Bond Polarity and Inter-molecular interactions

Polarity is due to the unequal sharing of electrons between the atoms. Sharing of electrons between the atoms can form either covalent or non-covalent bonds. Most of the covalent bonds involve in electron sharing irrespective of the nature of bonds between them.  The number of atoms joining together determines the bond character of the molecules, and this, in turn, determines the molecular properties of given atoms.

Bond formation in chemistry

A chemical bond is a permanent attraction between atoms, ions or molecules that facilitates the formation of chemical compounds. The electrostatic force of attraction between oppositely charged ions forms bonds. The electrostatic forces are formed due to the sharing of electrons. The 4 common types of bonds are ionic bonds, covalent bonds, hydrogen bonds, and van der Waals interactions. These are again subdivided into many categories based on polarity and non-polarity.  Let us discuss very commonly used bonds in chemistry.

1. Covalent bonds

Polar covalent bonds

In polar covalent bonds, the bonding pairs of the electron are pulled between the nuclei of the atoms that help in sharing the atoms. Such a pull can create electronegativity which in turn adds some more imbalance between the atoms so that an opposing electron pull starts acting. So a polar molecule is formed usually when the one end of the molecule turned almost positive ant the opposite end of the molecule has negative charges. Such a difference in electric potential ends up creating an electrical pole.

Non-Polar covalent bonds

A non-polar covalent bond is formed when the atoms experience an equal bond pull, and the bonding electrons are also shared equally.  Molecules of hydrogen, oxygen, and nitrogen have non-polar covalent bonds.  The table below helps to understand how bonds can be predicted based on the electronegativity. The degree of Polarity of bonds depends upon the relative electronegativity of participating atoms and the spatial arrangement of various bonds in the Atom. Because there is an electric difference between the 2 sides, it eventually leads to the movement of electrons towards the more electronegative element. The table below provides a clue about how electronegativity differences lead to different bond types. 

Electronegativity difference

Type of bond

Example

0.00-0.40

Covalent(       non-polar)

H-H

0.41-1.00

Covalent( slightly polar)

H-Cl

 

2.Ionic bonds

Ionic bonding completely transfers the valence electron(s) between atoms, consequently, it generates two oppositely charged ions. An ionic bond is selective and is commonly seen in metals and non-metals where the metal loses electrons to become a positively charged cation, while the nonmetal accepts electrons to turn to form a negatively charged anion. The difference between the covalent and ionic bond is that, in covalent bonds, atoms share electrons while in ionic bonds atoms transfer electrons. 

3.Hydrogen bonds

A hydrogen bond depicts a dipole-dipole attraction between molecules.  It occurs due to the attractive force between a hydrogen atom that covalently bonded to a very electronegative atom such as nitrogen, oxygen etc. The best example of a hydrogen bond is water molecules bonding together in the form of ice.

4. Van der Waals interactions

Van der Waals interactions occur when 2 or more adjacent atoms become close enough that their outer electrons might just barely touch. Such slight contact between neighbouring atoms induces charge fluctuations that result in a nonspecific and nondirectional attraction. However, when two atoms get too close to each other they strongly repel. Unlike ionic or covalent bonds, these attractions are not the consequence of chemical electronic bond or sharing of electrons hence they are relatively weak and more susceptible to disturbance. Therefore, the force quickly vanishes at longer distances between interacting molecules. 

Polarity and Intermolecular Interactions

In the molecular polarity, a molecule can be polar as a result of the formation of polar bonds with differences in their electronegativity. Polarity can also result from the asymmetric arrangement of nonpolar covalent bonds and non-bonding pairs of electrons. Intermolecular force refers to the force that exists between the molecules of substances.

Intermolecular forces (IMF)

Intermolecular forces drive the interaction between molecules either by attraction or repulsion. To create these 2 forces, there must be action between molecules and other types of neighbouring particles such as atoms or ions. Intermolecular forces exist within the molecules while intramolecular forces exist between 2 or more molecules. Intermolecular forces are generally weak when compared to intramolecular forces, for example, the covalent bond involving the sharing of electron pairs between atoms is significantly stronger than the forces that exist between neighbouring molecules.  The forces are frequently used in molecular mechanics. There are 4 types of intermolecular interactions as a result of the polarity of molecules as explained below. 

1. Dipole-Dipole interactions

Dipole-Dipole interaction forces mainly occur in polar molecules that have permanent dipoles. Here, the positive pole of the known molecule is normally attracted by the negative pole of the other molecule. For example, in H20, oxygen is considered to be more electronegative while the hydrogen atom is positively charged. The figure below shows how dipole-dipole interactions take place in water molecules.

2. Ion-Dipole Interactions

Ion-Dipole interaction is the attraction between cations/anions and the polar molecules. Ion-dipole interactions slightly differ from dipole-dipole interactions. A dipole-dipole force exists when the positive side of a polar molecule attracts the negative side of another polar molecule whereas an ion-dipole force is a force between an ion and a polar molecule. Dissolving sodium chloride in water is a good example of ion-dipole interaction where the polar water molecules are attracted towards the sodium ions and the chloride ions.

                              

3. Ion-induced dipole interactions

An ion-induced dipole force occurs when an ion comes in contact with a non-polar molecule. Similar to a dipole-induced dipole force, the charge of the ion causes a distortion of the electron cloud in the non-polar molecule, resulting in a temporary partial charge. The temporary partially charged dipole and the ion are attracted to each other and form a fleeting interaction.

              

4. Dipole-induced dipole interactions

Dipole-induced dipole interactions are formed if a polar molecule induces a dipole in an atom or in a nonpolar molecule which distorts the arrangement of the electrons in the nonpolar species.

Read More

  1. Covalent bond

  2. Ionic bonds and ionic compounds

  3. Bond Polarity and Inter-molecular interactions

Questions

  1. Explain the polarity of molecules and Intermolecular interactions.

  2. What is n der Waals forces of attraction?

  3. Name the 4 types of intermolecular interactions.

  4. What is ion-induced dipole interactions?

 



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