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Molecular Polarity

We know that atoms are bonded with chemical bonds to form molecules. Atoms can form ionic as well as covalent bonds that depend on their nature and also on the number valence electrons present in bonding atoms. Bonding between metal and non-metal is usually ionic type in which metal atoms change to metal ion (cation) and non-metallic atom accept electrons to form anions. The electrostatic force of attraction between cation and anion results the formation of ionic molecule. Sodium chloride is one of the best examples of ionic molecule in which sodium ion (cation) and chloride ion (anion) are bonded with ionic bonding.

Non-metals or elements with almost same electro negativity involve in covalent bonding by equal sharing of valence electrons. For example in the formation of chlorine molecule, two chlorine atoms share their valence electrons (one from each chlorine atom) to form single covalent bond. The covalent bonds could be multiple bonds that depend on the number of valence electrons of the bonding atoms.

For example in the nitrogen  atom requires three electrons to get the noble gas configuration therefore two nitrogen atoms share their 3 valence electrons to form triple covalent bond and form nitrogen molecule. In ionic molecules, there are cations and anions so there must be positive and negative charge on the molecule. The covalent molecules which are formed by same kind of atoms or atoms with least difference in electro negativity do not have any charge and called as non-polar molecules.

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Molecular Polarity Definition

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The covalent molecules which are formed with atoms have large difference in electro negativity get some partial charges on bonding atom that induced some polarity in the molecules.  Polarity is a physical property which affects other physical properties like melting and boiling points, solubility, and intermolecular interactions etc. The molecular polarity depends on the polarity of all bonds in that molecule. Some molecules have polar covalent bond but also have symmetrical arrangement of atoms that makes the molecule non-polar in nature.

Hydrogen chloride molecule is polar molecule. It has covalent bond between hydrogen and chlorine   atoms. We know that chlorine atom is more electro negative compare to hydrogen so it pulls the bonding electrons and acquires partial negative charge and also creates partial negative charge on hydrogen atom. So the unequal distribution of bonding electrons causes induction of polarity in the molecule. It is not necessary that the molecule with polar covalent bonds will be polar in nature.

Carbon dioxide molecule has two carbon-oxygen bonds which are polar in nature as oxygen is more electro negative in nature. But the symmetrical arrangement of both C=O bonds cancel the effect of each other and makes the molecule non-polar in nature.

Molecular Polarity Examples

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Hydrocarbons and molecules with non-metals are usually non-polar in nature as they have least difference in their electro negativity. Boron trifluoride (BF3) is non-polar molecule with three polar B-F bonds. All three B-F bonds are arranged in symmetrical manner that makes the molecule non-polar in nature. 
BF3  Molecular Geometry

On the contrary, water is polar molecule with two O-H bonds which are arranged in V-shape. Since molecule is asymmetric with bent geometry, both polar covalent bonds contributes in molecular polarity. Both hydrogen atoms acquire partial positive charges and oxygen atom acquires partial negative charge. 
H2O  Molecular Geometry

How to Find Molecular Polarity

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For the given molecule, first draw a reasonable Lewis structure and try to identify polarity for each bond. Remember if there is a difference in electro negativity, the bond will be polar otherwise it will be non-polar in nature.

Generally the electro negativity difference with more than 0.4 is considered as polar while less than 0.4 is considered for non-polar bonds. If all the covalent bonds in the molecule are non-polar, then the molecule will be non-polar. If there are few or all polar bonds, examine the center atom. Check the position of all bonds with center atom and also orientation of lone pairs on center atom. If there are no lone pairs and all bonds are arranged symmetrically then the molecule will be non-polar in nature.

The presence of polar bonds and orientation of lone pairs will decide the polarity of the molecule. Draw a geometric sketch of the molecule and try to determine the symmetry of the molecule. Notice the direction of arrows of polar bonds and their length which shows the relative polarities of the different bonds. Find out if any symmetry is present in the molecule that can make the molecule nonpolar.

Molecular Polarity Chart

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The table below shows different geometry with dipole moment of the molecule. You can notice the presence of lone pair and orientation of molecules affects the dipole moment of the molecule.

 Lone pair on center atom   Geometry 
 Polar or non-polar 
 CO2 Molecular Geometry  0  Linear  Non-polar
 BF3 Molecular Geometry  0  Triangle  Non-polar
 H2O Molecular Geometry  2  V-shape or bent   Polar
 CCl4 Molecular Geometry  0  tertrahedral  Non-polar
 F2O Molecular Geometry  2  Bent geometry  Polar
 CH2Cl2 Molecular Geometry  0  tertrahedral  Polar
 HCN Molecular Geometry  0  Linear  Polar
 SO2 Molecular Geometry  1  Bent  Polar

Bond Polarity and Molecular Polarity

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You can see in above table that only presence of polar bonds can’t make the molecule polar in nature. Carbon dioxide molecule contains two polar bonds which are arranged in linear direction and cancel the dipole of each other that makes the molecule non-polar in nature. Although the difference in electro negativity of C and O is 0.89 that is more than 0.4, hence C=O is a polar bond yet the molecular polarity is zero.

Similarly in OF2 molecule the difference in electro negativity of O and F is around 0.54 which makes the bond polar in nature. If the molecular geometry would be linear then the molecular could be non-polar. But like water molecule, this molecule also shows bent geometry that makes the molecule polar because in bent geometry, polar bonds can’t cancel the effect of each other.   
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