A Molecule /ˈmɒlɪkjuːl/ 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 despite being composed of a single non-bonded
atom.
A molecule may be homonuclear,
that is, it consists of atoms of a single chemical
element, as with oxygen (O2); or it may be a chemical
compound composed of more than one element,
as with water (H2O). Atoms and complexes
connected by non-covalent bonds such as hydrogen
bonds or ionic bonds are generally not considered single
molecules.
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 without presence of any
definable molecule, but also without any of the regularity of repeating units
that characterises crystals.
Molecular science
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
John Dalton
According to 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 Fr. molécule (1678), from modern Latin. molecula, diminutive of Latin moles "mass, barrier". A vague meaning at first; the vogue for the word (used until 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.
Molecular size
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. 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 supermolecules.
Smallest molecule diameter
Largest molecule diameter
Mesoporous silica have been produced with a
diameter of 1000 Å (100 nm)
Radius
Effective molecular radius is the size a molecule
displays in solution. The table of
permselectivity for different substances contains examples.
Formulas for molecules
Chemical formula types
The chemical
formula for a molecule uses a single 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 a single 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 different molecules.
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.
Structural formula
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.
Molecular geometry
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 isomers, may have
very similar physico-chemical properties and at the same time different biochemical
activities.
Molecular spectroscopy
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).
Theoretical aspects
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 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, 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.
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