Chemical Bonding and Molecular Structure
Valence electrons, ionic and covalent bonds, bond parameters, Lewis structures, polar character of covalent bond, dipole moment, VSEPR theory, hybridization involving s, p, and d orbitals.
As a result of polarisation, the molecule possesses a dipole moment which may be defined as the product of the magnitude of the charge and the distance between the centres of positive and negative charge. It is usually designated by a Greek letter 'μ'. Mathematically, dipole moment is expressed as: Dipole moment (μ) = charge (Q) × distance of separation (r). Dipole moment is a vector quantity and by convention it is depicted by a small arrow with tail on the negative centre and head pointing towards the positive centre. But in chemistry, the arrow is drawn with the tail on the positive centre and head pointing towards the negative centre. In case of polyatomic molecules the dipole moment of the molecule is the vector sum of the dipole moments of various bonds. In both ammonia (NH₃) and nitrogen trifluoride (NF₃), the molecule has a pyramidal shape with a lone pair of electrons on nitrogen atom. Although fluorine is much more electronegative than nitrogen, the resultant dipole moment of NH₃ (1.47 D) is much greater than that of NF₃ (0.24 D). This is because, in case of NH₃ the orbital dipole due to lone pair is in the same direction as the resultant dipole moment of the three N–H bonds, whereas in NF₃ the orbital dipole is in the direction opposite to the resultant dipole moment of the three N–F bonds.
The Valence Shell Electron Pair Repulsion (VSEPR) theory provides a simple procedure to predict the shapes of covalent molecules. Sidgwick and Powell in 1940 proposed that the geometry of a molecule depends upon the number of valence shell electron pairs (bonded or nonbonded) around the central atom. The repulsive interaction of electron pairs decreases in the order: Lone pair (lp) – Lone pair (lp) > Lone pair (lp) – Bond pair (bp) > Bond pair (bp) – Bond pair (bp). Nyholm and Gillespie (1957) refined the VSEPR model. While the lone pairs are localized on the central atom, each bonded pair is shared between two atoms. As a result, the lone pair electrons in a molecule occupy more space as compared to the bonding pairs of electrons. This results in greater repulsion between electron pairs containing a lone pair compared to the bond pairs.
Although both NH₃ and NF₃ molecules have pyramidal geometry, the dipole moment of NH₃ (1.47 D) is significantly higher than that of NF₃ (0.24 D). This difference is primarily attributed to:
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