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chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, properties ...
, polarity is a separation of
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electron ...
leading to a
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
or its chemical groups having an
electric dipole moment The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, that is, a measure of the system's overall Chemical polarity, polarity. The International System of Units, SI unit for electric ...
, with a negatively charged end and a positively charged end. Polar molecules must contain one or more polar bonds due to a difference in
electronegativity Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...
between the bonded atoms. Molecules containing polar bonds have no molecular polarity if the bond dipoles cancel each other out by symmetry. Polar molecules interact through dipole–dipole
intermolecular force An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the Electromagnetism, electromagnetic forces of attraction or repulsion which act between atoms and other types of neighbouring ...
s and
hydrogen bonds In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, str ...
. Polarity underlies a number of physical properties including
surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...
,
solubility In chemistry, solubility is the ability of a chemical substance, substance, the solute, to form a solution (chemistry), solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form su ...
, and melting and boiling points.


Polarity of bonds

Not all atoms attract electrons with the same force. The amount of "pull" an atom exerts on its electrons is called its
electronegativity Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...
. Atoms with high electronegativitiessuch as
fluorine Fluorine is a chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that species. Unlike chemical c ...
,
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
, and
nitrogen Nitrogen is the chemical element with the Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the pnictogens. It is a common element in the ...
exert a greater pull on electrons than atoms with lower electronegativities such as
alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...
s and
alkaline earth metal The alkaline earth metals are six chemical element A chemical element is a species of atoms that have a given number of protons in their atomic nucleus, nuclei, including the pure Chemical substance, substance consisting only of that spec ...
s. In a bond, this leads to unequal sharing of electrons between the atoms, as electrons will be drawn closer to the atom with the higher electronegativity. Because electrons have a negative charge, the unequal sharing of electrons within a bond leads to the formation of an electric dipole: a separation of positive and negative electric charge. Because the amount of charge separated in such dipoles is usually smaller than a
fundamental charge The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
, they are called
partial charges A partial charge is a non-integer charge value when measured in electric charge, elementary charge units. Partial charge is more commonly called net atomic charge. It is represented by the Greek lowercase letter 𝛿, namely 𝛿− or 𝛿+. Parti ...
, denoted as δ+ ( delta plus) and δ− (delta minus). These symbols were introduced by Sir Christopher Ingold and Dr. Edith Hilda (Usherwood) Ingold in 1926. The bond dipole moment is calculated by multiplying the amount of charge separated and the distance between the charges. These dipoles within molecules can interact with dipoles in other molecules, creating dipole-dipole intermolecular forces.


Classification

Bonds can fall between one of two extremescompletely nonpolar or completely polar. A completely nonpolar bond occurs when the electronegativities are identical and therefore possess a difference of zero. A completely polar bond is more correctly called an
ionic bond Ionic bonding is a type of chemical bonding that involves the Coulomb's law, electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ion ...
, and occurs when the difference between electronegativities is large enough that one atom actually takes an electron from the other. The terms "polar" and "nonpolar" are usually applied to
covalent bond A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of Molecule, molecules and crystals. The bond may result from the Coulomb's law, electrostatic force between oppos ...
s, that is, bonds where the polarity is not complete. To determine the polarity of a covalent bond using numerical means, the difference between the electronegativity of the atoms is used. Bond polarity is typically divided into three groups that are loosely based on the difference in electronegativity between the two bonded atoms. According to the
Pauling scale Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...
: * ''Nonpolar bonds'' generally occur when the difference in
electronegativity Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...
between the two atoms is less than 0.5 * ''Polar bonds'' generally occur when the difference in electronegativity between the two atoms is roughly between 0.5 and 2.0 * ''
Ionic bonds Ionic bonding is a type of chemical bonding that involves the Coulomb's law, electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ion ...
'' generally occur when the difference in electronegativity between the two atoms is greater than 2.0 Pauling based this classification scheme on the ''partial ionic character'' of a bond, which is an approximate function of the difference in electronegativity between the two bonded atoms. He estimated that a difference of 1.7 corresponds to 50% ionic character, so that a greater difference corresponds to a bond which is predominantly ionic. As a quantum-mechanical description, Pauling proposed that the
wave function A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex number, complex-valued probability amplitude, and the probabilities for the possible results of ...
for a polar molecule AB is a linear combination of wave functions for covalent and ionic molecules: ψ = aψ(A:B) + bψ(A+B). The amount of covalent and ionic character depends on the values of the squared coefficients a2 and b2.


Bond dipole moments

The bond dipole moment uses the idea of
electric dipole moment The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, that is, a measure of the system's overall Chemical polarity, polarity. The International System of Units, SI unit for electric ...
to measure the polarity of a chemical bond within a
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
. It occurs whenever there is a separation of positive and negative charges. The bond dipole μ is given by: :\mu = \delta \, d. The bond dipole is modeled as δ+ — δ with a distance ''d'' between the
partial charges A partial charge is a non-integer charge value when measured in electric charge, elementary charge units. Partial charge is more commonly called net atomic charge. It is represented by the Greek lowercase letter 𝛿, namely 𝛿− or 𝛿+. Parti ...
δ+ and δ. It is a vector, parallel to the bond axis, pointing from minus to plus, as is conventional for
electric dipole moment The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, that is, a measure of the system's overall Chemical polarity, polarity. The International System of Units, SI unit for electric ...
vectors. Chemists often draw the vector pointing from plus to minus. This vector can be physically interpreted as the movement undergone by electrons when the two atoms are placed a distance ''d'' apart and allowed to interact, the electrons will move from their free state positions to be localised more around the more
electronegative Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...
atom. The
SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms and initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wid ...
for electric dipole moment is the coulomb–meter. This is too large to be practical on the molecular scale. Bond dipole moments are commonly measured in
debye The debye (symbol: D) (; ) is a CGS unit (a non-SI metric system, metric unit) of electric dipole momentTwo equal and opposite charges separated by some distance constitute an electric dipole. This dipole possesses an electric dipole moment whose ...
s, represented by the symbol D, which is obtained by measuring the charge \delta in units of 10−10
statcoulomb The franklin (Fr) or statcoulomb (statC) electrostatic unit of charge (esu) is the units of measurement, physical unit for electrical charge used in the Centimetre–gram–second system of units#Electrostatic units (ESU), cgs-esu and Gaussian uni ...
and the distance ''d'' in
Angstrom The angstromEntry "angstrom" in the Oxford online dictionary. Retrieved on 2019-03-02 from https://en.oxforddictionaries.com/definition/angstrom.Entry "angstrom" in the Merriam-Webster online dictionary. Retrieved on 2019-03-02 from https://www.m ...
s. Based on the conversion factor of 10−10 statcoulomb being 0.208 units of elementary charge, so 1.0 debye results from an electron and a proton separated by 0.208 Å. A useful conversion factor is 1 D = 3.335 64 C m. For diatomic molecules there is only one (single or multiple) bond so the bond dipole moment is the molecular dipole moment, with typical values in the range of 0 to 11 D. At one extreme, a symmetrical molecule such as
chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...
, , has zero dipole moment, while near the other extreme, gas phase
potassium bromide Potassium bromide (Potassium, KBromine, Br) is a salt (chemistry), salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the US. Its action is due to the b ...
, KBr, which is highly ionic, has a dipole moment of 10.41 D. For polyatomic molecules, there is more than one bond. The total
molecular dipole moment In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
may be approximated as the
vector sum In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...
of the individual bond dipole moments. Often bond dipoles are obtained by the reverse process: a known total dipole of a molecule can be decomposed into bond dipoles. This is done to transfer bond dipole moments to molecules that have the same bonds, but for which the total dipole moment is not yet known. The vector sum of the transferred bond dipoles gives an estimate for the total (unknown) dipole of the molecule.


Polarity of molecules

While the molecules can be described as "polar covalent", "nonpolar covalent", or "ionic", this is often a relative term, with one molecule simply being ''more polar'' or ''more nonpolar'' than another. However, the following properties are typical of such molecules. A molecule is composed of one or more chemical bonds between
molecular orbital In chemistry, a molecular orbital is a Function (mathematics), mathematical function describing the location and Matter wave, wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical propertie ...
s of different atoms. A molecule may be polar either as a result of polar bonds due to differences in
electronegativity Electronegativity, symbolized as ''Chi (letter), χ'', is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its ato ...
as described above, or as a result of an asymmetric arrangement of nonpolar covalent bonds and non-bonding pairs of electrons known as a full
molecular orbital In chemistry, a molecular orbital is a Function (mathematics), mathematical function describing the location and Matter wave, wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical propertie ...
.


Polar molecules

A polar molecule has a net
dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically.
Water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
(H2O) is an example of a polar molecule since it has a slight positive charge on one side and a slight negative charge on the other. The dipoles do not cancel out, resulting in a net dipole. Due to the polar nature of the water molecule itself, other polar molecules are generally able to dissolve in water. The dipole moment of water depends on its state. In the gas phase the dipole moment is ≈ 1.86
debye The debye (symbol: D) (; ) is a CGS unit (a non-SI metric system, metric unit) of electric dipole momentTwo equal and opposite charges separated by some distance constitute an electric dipole. This dipole possesses an electric dipole moment whose ...
(D), whereas liquid water (≈ 2.95 D) and ice (≈ 3.09 D) are higher due to differing hydrogen-bonded environments. Other examples include sugars (like
sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...
), which have many polar oxygen–hydrogen (−OH) groups and are overall highly polar. If the bond dipole moments of the molecule do not cancel, the molecule is polar. For example, the water molecule (H2O) contains two polar O−H bonds in a bent (nonlinear) geometry. The bond dipole moments do not cancel, so that the molecule forms a
molecular dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole moment, electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple ...
with its negative pole at the oxygen and its positive pole midway between the two hydrogen atoms. In the figure each bond joins the central O atom with a negative charge (red) to an H atom with a positive charge (blue). The
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an imp ...
, HF, molecule is polar by virtue of polar covalent bondsin the covalent bond electrons are displaced toward the more electronegative fluorine atom.
Ammonia Ammonia is an inorganic chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a dis ...
, NH3, is a molecule whose three N−H bonds have only a slight polarity (toward the more electronegative nitrogen atom). The molecule has two lone electrons in an orbital that points towards the fourth apex of an approximately regular tetrahedron, as predicted by the
VSEPR theory Valence shell electron pair repulsion (VSEPR) theory ( , ), is a model used in chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter ...
. This orbital is not participating in covalent bonding; it is electron-rich, which results in a powerful dipole across the whole ammonia molecule. In
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic Allotropy, allotrope , breaking down i ...
(O3) molecules, the two O−O bonds are nonpolar (there is no electronegativity difference between atoms of the same element). However, the distribution of other electrons is unevensince the central atom has to share electrons with two other atoms, but each of the outer atoms has to share electrons with only one other atom, the central atom is more deprived of electrons than the others (the central atom has a
formal charge In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, struct ...
of +1, while the outer atoms each have a formal charge of −). Since the molecule has a bent geometry, the result is a dipole across the whole ozone molecule. When comparing a polar and nonpolar molecule with similar molar masses, the polar molecule in general has a higher boiling point, because the dipole–dipole interaction between polar molecules results in stronger intermolecular attractions. One common form of polar interaction is the
hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily Electrostatics, electrostatic force of attraction between a hydrogen (H) atom which is Covalent bond, covalently bound to a more electronegativity, electronegative "donor" atom or group ( ...
, which is also known as the H-bond. For example, water forms H-bonds and has a molar mass M = 18 and a boiling point of +100 °C, compared to nonpolar
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
with M = 16 and a boiling point of –161 °C.


Nonpolar molecules

A molecule may be nonpolar either when there is an equal sharing of electrons between the two atoms of a diatomic molecule or because of the symmetrical arrangement of polar bonds in a more complex molecule. For example,
boron trifluoride Boron trifluoride is the inorganic compound with the formula BF3. This pungent, colourless, and toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds. Structure and b ...
(BF3) has a trigonal planar arrangement of three polar bonds at 120°. This results in no overall dipole in the molecule.
Carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
(CO2) has two polar C=O bonds, but the geometry of CO2 is linear so that the two bond dipole moments cancel and there is no net molecular dipole moment; the molecule is nonpolar. Examples of household nonpolar compounds include fats, oil, and petrol/gasoline. Most nonpolar molecules are water-insoluble (
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly intermolecular force, repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend t ...
) at room temperature. Many nonpolar
organic solvent A solvent (s) (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a ...
s, such as
turpentine Turpentine (which is also called spirit of turpentine, oil of turpentine, terebenthene, terebinthine and (colloquially) turps) is a fluid obtained by the distillation of resin harvested from living trees, mainly pines. Mainly used as a special ...
, are able to dissolve non-polar substances. In the
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
molecule (CH4) the four C−H bonds are arranged tetrahedrally around the carbon atom. Each bond has polarity (though not very strong). The bonds are arranged symmetrically so there is no overall dipole in the molecule. The diatomic
oxygen Oxygen is the chemical element with the chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen Group (periodic table), group in the periodic table, a highly Chemical reaction, reactive nonmetal, and an oxidizing a ...
molecule (O2) does not have polarity in the covalent bond because of equal electronegativity, hence there is no polarity in the molecule.


Amphiphilic molecules

Large molecules that have one end with polar groups attached and another end with nonpolar groups are described as
amphiphile An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
s or ''amphiphilic'' molecules. They are good
surfactant Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsion#Emulsifiers , ...
s and can aid in the formation of stable emulsions, or blends, of water and fats. Surfactants reduce the interfacial tension between oil and water by adsorbing at the liquid–liquid interface. File:CHCA cleavable detergent.png, This amphiphilic molecule has several polar groups (
hydrophilic A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clar ...
, ''water-loving'') on the right side and a long nonpolar chain (
lipophilic Lipophilicity (from Greek λίπος "fat" and φίλος "friendly"), refers to the ability of a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) c ...
, ''fat-loving'') at the left side. This gives it
surfactant Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsion#Emulsifiers , ...
properties File:Micelle scheme-en.svg, A
micelle A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloid, colloidal suspension (also known as associat ...
the
lipophilic Lipophilicity (from Greek λίπος "fat" and φίλος "friendly"), refers to the ability of a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) c ...
ends of the surfactant molecules dissolve in the oil, while the
hydrophilic A hydrophile is a molecule or other molecular entity that is intermolecular force, attracted to water molecules and tends to be dissolution (chemistry), dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clar ...
charged ends remain outside in the water phase, shielding the rest of the
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly intermolecular force, repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend t ...
micelle. In this way, the small oil droplet becomes water-soluble. File:Phospholipid schematic representation.png,
Phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophile, hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an Alcohol (chemistry), alcohol residue (usually a glycerol mol ...
s are effective natural surfactants that have important biological functions File:Phospholipids aqueous solution structures.svg, Cross section view of the structures that can be formed by
phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophile, hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an Alcohol (chemistry), alcohol residue (usually a glycerol mol ...
s. They can form a
micelle A micelle () or micella () (plural micelles or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloid, colloidal suspension (also known as associat ...
and are vital in forming
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the cytoplasm, interior of all Cell (biology), cells from th ...
s


Predicting molecule polarity

Determining the
point group In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...
is a useful way to predict polarity of a molecule. In general, a molecule will not possess dipole moment if the individual bond dipole moments of the molecule cancel each other out. This is because dipole moments are
euclidean vector In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...
quantities with magnitude and direction, and a two equal vectors who oppose each other will cancel out. Any molecule with a centre of inversion ("i") or a horizontal mirror plane ("σh") will not possess dipole moments. Likewise, a molecule with more than one C''n'' axis of rotation will not possess a dipole moment because dipole moments cannot lie in more than one
dimension In physics and mathematics, the dimension of a Space (mathematics), mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any Point (geometry), point within it. Thus, a Line (geometry), lin ...
. As a consequence of that constraint, all molecules with
dihedral symmetry In mathematics, a dihedral group is the group (mathematics), group of symmetry, symmetries of a regular polygon, which includes rotational symmetry, rotations and reflection symmetry, reflections. Dihedral groups are among the simplest example ...
(D''n'') will not have a dipole moment because, by definition, D point groups have two or multiple C''n'' axes. Since C1, Cs,C∞h C''n'' and C''n''v
point groups In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...
do not have a centre of inversion, horizontal mirror planes or multiple C''n'' axis, molecules in one of those point groups will have dipole moment.


Electrical deflection of water

Contrary to popular misconception, the electrical deflection of a stream of water from a charged object is not based on polarity. The deflection occurs because of electrically charged droplets in the stream, which the charged object induces. A stream of water can also be deflected in a uniform electrical field, which cannot exert force on polar molecules. Additionally, after a stream of water is grounded, it can no longer be deflected. Weak deflection is even possible for nonpolar liquids.


See also

*
Chemical properties A chemical property is any of a material's properties that becomes evident during, or after, a chemical reaction; that is, any quality that can be established only by changing a substance's chemical substance, chemical identity.William L. Masterto ...
*
Colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others ext ...
*
Detergent A detergent is a surfactant Surfactants are chemical compounds that decrease the surface tension between two liquids, between a gas and a liquid, or interfacial tension between a liquid and a solid. Surfactants may act as detergents, wettin ...
*
Electronegativities of the elements (data page) Electronegativity (Pauling scale) Notes * Separate values for each source are only given where one or more sources differ. * Electronegativity is not a uniquely defined property and may depend on the definition. The suggested values are all ...
*
Polar point group In geometry, a polar point group is a point group In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in co ...


References


External links


Chemical Bonding



Molecule Polarity
{{chemical bonds Physical chemistry Chemical properties Dimensionless numbers of chemistry