Chemical bonding, class XI, Bond parameters

Bond parameters

1. UNIT 4 CHEMICAL BONDING AND MOLECULAR STRUCTURE

2. Match the following! Lattice Enthalpy Bond length Bond angle Bond enthalpy Bond order energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions The number of bonds between the two atoms in a molecule the overall size of the atom which includes its valence shell in a non-bonded situation The amount of energy liberated when one mole of a gaseous molecule is formed from gaseous atoms angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion the equilibrium distance between the nuclei of two bonded atoms in a molecule

3. BOND LENGTH The equilibrium distance between the nuclei of two bonded atoms in a molecule As bond length increases, bond strength or stability decreases

4. BOND ANGLE The angle between the orbitals containing bonding electron pairs around the central atom in a molecule/complex ion As the electronegativity of the central atom decreases, bond angle decreases

5. BOND ENTHALPY The amount of energy liberated when one mole of a gaseous molecule is formed from gaseous atoms Larger the bond dissociation enthalpy, stronger will be the bond

6. BOND ORDER The number of bonds between the two atoms in a molecule With increase in bond order, bond strength increases Isoelectronic molecules and ions have identical bond orders; N 2 , CO and NO+ have bond order 3

7. RESONANCE STRUCTURE a number of structures with similar energy are taken as the canonical structures of a hybrid which describes the molecule accurately

8. Resonance stabilizes the molecule as the energy of the resonance hybrid is less than the energy of any single canonical structure Resonance averages the bond characteristics as a whole The molecule as such has a single structure which is the resonance hybrid of the canonical forms

9. POLARITY OF BONDS In reality no bond or a compound is either completely covalent or ionic!! When covalent bond is formed between two similar atoms, the shared pair of electrons is equally attracted by the two atoms: nonpolar covalent bond In case of a hetero-nuclear molecule like HF, the shared electron pair between the two atoms gets displaced more towards fluorine. The resultant covalent bond is a polar covalent bond As a result of polarisation, the molecule possesses the dipole moment ( the product of the magnitude of the charge and the distance between the centres of positive and negative charge) (µ)

10. Salient features Dipole moment (µ) = charge (Q) × distance of separation (r) vector quantity Dipole moment is usually expressed in Debye units (D) 1 D = 3.33564 × 10^(–30) C m represented by a crossed arrow put above Lewis structure of the molecule

11. ILLUSTRATIONS For example in H2O molecule, The dipole moment in case of BeF 2 BF 3 ?

12. Fajan’s rules: - 1. The smaller the size of the cation and the larger the size of the anion, the greater the covalent character of an ionic bond NaCl KCl CsCl AgF AgBr AgCl AgI 2. The greater the charge on the cation, the greater the covalent character of the ionic bond NaCl MgCl 2 AlCl 3 SiCl 4

13. Valence Shell Electron Pair Repulsion (VSEPR)Theory Sidgwick and Powell, 1940 based on the repulsive interactions of the electron pairs in the valence shell of the atoms The shape of a molecule depends upon the number of valence shell electron pairs around the central atom Pairs of electrons in the valence shell repel one another. These pairs of electrons tend to occupy such positions that minimize repulsion and thus maximize distance between them. A multiple bond is treated a single super electron pair.

14. What can be the order of repulsive interactions of the electron pairs in the valence shell of the atoms? Which among the following have zero dipole moment? HCl, I 2 , CO 2 , H 2 S, CHCl 3 , CCl 4 Why is the dipole moment of NF 3 less than NH 3 ?