The chemical bond can be defined as the interaction between two elements which helps them close to each other. Elements involved in the formation of chemical bonds with the help of their valence electrons. There are mainly two types of chemical bonds; Ionic and covalent. Ionic bonds are formed between two oppositely charged ions by the complete transfer of electrons. When an atom loses its electrons to form a cation, it acquires a positive charge.
Similarly, an anion is formed by accepting an electron and has a negative charge. The electrostatic force of attraction between cation and anion is known as electrovalent interaction or Ionic bond. For example, the bond between sodium ion and chloride ion leads to the formation of sodium chloride that is an ionic compound.
The covalent bond is formed by the complete transfer of valence electrons between bonded atoms. Such type of bond is formed by the equal sharing of electrons between two bonded atoms. These atoms have equal contribution to the formation of the covalent bond. On the basis of the number of bonds; covalent bond can be classified as single, double and triple covalent bond. A single covalent bond is formed by one sigma bond; double bond is formed by one sigma and one pi whereas triple bond is formed by one sigma and two pi bonds. On the basis of the polarity of a covalent bond, it can be classified as polar and non-polar covalent bonds.
Let us take the example of hydrogen chloride (HCl) molecule. The bonding of hydrogen and chlorine atoms leans more towards Cl atom.
- This is because Cl is more electronegative in nature than the hydrogen. Thus the shared pair of electrons lies not exactly in between the bond but lean towards chlorine.
- This causes the Cl atom to acquire a slight negative charge, and H atom a slight positive charge.
- This will make the covalent bond between H and Cl to possess an appreciable ionic character.
Hº + •Cl::: → Hº •Cl:::
When two atoms of the same element involve in a covalent bond to form a molecule, the shared pair of electrons will be at the exact midway between the two atoms. This means the electrons are equally shared by the atoms. The resulting molecule will be neutral electrically in all respects.
Such a covalent bond is called as a non-polar covalent bond. The examples of non-polar covalent bonds are the molecules of H2, O2, Cl2 etc.
O: + :O $\to$ O::O or O=O
- Polar covalent bond can be defined as the bond which is primarily a covalent bond but shows a degree of polarity. This partial polarity of the molecule is attributed to the covalent bond formed between two different atoms.
- In such cases, the bonded pair of electrons tends to be more towards the atom which has more negativity or an affinity to attract the electron pair more towards it than the other.
- This results in that atom to attain a partial negative charge and the other an equivalent positive charge.
- For example, the bond between Hydrogen and Chlorine to form the hydrogen chloride gas is covalent. But the difference in the electronegativities between the two makes the molecule polar.
In water molecule one atom of oxygen bonds with two atoms of hydrogen. Oxygen has 6 electrons in its valence shell and it shares with two hydrogen atoms to complete its octet structure. In turn, each hydrogen shares one of the oxygen electrons and attains helium configuration.
The electronegativity on Pauling scale for hydrogen is 1 and that of oxygen is 3.5. This suggests that oxygen attracts the shared pair of electrons more towards itself and attains a partial negative charge while each hydrogen atom attains a partial positive charge.
The presence of 2 lone pairs of electrons on the central oxygen atom pushes the two hydrogens to an angle and the angle between the two hydrogen atoms bonded to oxygen is 104.5º ( a V-shaped molecule)
Some of the polar covalent bonded compounds are Hydrogen chloride H−Cl. The electronegativity of hydrogen is 1 and the electronegativity of chlorine is 3. This makes the bond a polar covalent.
Lithium Fluoride LiF Li− F. The electronegativity of lithium is 1 and that of fluorine, the most electronegative element is 3.5. This leaves a partial positive charge on lithium because of the pull by the more electronegative fluorine of the shared pair of electrons. Fluorine gets a partial negative charge.
Unsymmetrical compounds exhibit polar covalent characteristics. An organic compound with functional groups having a difference of electronegativity shows polarity. For example, 1-chlorobutane (CH3-CH2-CH2-CH2Cl) shows partial negative charge on Cl and the partial positive charge distributed over the carbon atoms. This is called inductive effect.
Even double bond present in unsymmetrical hydrocarbon also exhibits polarity. Example CH3-CH2-CH=CH2 shows partial -ve charge on the carbon atom having a double bond.
Molecules formed by the combination of different elements with sharing of electrons between them results in polar covalent molecules. Ammonia molecule NH3 is an example of the polar covalent molecule. Similarly, the molecules of CH3Cl,CH2Cl2, and CHCl3 are also polar covalent molecules.
- Fajans rule says that the electrovalent and covalent bonds can be identified by the electronegativities of the bonded atoms.
- If the electronegativity is same the bond formed is the covalent bond which is non-polar due to the balancing of electronegativities.
- If the difference of electronegativity is more than 1.7 the bond is electrovalent and the ionization takes place if the compound is dissolved in a polar solvent like water.
- While the difference less than 1.7 till 0.1 the bonds are called polar covalent bonds with different degrees of polarity.
It is generally believed that all covalent compounds where there is a bond between elements with different electronegativities will be polar. But it is not true there are certain molecules which are non-polar even though there exists a difference in electronegativity. This is explained by the geometry of the molecule.
- If the molecule is symmetrical the polarities will nullify and the resulting molecule is nonpolar.
- Carbon tetrachloride CCl4 Carbon electronegativity is 2.5 and that of chlorine is 3. Thus all the four chlorine atoms try to attract the shared pair of electrons towards themselves. This is expected to leave a partial positive charge on carbon and negative charges on the chlorine.
- The geometry of the molecule is a perfect tetrahedral shape with a bond angle of 109º.27'. The equal pull forces from all sides nullify the polar effect on the molecule and so it is a non-polar molecule. However, if one of the chlorine is replaced by a hydrogen ( CHCl3) then there exists a polarity.
- Similarly, we can explain the methane (CH4) molecule. the hybridization of carbon molecule happens in three ways, sp,sp2 and sp3.
- If the hybridization is sp the shape of the molecule is linear and if there is asymmetry in the molecule the molecule becomes non-polar eg. Acetylene H-C=C-H.
- If the molecule is H-C=C-Cl, it behaves as a polar molecule. sp2 hybridized molecules will have a trigonal configuration and even in this case if the symmetry is not maintained the polarity exists. Eg H2C=CH2 is non-polar and Cl2C=CCl2 is also non-polar but if it is ClHC=CH2 it is polar.
- The molecule Cl2C=CBr2 is polar but ClBrC=CClBr is non polar. This gives an idea of the polarity in many more covalent bonded compounds.