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.
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°.
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.
Partial Covalent Character of Ionic Bonds: For this, Fajans proposed some rules which are as follows:
- The smaller the size of the cation and the greater the size of the anion, covalent character of an ionic bond is greater.
- Greater charge on cation results in greater covalent character of ionic bond.
- In case of cations of the same size and charge, the one with electronic configuration (n-1)dnns0 (typical of transition metals) is more polarizing than the one with electronic configuration ns2np6 (typical of alkali and alkaline earth metals).
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:
- The polarizing power of cation
- Polarisability of anion
- Extent of distortion (polarization) of anion
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 shape of molecule depends upon the number of valence shell electron pairs (bonded or nonbonded) around the central atom.
- Pairs of electrons in valence shell have negatively charged electron clouds. So, they repel one another.
- Pairs of electrons in valence shell tend to occupy such positions in space that repulsion is minimized and distance between them is maximized.
- The valence shell is considered to be spherical with electron pairs on the spherical surface at maximum distance from one another.
- A multiple bond is treated as a single electron pair and two or three pairs of electron of a multiple bond are treated as a single super pair.
- In case of two or more resonance structures of a molecule, the VSEPR model is applicable to any such structure.
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.
(b) Molecules in which the central atom has one or more lone pairs. Following list shows various shapes of such molecules.
- Molecule Type: AB2E, No. of bonding pairs: 2, No. of lone pairs: 1, Arrangement of Electron Pairs: Trigonal planar, Shape: Bent, Examples: SO2, O3
- Molecule Type: AB3E, No. of bonding pairs: 3, No. of lone pairs: 1, Arrangement of Electron Pairs: Tetrahedral, Shape: Trigonal pyramidal, Examples: NH3
- Molecule Type: AB2E2, No. of bonding pairs: 2, No. of lone pairs: 2, Arrangement of Electron Pairs: Tetrahedral, Shape: Bent, Examples: H2O
- Molecule Type: AB4E, No. of bonding pairs: 4, No. of lone pairs: 1, Arrangement of Electron Pairs: Trigonal bipyramidal, Shape: See saw, Examples: SF4
- Molecule Type: AB3E2, No. of bonding pairs: 3, No. of lone pairs: 2, Arrangement of Electron Pairs: Trigonal bipyramidal, Shape: T-shape, Examples: ClF3
- Molecule Type: AB5E, No. of bonding pairs: 5, No. of lone pairs: 1, Arrangement of Electron Pairs: Octahedral, Shape: Square pyramid, Examples: BrF5
- Molecule Type: AB4E2, No. of bonding pairs: 4, No. of lone pairs: 2, Arrangement of Electron Pairs: Octahedral, Shape: Square planar, Examples: XeF4
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.