A molecule is an electrically neutral group of two or more atoms held
together by chemical bonds. Molecules are distinguished
from ions by their lack of electrical charge. However, in quantum
physics, organic chemistry, and biochemistry, the term molecule is
often used less strictly, also being applied to polyatomic ions.
In the kinetic theory of gases, the term molecule is often used for
any gaseous particle regardless of its composition. According to this
definition, noble gas atoms are considered molecules as they are
A molecule may be homonuclear, that is, it consists of atoms of one
chemical element, as with oxygen (O2); or it may be heteronuclear, a
chemical compound composed of more than one element, as with water
(H2O). Atoms and complexes connected by non-covalent interactions,
such as hydrogen bonds or ionic bonds, are generally not considered
Molecules as components of matter are common in organic substances
(and therefore biochemistry). They also make up most of the oceans and
atmosphere. However, the majority of familiar solid substances on
Earth, including most of the minerals that make up the crust, mantle,
and core of the Earth, contain many chemical bonds, but are not made
of identifiable molecules. Also, no typical molecule can be defined
for ionic crystals (salts) and covalent crystals (network solids),
although these are often composed of repeating unit cells that extend
either in a plane (such as in graphene) or three-dimensionally (such
as in diamond, quartz, or sodium chloride). The theme of repeated
unit-cellular-structure also holds for most condensed phases with
metallic bonding, which means that solid metals are also not made of
molecules. In glasses (solids that exist in a vitreous disordered
state), atoms may also be held together by chemical bonds with no
presence of any definable molecule, nor any of the regularity of
repeating units that characterizes crystals.
1 Molecular science
2 History and etymology
4 Molecular size
5 Molecular formulas
Chemical formula types
5.2 Structural formula
6 Molecular geometry
7 Molecular spectroscopy
8 Theoretical aspects
9 See also
11 External links
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 bonds, 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 atoms. Polyatomic ions
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 ions, Rydberg molecules,
transition states, van der Waals complexes, or systems of colliding
atoms as in Bose–Einstein condensate.
History and etymology
Main article: History of molecular theory
Merriam-Webster and the Online Etymology Dictionary, the
word "molecule" derives from the
Latin "moles" or small unit of mass.
Molecule (1794) – "extremely minute particle", from French
molécule (1678), from New
Latin molecula, diminutive of
"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
substances that still retain their composition and chemical
properties. This definition often breaks down since many
substances in ordinary experience, such as rocks, salts, and metals,
are composed of large crystalline networks of chemically bonded atoms
or ions, 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
Main article: Covalent bonding
A covalent bond forming H2 (right) where two hydrogen atoms share the
A covalent bond is a chemical bond that involves the sharing of
electron pairs between atoms. 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
Main article: Ionic bonding
Sodium and fluorine undergoing a redox reaction to form sodium
Sodium loses its outer electron to give it a stable electron
configuration, and this electron enters the fluorine atom
Ionic bonding is a type of chemical bond that involves the
electrostatic attraction between oppositely charged ions, and is the
primary interaction occurring in ionic compounds. The ions are atoms
that have lost one or more electrons (termed cations) and atoms that
have gained one or more electrons (termed anions). 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+ or SO42−. Basically, an ionic bond is the
transfer of electrons from a metal to a non-metal for both atoms to
obtain a full valence shell.
Most molecules are far too small to be seen with the naked eye, but
there are exceptions. DNA, a macromolecule, can reach macroscopic
sizes, as can molecules of many polymers. Molecules commonly used as
building blocks for organic synthesis have a dimension of a few
angstroms (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 macromolecules or
The smallest molecule is the diatomic hydrogen (H2), 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 contains examples.
Chemical formula types
Main article: Chemical formula
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 elements
that constitute it. For example, water is always composed of a 2:1
ratio of hydrogen to oxygen atoms, and ethyl alcohol or ethanol is
always composed of carbon, hydrogen, 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 atoms in different arrangements are
called isomers. 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
The empirical formula is often the same as the molecular formula but
not always. For example, the molecule acetylene has molecular formula
C2H2, 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 units equal to 1/12 of the mass
of a neutral carbon-12 (12C isotope) atom. For network solids, the
term formula unit is used in stoichiometric calculations.
Main article: Structural formula
3D (left and center) and 2D (right) representations of the terpenoid
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.
Main article: Molecular geometry
Structure and STM image of a "cyanostar" dendrimer molecule.
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.
Isomers share a chemical
formula but normally have very different properties because of their
different structures. Stereoisomers, a particular type of isomer, may
have very similar physico-chemical properties and at the same time
different biochemical activities.
Main article: Spectroscopy
Hydrogen can be removed from individual H2TPP molecules by applying
excess voltage to the tip of a scanning tunneling microscope (STM, a);
this removal alters the current-voltage (I-V) curves of TPP molecules,
measured using the same STM tip, from diode like (red curve in b) to
resistor like (green curve). Image (c) shows a row of TPP, H2TPP and
TPP molecules. While scanning image (d), excess voltage was applied to
H2TPP at the black dot, which instantly removed hydrogen, as shown in
the bottom part of (d) and in the rescan image (e). Such manipulations
can be used in single-molecule electronics.
Molecular spectroscopy deals with the response (spectrum) of molecules
interacting with probing signals of known energy (or frequency,
according to Planck's formula). Molecules have quantized energy levels
that can be analyzed by detecting the molecule's energy exchange
through absorbance or emission.
Spectroscopy does not generally
refer to diffraction studies where particles such as neutrons,
electrons, or high energy X-rays interact with a regular arrangement
of molecules (as in a crystal).
Microwave spectroscopy commonly measures changes in the rotation of
molecules, and can be used to identify molecules in outer space.
Infrared spectroscopy measures changes in vibration of molecules,
including stretching, bending or twisting motions. It is commonly used
to identify the kinds of bonds or functional groups 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 actually measures the
environment of particular nuclei in the molecule, and can be used to
characterise the numbers of atoms in different positions in a
The study of molecules by molecular physics and theoretical chemistry
is largely based on quantum mechanics and is essential for the
understanding of the chemical bond. The simplest of molecules is the
hydrogen molecule-ion, H2+, and the simplest of all the chemical bonds
is the one-electron bond. H2+ is composed of two positively charged
protons 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
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, He2, 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.
List of compounds
List of interstellar and circumstellar molecules
Molecular design software
Periodic systems of small molecules
Comparison of software for molecular mechanics modeling
Van der Waals molecule
World Wide Molecular Matrix
Molecular and cell biology portal
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