Polar molecule

Information about Polar molecule

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A commonly-used example of a polar compound is water (H2O). The electrons of water's hydrogen atoms are strongly attracted to the oxygen atom, and are actually closer to oxygen's nucleus than to the hydrogen nuclei; thus, water has a relatively strong negative charge in the middle (red shade), and a positive charge at the ends (blue shade).


Chemical polarity, also known as bond polarity or simply polarity, is a concept in chemistry which describes how equally bonding electrons are shared between atoms. It is a physical property of compounds and affects other physical properties such as solubility, melting points and boiling points. Polarity also affects intermolecular forces, leading to some compounds or molecules within compounds being labelled as polar or non-polar.

Polarity refers to the dipole-dipole intermolecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. For example, water is thought to be polar because of the uneven sharing of its electrons. However, methane is considered non-polar because the carbon shares the hydrogen molecules uniformally.

Theory

Diagram showing the net effect of symmetrical polar bonds (direction of yellow arrows show the migration of electrons) within boron trifluoride cancelling out to give a net polarity of zero. δ- shows an increase in negative charge and δ+ shows an increase in positive charge.


Electrons are not always shared equally between two bonding atoms: one atom might exert more of a force on the electron cloud than the other. This "pull" is termed electronegativity and measures the attraction for electrons a particular atom has. The unequal sharing of electrons within a bond leads to the formation of an electric dipole: a separation of positive and negative electric charge.

Atoms with high electronegativities - such as fluorine, oxygen, and nitrogen - exert a greater pull on electrons than atoms with lower electronegativities. In a bonding situation this can lead to unequal sharing of electrons between atoms as electrons will spend more time closer to the atom with the higher electronegativity.

Bonds can fall between one of two extremes - being completely non-polar or completely polar. A completely non-polar bond occurs when the electronegativities are identical and therefore possess a difference of zero. A completely polar bond is more correctly termed ionic bonding and occurs when the difference between electronegativities is large enough that one atom takes an electron from the other. The terms "polar" and "non-polar" bonds usually refer to covalent bonds. To determine the polarity of a covalent bond using numerical means, the difference between the electronegativity of the atoms is taken. If the result is between 0.5 and 2 then, generally, the bond is polar.

Polarity of molecules

A compound is composed of one or more chemical bonds between different atoms. The polarity of each bond within the compound may determine the overall polarity of the compound: how polar or non-polar it is.

A polar molecule may be polar as a result of polar bonds or as a result of an asymmetric arrangement of non-polar bonds and non bonding pairs of electrons.

Example 1. A polar molecule by virtue of polar bonds (bonds which have unequal sharing of electrons between the two atoms involved in bonding), e.g hydrogen fluoride, HF, where the bonding pair of electron is displaced towards the more electronegative fluorine atom.

Example 2. In ammonia, NH3, the three N-H bonds have only a slight polarity (towards the more electronegative nitrogen atom), however the lone pair of electrons (pointing towards the fourth apex of the approximate tetrahedron, (VSEPR) is electron rich and results in a powerful dipole across the whole ammonia molecule.

A non-polar compound may be non polar because there is (almost) no polarity in the bonds or because of the symmetrical arrangement of polar bonds.

Example 3. Methane, CH4 The four C-H bonds, arranged tetrahedrally around the carbon atom, has very little polarity in the bonds and so there is no dipole in the molecule.

Example 4. BF3, boron trifluoride has a trigonal planar arrangement of three polar bonds at 120o This results in no overall dipole in the molecule.

Properties and examples

While molecules can be described as "polar," "non-polar," or "semi-polar," it must be noted that this is often a relative term, with one molecule simply being more polar or more non-polar than another. As such, there are no ultimate properties which can be ascribed to polar or non-polar molecules. However, the following properties are typical of such molecules.

Polar molecules

Examples of household polar molecules include ammonia and sugar (glucose). Polar molecules are generally able to dissolve in water (hydrophilic) due to the polar nature of water. Polar molecules have slightly positive and slightly negatively charged ends.

Non-polar molecules

Examples of household non-polar compounds include fats, oil and petrol. Most non-polar molecules are water insoluble (hydrophobic) at room temperature. However many non-polar organic solvents, such as turpentine, are able to dissolve non-polar substances - like dissolves like.

When comparing a polar and non-polar molecule with similar molar masses, the polar molecule generally has a higher boiling point, because of the dipole-dipole interaction between their molecules.

Predicting molecule polarity

FormulaDescriptionExample
PolarABLinear Molecules CO
HAxMolecules with a single HHCl
AxOHMolecules with an OH at one endC2H5OH
OxAyMolecules with an O at one endH2O
NxAyMolecules with an N at one endNH3
NonpolarAx All elements O2
CxAyMost carbon compoundsCO2

Electronegativity difference predictions

Non-polar covalent bond: 0.0-0.4

Slightly polar bond: 0.5-0.9

Moderately polar bond: 1-1.3

Highly polar bond: 1.4-1.7

Slightly ionic bond: 1.8-2.2

Ionic Bond: 2.3+
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A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds.
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Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
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atom (Greek ἄτομος or átomos meaning "indivisible") is the smallest particle still characterizing a chemical element.
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Solubility is a physical property referring to the ability for a given substance, the solute, to dissolve in a solvent.[1] It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution.
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The melting point of a crystalline solid is the temperature range at which it changes state from solid to liquid. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds
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boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.[1][2][3][4]
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In physics, chemistry, and biology, intermolecular forces are forces that act between stable molecules or between functional groups of macromolecules. These non-covalent forces, which give rise to bonding energies of less than a few kcal/mol, are generally much weaker than the
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dipoles (Hellènic: di(s)- = twi- and pòla = pivot, hinge). An electric dipole is a separation of positive and negative charge. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some, usually small,
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In physics, chemistry, and biology, intermolecular forces are forces that act between stable molecules or between functional groups of macromolecules. These non-covalent forces, which give rise to bonding energies of less than a few kcal/mol, are generally much weaker than the
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Flavour in particle physics
 

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Electronegativity, symbol χ, is a chemical property which describes the power of an atom (or, more rarely, a functional group) to attract electrons towards itself.[1] First proposed by Linus Pauling in 1932 as a development of valence bond theory,[2]
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Asymmetry is the absence of, or a violation of, a symmetry.

Asymmetry in organisms

Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension.
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Water (H2O, HOH) is the most abundant molecule on Earth's surface, composing of about 70% of the Earth's surface as liquid and solid state in addition to being found in the atmosphere as a vapor.
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Electron

Theoretical estimates of the electron density for the first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density
Composition: Elementary particle
Family: Fermion
Group: Lepton
Generation: First
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Methane is a chemical compound with the molecular formula CH4. It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.
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4, 2
(mildly acidic oxide)
Electronegativity 2.55 (Pauling scale)
Ionization energies
(more) 1st: 1086.5 kJmol−1
2nd: 2352.6 kJmol−1
3rd: 4620.5 kJmol−1

Atomic radius 70 pm
Atomic radius (calc.
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1, −1
(amphoteric oxide)
Electronegativity 2.20 (Pauling scale) More

Atomic radius 25 pm
Atomic radius (calc.) 53 pm
Covalent radius 37 pm
Van der Waals radius 120 pm
Miscellaneous

Thermal conductivity (300 K) 180.
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Electronegativity, symbol χ, is a chemical property which describes the power of an atom (or, more rarely, a functional group) to attract electrons towards itself.[1] First proposed by Linus Pauling in 1932 as a development of valence bond theory,[2]
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dipoles (Hellènic: di(s)- = twi- and pòla = pivot, hinge). An electric dipole is a separation of positive and negative charge. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some, usually small,
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100% F is stable with 10 neutrons
References
This box:  •

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2, −1
(neutral oxide)
Electronegativity 3.44 (Pauling scale)
Ionization energies
(more) 1st: 1313.9 kJmol−1
2nd: 3388.3 kJmol−1
3rd: 5300.5 kJmol−1

Atomic radius 60 pm
Atomic radius (calc.
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3, 5, 4, 2
(strongly acidic oxide)
Electronegativity 3.04 (Pauling scale)
Ionization energies
(more) 1st: 1402.3 kJmol−1
2nd: 2856 kJmol−1
3rd: 4578.1 kJmol−1

Atomic radius 65 pm
Atomic radius (calc.
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ionic bond (or electrovalent bond) is a type of chemical bond based on electrostatic forces between two oppositely-charged ions. In ionic bond formation, a metal donates an electron, due to a low electronegativity, to form a positive ion or cation.
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Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds.
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Ammonia is a compound with the formula NH3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of the planet as a precursor to foodstuffs and fertilizers.
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Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. The living cell uses it as a source of energy and metabolic intermediate.
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Hydrophile, from the Greek (hydros) "water" and φιλια (philia) "friendship," refers to a physical property of a molecule that can transiently bond with water (H2O) through hydrogen bonding.
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hydrophobicity (from the combining form of water in Attic Greek hydro- and for fear phobos) refers to the physical property of a molecule (known as a hydrophobe) that is repelled from a mass of water [1].
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A solvent is a liquid that dissolves a solid, liquid, or gaseous solute, resulting in a solution. The most common solvent in everyday life is water. Most other commonly-used solvents are organic (carbon-containing) chemicals. These are called organic solvents.
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