Bond polarity
If a covalent bond is formed between two atoms of the same element homo nuclear) or two atoms whose electron affinity are nearly same, then the electron pair is equally shared between the two atoms. Such molecules are said to be non polar molecules and the bond formed is called non polar covalent bond. Examples: H2, CI2, 02, N2 etc.
If the covalent bond is formed between two different atoms, the electron pair between the two atoms shifts slightly towards an atom of greater electronegativity. Hence one end of the molecule acquires a small positive charge and the other a small negative charge. Such molecules where there is unequal sharing of the electron pair are called polar molecules and the bond formed between the atoms is known as a polar covalent bond. Example-. HCI. In this, chlorine atom being more electronegative gets a larger share of the electron pair and acquires a small negative charge and hydrogen atom acquires a small positive charge as shown below.
The polarity of the bond depends on the electronegativity difference between the two atoms. It is expressed in terms of dipole moment. Higher the dipole moment, greater the polarity of the bond. In symmetrical molecules like, carbon dioxide, carbon tetrachloride, methane etc. the dipole moment is zero since the polarity is mutually cancelled within the molecule and hence such molecules are nonpolar. In asymmetric molecules like water, ammonia etc. the polarity is not cancelled and such molecules possess dipole moment and hence are polar.
Dipole moment
The degree of polarity of a polar molecule is measured in terms of dipole moment.
Dipole moment is defined as the product of magnitude of a charge and the distance between two opposite charges
Dipole moment, (a = (charge on the polar ends) x distance between the ends H = q x d
The unit of dipole moment is debye (D).
Dipole moment is represented by an arrow (with + on the tail) drawn above the molecule pointing towards the negative end.
ID = 3.355 x 10 30 cm, where C = Coulomb, m = distance between the charges of separation.
Example:
(1) Dipole moment of NH$
NH3 has a pyramid{al} structure. Dipole moment is less than that of water because N is less electronegative than oxygen and charge separation is less. Resultant dipole moment of NH-* is 1.49D.
(2) Dipole moment of water
Water has a bent structure. Hence, it has a resultant dipole moment of 1.84 D. This shows that the molecule is not linear.
(3) Carbon di-oxide
A molecule of carbon dioxide is linear. The dipole moments in both the directions are equal and opposite.
Hence, the net dipole moment is zero.
(4) Boron triflouride
Boron triflouride has a triangular planar structure. The molecule is symmetrical and the resultant dipole moment in any two directions is equal and opposite to in the third direction. Hence the net dipole moment is zero.
Uses of Dipole moment
Measurement of dipole moment is useful in predicting the geometry of the molecule.
• zero dipole moment of C02 indicates linear structure.
• zero dipole moments for BF3, C6H6, and CH4 account for their symmetrical structure.
• finite value of dipole moment of NH3 indicates trigonal pyramid{al} structure.
Dipole moment is a vector quantity. It helps to predict whether a molecule is polar or non polar and also the geometry of the molecule.
Example: Calculation of percentage of ionic character using dipole moment.
Calculate the percentage of ionic character of a diatomic molecule having a dipole moment of 1.92 Debye and a bond length of 2.0 A. \i = q x d
1.92 x 10~18 esu cm = q x 2 x 10~8 cm
1.92X10"18 An. 10
or q = --3— = 0.96 x 10 esu
2x10
Since charge on an isolated ion = 4.8 x 10"'° esu
,T, r. . , 0.96 xlO-10
% of ionic character =-— x 100 - 20%
4.8x10
(5) Hydration of ions solution
Ionic, substances dissolve in polar solvents. Although the ionic bond is relatively strong, it is easily broken by solvent molecules. The ions formed are surrounded by solvent molecules and the process is called hydration or solvation.
The ionic compounds, under the influence of polar water molecules split up into negatively and positively charged ions. Each ion is then surrounded by water molecules. The negative ion is attracted by the positive end (hydrogen end) of water molecule and the positive ion is attracted by the negative end (oxygen end) of water molecules. Thus the ions get hydrated.
When an ionic compound dissolves in water the ions are hydrated and the energy evolved in the process Is called hydration energy. Thus, for an ionic solid to be soluble in water, the necessary condition is hydration energy > lattice energy (AH hydration > U).