
is made out of two or more atoms of a single element.
While some people say a metallic crystal can be considered a single giant molecule held together by metallic bonding
others point out that metals act very differently than molecules.
A covalent bond is a chemical bond
that involves the sharing of electron pair
s between atom
s. These electron pairs are termed ''shared pairs'' or ''bonding pairs'', and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is termed ''covalent bonding''.
Ionic bonding is a type of chemical bond
that involves the electrostatic
attraction between oppositely charged ion
s, and is the primary interaction occurring in ionic compound
s. The ions are atoms that have lost one or more electron
s (termed cation
s) and atoms that have gained one or more electrons (termed anion
s). This transfer of electrons is termed ''electrovalence'' in contrast to covalence
. In the simplest case, the cation is a metal
atom and the anion is a nonmetal
atom, but these ions can be of a more complicated nature, e.g. molecular ions like NH4+
At normal temperatures and pressures, ionic bonding mostly creates solids (or occasionally liquids) without separate identifiable molecules, but the vaporization/sublimation such materials does produce small separate molecules where electrons are still transferred fully enough for the bonds to be considered ionic rather than covalent.
Most molecules are far too small to be seen with the naked eye, although molecules of many polymer
s can reach macroscopic
sizes, including biopolymer
s such as DNA
. Molecules commonly used as building blocks for organic synthesis have a dimension of a few angstrom
s (Å) to several dozen Å, or around one billionth of a meter. Single molecules cannot usually be observed by light
(as noted above), but small molecules and even the outlines of individual atoms may be traced in some circumstances by use of an atomic force microscope
. Some of the largest molecules are macromolecule
s or supermolecule
The smallest molecule is the diatomic hydrogen
), with a bond length of 0.74 Å.
Effective molecular radius is the size a molecule displays in solution.
The table of permselectivity for different substances
Chemical formula types
The chemical formula
for a molecule uses one line of chemical element
symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, and ''plus'' (+) and ''minus'' (−) signs. These are limited to one typographic line of symbols, which may include subscripts and superscripts.
A compound's empirical formula
is a very simple type of chemical formula. It is the simplest integer ratio
of the chemical element
s that constitute it. For example, water is always composed of a 2:1 ratio of hydrogen
atoms, and ethanol
(ethyl alcohol) is always composed of carbon
, and oxygen
in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely – dimethyl ether
has the same ratios as ethanol, for instance. Molecules with the same atom
s in different arrangements are called isomer
s. Also carbohydrates, for example, have the same ratio (carbon:hydrogen:oxygen= 1:2:1) (and thus the same empirical formula) but different total numbers of atoms in the molecule.
The molecular formula
reflects the exact number of atoms that compose the molecule and so characterizes different molecules. However different isomers can have the same atomic composition while being different molecules.
The empirical formula is often the same as the molecular formula but not always. For example, the molecule acetylene
has molecular formula C2
, but the simplest integer ratio of elements is CH.
The molecular mass
can be calculated from the chemical formula
and is expressed in conventional atomic mass unit
s equal to 1/12 of the mass of a neutral carbon-12 (12C isotope
) atom. For network solid
s, the term formula unit
is used in stoichiometric
For molecules with a complicated 3-dimensional structure, especially involving atoms bonded to four different substituents, a simple molecular formula or even semi-structural chemical formula
may not be enough to completely specify the molecule. In this case, a graphical type of formula called a structural formula
may be needed. Structural formulas may in turn be represented with a one-dimensional chemical name, but such chemical nomenclature
requires many words and terms which are not part of chemical formulas.
Molecules have fixed equilibrium
geometries—bond lengths and angles— about which they continuously oscillate through vibrational and rotational motions. A pure substance is composed of molecules with the same average geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity
share a chemical formula but normally have very different properties because of their different structures. Stereoisomer
s, a particular type of isomer, may have very similar physico-chemical properties and at the same time different biochemical
Molecular spectroscopy deals with the response (spectrum
) of molecules interacting with probing signals of known energy
, according to Planck's formula
). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through absorbance
Spectroscopy does not generally refer to diffraction
studies where particles such as neutron
s, or high energy X-ray
s interact with a regular arrangement of molecules (as in a crystal).
commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space. Infrared spectroscopy
measures the vibration of molecules, including stretching, bending or twisting motions. It is commonly used to identify the kinds of bonds or functional group
s in molecules. Changes in the arrangements of electrons yield absorption or emission lines in ultraviolet, visible or near infrared
light, and result in colour. Nuclear resonance spectroscopy measures the environment of particular nuclei in the molecule, and can be used to characterise the numbers of atoms in different positions in a molecule.
The study of molecules by molecular physics
and theoretical chemistry
is largely based on quantum mechanic
s and is essential for the understanding of the chemical bond
. The simplest of molecules is the hydrogen molecule-ion
, and the simplest of all the chemical bonds is the one-electron bond
is composed of two positively charged proton
s and one negatively charged electron
, which means that the Schrödinger equation
for the system can be solved more easily due to the lack of electron–electron repulsion. With the development of fast digital computers, approximate solutions for more complicated molecules became possible and are one of the main aspects of computational chemistry
When trying to define rigorously whether an arrangement of atoms is ''sufficiently stable'' to be considered a molecule, IUPAC suggests that it "must correspond to a depression on the potential energy surface
that is deep enough to confine at least one vibrational state".
This definition does not depend on the nature of the interaction between the atoms, but only on the strength of the interaction. In fact, it includes weakly bound species that would not traditionally be considered molecules, such as the helium dimer
, which has one vibrational bound state
and is so loosely bound that it is only likely to be observed at very low temperatures.
Whether or not an arrangement of atoms is ''sufficiently stable'' to be considered a molecule is inherently an operational definition. Philosophically, therefore, a molecule is not a fundamental entity (in contrast, for instance, to an elementary particle
); rather, the concept of a molecule is the chemist's way of making a useful statement about the strengths of atomic-scale interactions in the world that we observe.
* Chemical polarity
* Covalent bond
* Diatomic molecule
* List of compounds
* List of interstellar and circumstellar molecules
* Molecular biology
* Molecular design software
* Molecular engineering
* Molecular geometry
* Molecular Hamiltonian
* Molecular ion
* Molecular modelling
* Molecular promiscuity
* Molecular orbital
* Non-covalent bonding
* Periodic systems of small molecules
* Small molecule
* Comparison of software for molecular mechanics modeling
* Van der Waals molecule
* World Wide Molecular Matrix
Molecule of the MonthSchool of Chemistry, University of Bristol