How to know if compound is polar or nonpolar?
Polarity is an essential factor in controlling the physicochemical properties of a substance. Some chemical substances are inert in nature while others react vigorously. Some dissolve in water while some stay insoluble.
Similarly, water and oil do not mix with each other. Why is all that so? Well, the answers to all these questions and many more are somewhat hidden in determining whether a molecule is polar or nonpolar in nature.
The electron cloud distribution around a molecule affects its polarity which in turn controls a diverse range of its properties.
Thus, considering the importance of the ‘polarity concept’ this article is written to help you in understanding everything there is to know about polar versus non-polar molecules.
How to tell if a molecule is polar or non-polar?
Polarity determines the charge separation present in a molecule. The polarity of a molecule is controlled by its electronic cloud distribution.
A molecule is polar if it has an asymmetric charge distribution and a molecule is nonpolar if the charge is symmetrically spread over the molecule.
A bond is said to polar if bonded atoms have specific electronegativity difference.
Polar bonds have a particular dipole moment value. If the dipole moments of the polar bonds do not get canceled in the overall molecular arrangement, then the molecule is polar. Otherwise, if the dipole moments of individually polar bonds get canceled in the molecule overall then it is considered a non-polar molecule.
Difference between polar and non-polar molecules
All the major differences present between polar and non-polar molecules are summarized in the table given below.
|How do you know if the molecule is polar or nonpolar?|
|Made up of bonds containing dissimilar atoms||The bonds may consist of identical or dissimilar atoms|
|Constituent atoms have a difference in electronegativity||Constituent atoms may or may not have a difference in electronegativity|
|The asymmetric arrangement of atoms in the molecules||Symmetric arrangement of atoms in the molecule|
|Usually, an odd number of lone electron pairs present on the central atom in the molecule||The lone pair of electrons are either not present at all or are evenly distributed throughout the molecule|
|Overall charged electron cloud distribution is unbalanced in the molecule||The overall charged electron cloud distribution is balanced|
|The individual dipole moments do not get canceled in the molecule overall||The individual dipole moments if exist still get canceled in the overall molecule|
|Non-zero net dipole moment||Zero net dipole moment|
|Examples: CHCl3, NH3, H2O, SO2, HCN, etc.||Examples: H2, CO2, CCl4, BF3, SO3, etc.|
So, by the above discussion, we can infer that there are three main factors that control the polarity of covalently bonded molecules i.e.,
- Dipole moments
- Arrangement of atoms in the molecule/ molecular shape
Now, let us discuss each of the three factors one by one so that you can have a better idea of how each factor controls molecular polarity.
What factors determine whether a molecule is polar or non-polar?
Electronegativity is defined as the tendency of an atom to attract a shared pair of electrons from a covalent bond.
The electronegativity increases across a period while decreases down the group in the Periodic Table of elements. Fluorine (F) which lies at the far-right corner of the periodic table, at the top of Group VII A is the most electronegative element known so far.
According to Fajan’s rule, we know that most chemical bonds possess both ionic as well as covalent characteristics. Similarly, no covalent bond can be purely non-polar unless it is made up of two identical atoms such as H2, O2, or Cl2, etc. where there is no electronegativity difference between the bonded atoms. Contrarily, covalent bonds formed by two dissimilar atoms such as a C=O bond or a C-Cl bond are polar owing to the electronegativity difference between the concerned atoms.
According to Linus Pauling’s electronegativity scale, a covalent bond is actually considered polar if the electronegativity difference between the bonded atoms lies between 0.5 to 1.6 units. This is called bond polarity.
One of the two bonded atoms more strongly attracts the shared pair of electrons. It develops a partial negative charge due to this slight excess of electrons while the other atom gains a partial positive charge due to a slight electron deficiency. Oppositely charged poles develop in the molecule.
If the bonded atoms have an electronegativity difference of fewer than 0.5 units then the bond will be only weakly polar because none of the two atoms will be able to strongly attract the shared electron cloud towards itself.
Conversely, for an electronegativity difference greater than 1.6 units, complete transference of electrons occurs, and the bond changes from a polar covalent to an ionic bond.
How to know if a bond is polar or nonpolar using electronegativity?
|Electronegativity difference||Type of bond||Examples|
|< 0.5||Nonpolar||Br2, H2|
|0.5 to 1.6||Polar||NH3, H2O, etc.|
|> 1.6||Ionic||NaCl, MgO, etc.|
The dipole moment (μ) is a vector quantity. It represents both the magnitude (measured in Debye, D) as well as the direction of a polar bond or molecule.
Mathematically, μ is calculated as the product of electric charge (Q) and bond length (r).
How to know if molecule is polar or nonpolar based on dipole moment?
The dipole moment points from the positive pole to the negative pole of a bond or molecule. For example, a C-Cl bond is polar. There is a difference of 0.61 units between carbon (E.N=2.55) and chlorine (E.N=3.16). So, Cl develops a partial negative charge (Clδ-) while C gains a partial positive charge (Cδ+). Thus, the dipole moment of the C-Cl bond points from Cδ+ to Clδ- and it is called a polar covalent bond.
A molecule may possess polar bonds but still be non-polar. This is because of the third main factor i.e., the arrangement of atoms in the molecule formally known as molecular shape or geometry.
If the atoms present in a molecule are symmetrically arranged for e.g., in the tetrahedral shape of the CCl4 molecule or in the linear shape of CO2, the dipole moments of individually polar C-Cl or C=O bonds get canceled equally in opposite directions. Thus, the molecule becomes non-polar overall with net μ=0. The overall charge distribution gets balanced in the molecule.
In contrast to that, in a molecule in which the atoms are asymmetrically arranged, the overall charge distribution does not get canceled, and the dipole moments get canceled weakly or do not get canceled at all. Such a molecule is overall polar with net μ > 0. This is known as molecular polarity.
Examples of different polar and non-polar molecules
An example of a polar molecule with polar C-Cl bonds is chloroform (CHCl3 ).
There is a small electronegativity difference between C and H atoms in a C-H bond, so it is weakly polar. The highly polar C-Cl bonds attract the shared electron cloud of this C-H bond as well in addition to attracting the C-Cl bonded electrons. The charged electron cloud does not get balanced equally in different parts of the molecule. Thus, CHCl3 is a polar molecule with a net μ= 1.08 D.
Water (H2O) is a polar molecule.
Each O-H bond in the H2O molecule is polar due to a high electronegativity difference between the bonded atoms. Additionally, water molecules adopt a bent/ V-shape with two lone pairs present on the central O atom. The electron cloud stays unevenly distributed in the molecule overall thus it is polar with a net μ value of 1.85 D.
Similarly, ammonia (NH3) is a polar molecule
This is because each N-H bond has an electronegativity difference of 0.84 units between the bonded N and H atoms. There is a lone pair of electrons present on the central Nitrogen thus NH3 adopts an asymmetric trigonal pyramidal shape and geometry. The dipole moments do not get canceled, rather their effect adds up in the molecule overall. Net μ (NH3) = 1.46 D.
On the other hand, chlorine ( Cl2 ) is a non-polar molecule.
It is made up of two identical Cl atoms. Both have an equal grip on the shared electrons in the linear shape of the molecule. The molecule has a zero net dipole moment.
Carbon tetrachloride (CCl4) has polar C-Cl bonds present, but it is also an example of a non-polar molecule
The dipole moments of individual C-Cl bonds get canceled equally in the symmetric tetrahedral shape of the CCl4 molecule. The electronic cloud gets balanced on opposite sides of the molecule thus it is non-polar with a net μ=0.
Carbon dioxide (CO2) is also a non-polar molecule.
This is because the polarity of C=O bonds gets canceled in opposite directions in the linear shape of the molecule.
We have just seen that CCl4 is a non-polar molecule due to its symmetric tetrahedral shape and geometry. Does that mean all tetrahedral molecules are non-polar? Let’s find out.
Are all tetrahedral molecules non-polar?
As chemistry students, we should know that tetrahedral is an ideal electronic geometry according to the valence shell electron pair repulsion (VSEPR) theory of chemical bonding. It symbolizes an AB4-type molecule that is formed by four bond pairs (B) around the central A atom.
- If all the atoms (B) surrounding the central atom are identical such as the four Cl atoms in CCl4 or the four H atoms in CH4 then the molecule is definitely non-polar.
- However, if different B atoms surround the central atom then the molecule can be polar. For instance, CHCl3, CH2Cl2, and CH3Cl also have a tetrahedral shape or geometry but they are polar molecules. This is because of the high electronegativity of Cl atoms. The Cl atoms attract each of the C-H bonded electrons in addition to attracting the C-Cl electrons. The charge electron cloud stays non-uniformly distributed in the molecules overall, so they are polar tetrahedral molecules.
- Similarly, methanol (CH3OH) has a tetrahedral shape and geometry, but it is polar because oxygen is more electronegative than both C and H atoms. It attracts the shared electron cloud of both O-H and each of the C-H bonds. The overall charge density stays unbalanced in the
- Iodoform (CHI3) and the ozone-depleting dichloro difluoro carbon (CF2Cl2) are also examples of polar tetrahedral molecules.
We have also discussed earlier that H2O is a polar molecule due to its bent asymmetric shape. Now, the question that arises is: Are all bent molecules polar? Let’s discuss that.
Are all bent molecules polar?
- Mostly all bent molecules are polar. A molecule obtains a bent shape in the presence of lone electron pairs. The bonded atoms arrange asymmetrically to accommodate lone pair-lone pair and lone pair-bond pair repulsions. Thus, bent molecules such as H2O, SO2, and NO2 are polar in nature.
Examples of polar bent molecules
Until here, we have learned that polar molecules must have polar bonds present in them and these polar bonds should be asymmetrically arranged overall. These are two fundamental principles of polarity. But we would like to mention a few molecules that deviate from these fundamentals as there is always room for exceptions. Especially when we talk about science!
Exceptions in polar versus non-polar concept
Dihydrogen sulfide (H2S)
There is a small electronegativity difference of 0.38 units between the sulfur (E.N= 2.58) and hydrogen (E.N= 2.20) atoms of each H-S bond in the H2S molecule. 0.38 units < 0.5 units are required for a polar covalent bond according to Pauling’s scale. Thus, an H-S bond is technically non-polar.
But as we have told you already that a bond made up of two dissimilar atoms cannot be purely non-polar. It always has some polar characteristics. The weak polarity effects of two H-S bonds add up in the overall bent shape of the molecule with two lone pairs present on the central S atom. Thus, H2S is a polar molecule overall with net μ=0.95 D.
Carbon monoxide (CO)
CO is a linear molecule that is a symmetrical shape, so the molecule is supposed to be non-polar. But in opposition to that, CO is a polar molecule (μ=0.122 D). This is because of the higher electronegativity of oxygen that strongly attracts the shared electron cloud towards itself. As a result of this, the electronic charge distribution stays unbalanced in the molecule overall.
Ozone (O3) is made up of three identical oxygen atoms with no electronegativity difference present between them but still, it is a polar molecule. This polarity exists due to the asymmetric, bent shape of O3. The oxygen-oxygen shared electron cloud is delocalized and shifts continuously from one position to the other in the molecule to generate different resonance hybrids. This unstable charge distribution present in the molecule makes it a polar molecule overall (net μ= 0.534 D).
In conclusion, the key factor that controls the polarity of chemical compounds and molecules is the presence and/or the absence of an unbalanced electronic cloud distribution.
This key factor dominates above bond polarity as well as the shape of the molecule in giving a final decision about the polar or non-polar nature of a molecule.
If you find difficulty in understanding the polar or nonpolar nature of a molecule, then go through the links given below –
- Is SF4 polar or nonpolar?
- Is CO2 polar or nonpolar?
- Is NH3 polar or nonpolar?
- Is SO2 polar or nonpolar?
- Is SO3 polar or nonpolar?
- Is H2O polar or nonpolar?
- Is H2S polar or nonpolar?
- Is HCN polar or nonpolar?
- Is CCl4 polar or nonpolar?
- Is XeF4 polar or nonpolar?
- Is CH2O polar or nonpolar?
- Is CHCl3 polar or nonpolar?
- Is BrF5 polar or nonpolar?
- Is SF6 polar or nonpolar?
- Is BF3 polar or nonpolar?
- Is PCl5 polar or nonpolar?
How can you tell if a molecule is polar or non-polar?
How the polarity of a molecule affects its solubility in water?
Like dissolves like. According to this general rule, polar molecules dissolve in polar solvents such as water (H2O) while non-polar molecules dissolve in non-polar organic solvents such as CCl4.
Polar molecules interact with water using their oppositely charged ends and become dissoluble in it.
Examples of water-soluble (also called hydrophilic) molecules are NH3 and CH3
In addition to opposite charge interaction, these molecules contain H atoms directly bonded to an electronegative O or N atom so they can additionally develop hydrogen bonding with water.
Why don’t water and oil mix with each other?
Oils are triglycerides i.e., fatty acids bonded to glycerol via ester linkages. Long alkyl chains present in their structures make oil a non-polar chemical compound.
Therefore, oils do not have oppositely charged poles to interact with polar H2O molecules.
Thus, oil is immiscible (does not mix) with water and vice versa. Rather, it forms an easily distinguishable, shiny layer above the water surface.
Is there any shortcut or trick to find the polarity of molecules?
However, a few exceptions do exist that we have already talked about.
How to tell if a molecule is polar or non-polar without electronegativity?
- Determine the electronegativity of bonded atoms.
- A covalent bond is polar if the bonded atoms have an electronegativity difference greater than 0.5 units.
- Draw the Lewis structure of the molecule and determine its molecular shape and geometry.
- If the atoms arrange symmetrically in a molecule and the charged electron cloud stays non-uniformly distributed in the molecule overall then it is a polar molecule.
- Polar molecules have net μ > 0.
- If dipole moments get equally canceled in the opposite direction and the electron cloud becomes uniformly distributed over the molecule then it is a non-polar molecule with a net μ =0.