Chemical Bonding

Polarity of Bonds

A hundred per cent ionic or covalent can exist only in ideal situation. But in reality, no bond is either completely covalent or ionic. A covalent bond has some ionic character, and an ionic bond has some covalent character.

Covalent Bond Between Two Similar Atoms: In case of covalent bond between two similar atoms, the shared pair of electrons is equally attracted by two atoms. Due to this, the electron pair is situated exactly between the two identical nuclei. The bond so formed is called nonpolar covalent bond. Examples: H2, O2, Cl2, etc.

Covalent Bond in Heteronuclear Molecule: In case of a heteronuclear molecule, the shared pair of electrons gets displaced more towards the more electronegative atom. The bond so formed is called polar covalent bond. For example; in case of HF, the shared pair of electrons is displaced towards fluorine.

Such a molecule (heteronuclear) possesses the dipole moment. The product of the magnitude of charge and the distance between the centres of positive and negative charges is called dipole moment. Dipole moment is expressed as follows:

Dipole Moment(μ) = Charge (Q) × Distance of Separation (r)

Dipole moment is generally expressed in Debye units (D).

1 D = 3.33564 × 10-30 C m or coulomb meter

Dipole moment is a vector quantity. By convention, it is depicted by a small arrow with a tail (↦) on the negative centre and head towards the positive centre. But in chemistry, dipole moment is depicted by a crossed arrow put on Lewis structure of molecule, in which the cross is on the positive end and arrow head is on the negative end (⇸). The arrow symbolizes the direction of the shift of electron density in the molecule.

Dipole moment in HF

Polyatomic Molecules: In case of polyatomic molecules, dipole moment depends not only upon the individual dipoles but also on the spatial arrangement of various bonds in the molecule. In this case, dipole moment of a molecule is the vector sum of dipole moments of various bonds.

Example: H2O molecule has a bent structure, in which two O-H bonds are at an angle of 104.5°.

Dipole moment in water molecule

Net dipole moment μ = 1.85 D

= 1.85 × 3.33564 × 10-30 C m

= 6.17 × 10-30 C m

In case of BeF2, net dipole moment is zero because two equal bond dipoles point in opposite directions and cancel the effect of each other.

In case of BF3, the dipole moment is zero because the three bonds are oriented at 120° and resultant vector is zero.

Dipole moment of NH3 (4.90 × 10-30 C m) is greater than that of NF3 (0.8 × 10-30 C m). This happens because in case of NH3 the orbital dipole due to lone pair is in same direction as the resultant dipole. But in case of NF3 the orbital dipole is in opposite direction to the resultant dipole.

Dipole moment in ammonia and nitrogen fluoride

Partial Covalent Character of Ionic Bonds: For this, Fajans proposed some rules which are as follows:

We know that the cation polarizes the anion, and pulls the electronic charge towards itself. This increases the charge between the two ions. So, following factors determine the percent covalent character of an ionic bond:

Valence Shell Electron Pair Repulsion (VSEPR) Theory

This theory was first proposed by Sidgwick and Powell in 1940 and was further developed by Nyholm and Gllespie in 1957. Main postulates of VSEPR theory are as follows:

The repulsion interaction of electron pairs decreases in following order:

Lon pair (lp) – lp > lp – Bond pair (bp) > bp – bp

While the lone pairs are localized on the central atom, each bonded pair is shared between two atoms. So, the lone pair occupies more space compared to the bonded pair. It results in greater repulsion between lone pairs compared to lp-bp and bp-bp repulsions. These repulsion effects result in deviations from idealized shapes and alterations in bond angles in molecules.

For prediction of geometrical shapes of molecules with the help of VSEPR theory, it is convenient to divide molecules into two categories:

(a) Molecules in which the central atom has no lone pair

Examples: compounds of AB2, AB3, AB4, AB5 and AB6, the shapes in that order are: linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral.

Shape of atom with lone pair electrons

(b) Molecules in which the central atom has one or more lone pairs. Following table shows various shapes of such molecules.

Molecule TypeNo. of bonding pairsNo. of lone pairsArrangement of Electron PairsShapeExamples
AB2E21Trigonal planarBentSO2, O3
AB3E31TetrahedralTrigonal pyramidalNH3
AB4E41Trigonal bipyramidalSee sawSF4
AB3E232Trigonal bipyramidalT-shapeClF3
AB5E51OctahedralSquare pyramidBrF5
AB4E242OctahedralSquare planarXeF4

VSEPR theory is able to predict the geometry of a large number of molecules, specially the compounds of p-block elements accurately. It also accurately gives the shape when energy difference between possible structures is very small. But the theoretical basis of VSEPR theory about the effects of electron pair repulsions on molecular shapes is not clear.

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