In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure. Polarity underlies a number of physical properties including surface tension, solubility, and melting- and boiling-points.
Polarity of bonds
|In a molecule of hydrogen fluoride (HF), the more electronegative atom (fluoride) is shown in yellow. Because the electrons spend more time by the fluorine atom in the H-F bond, the red represents partially negatively charged regions.|
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. In which case can make a dipole-dipole intermolecular force. 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. Partial charges are denoted as δ+ (delta plus) and δ− (delta minus). These symbols were introduced by Christopher Ingold and his wife Hilda Usherwood in 1926.
Atoms with high electronegativities — such as fluorine, oxygen, and nitrogen — exert a greater pull on electrons than atoms with lower electronegativities. In a bond, this can lead to unequal sharing of electrons between the atoms, as electrons will be drawn closer to the atom with the higher electronegativity.
Bonds can fall between one of two extremes — being completely 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 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 "nonpolar" 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. On the Pauling scale, if the result is between 0.4 and 1.7 then, generally, the bond is polar covalent.
Pauling considered that the partial ionic character of a bond 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.
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 orbitals of different atoms. A molecule may be polar either as a result of polar bonds due to differences in electronegativity 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.
A polar molecule has a net dipole as a result of the opposing charges (i.e. having partial positive and partial negative charges) from polar bonds arranged asymmetrically. Water (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, polar molecules are generally able to dissolve in water. Another example includes sugars (like sucrose), which have many polar oxygen–hydrogen (-OH) groups and are overall highly polar.
The hydrogen fluoride, HF, molecule is polar by virtue of polar covalent bonds — in the covalent bond electrons are displaced towards the more electronegative fluorine atom. Ammonia, NH3, molecule the three N–H bonds have only a slight polarity (toward the more electronegative nitrogen atom). However, the molecule has two lone electrons in an orbital, that points towards the fourth apex of the approximate tetrahedron, (VSEPR). This orbital is not participating in covalent bonding; it is electron-rich, which results in a powerful dipole across the whole ammonia molecule.
In the ozone, O3, molecule 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 uneven — since 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 of +1, while the outer atoms each have a formal charge of −1/2). Since the molecule has a bent geometry, the result is a dipole across the whole ozone molecule.
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, the boron trifluoride molecule (BF3) has a trigonal planar arrangement of three polar bonds at 120°. This results in no overall dipole in the molecule.
Examples of household nonpolar compounds include fats, oil, and petrol/gasoline. Therefore (per the "oil and water" rule of thumb), most nonpolar molecules are water-insoluble (hydrophobic) at room temperature. However, many nonpolar organic solvents, such as turpentine, are able to dissolve polar substances. 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 causes greater attachment. The most common form of such an interaction is the hydrogen bond, which is also known as the H-bond.
In the methane molecule (CH4) the four C–H bonds are arranged tetrahedrally around the carbon atom. Each bond has polarity (though not very strong). However, the bonds are arranged symmetrically so there is no overall dipole in the molecule. The diatomic oxygen molecule (O2) does not have polarity in the covalent bond because of equal electronegativity, hence there is no polarity in the molecule.
Large molecules that have one end with polar groups attached and another end with nonpolar groups are good surfactants. They 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.
Phospholipids are effective natural surfactants that have important biological functions
Predicting molecule polarity
- This classification table gives a good general understanding of predicting molecular dipole of some general molecular structures. However, one should not interpret it literally:
|HAx||Molecules with a single H||HF|
|AxOH||Molecules with an OH at one end||C2H5OH|
|OxAy||Molecules with an O at one end||H2O|
|NxAy||Molecules with an N at one end||NH3|
|Nonpolar||A2||Diatomic molecules of the same element||O2|
|CxAy||Most carbon compounds||CO2|
- Determining the point group 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 quantities with magnitude and direction, and a two equal vectors who oppose each other will cancel out.
Any molecule with an centre of inversion ( "i" ) or a horizontal mirror plane ( "σh ") will not possess dipole moments. Likewise, a molecule with more than one Cn axis will not possess dipole moment because dipole moments cannot lie in more than one dimension. As a consequence of that constraint, all molecules with D symmetry (Schönflies notation) will, therefore, not have dipole moment because, by definition, D point groups have two or multiple Cn axis.
Since C1, Cs,C∞h Cn and Cnv point groups do not have a centre of inversion, horizontal mirror planes or multiple Cn axis, molecules in one of those point groups will have dipole moment.
- Polar point group
- Bond dipole moment
- Covalent bond
- Chemical bonding
- Thorium monoxide has the highest known internal electric field of any molecule.
- The Origin of the "Delta" Symbol for Fractional Charges Jensen, William B. J. Chem. Educ. 2009, 86, 545. Link
- L. Pauling The Nature of the Chemical Bond (3rd ed., Oxford University Press 1960) p.98-100.
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