Chemical Bonding and Molecular Structure Chemistry - Chemical Bonding and Molecular Structure

Lewis dot structures, in general, do notrepresent the actual shapes of the molecules.In case of polyatomic ions, the net charge ispossessed by the ion as a whole and not by aparticular atom. It is, however, feasible toassign a formal charge on each atom. Theformal charge of an atom in a polyatomicmolecule or ion may be defined as thedifference between the number of valenceelectrons of that atom in an isolated or freestate and the number of electrons assignedto that atom in the Lewis structure. It isexpressed as :Generally the lowest energystructure is the one with the smallestformal charges on the atoms. The formalcharge is a factor based on a pure covalentview of bonding in which electron pairsare shared equally by neighbouring atoms. The octet rule, though useful, is not universal.It is quite useful for understanding thestructures of most of the organic compoundsand it applies mainly to the second periodelements of the periodic table. There are threetypes of exceptions to the octet rule.

• The incomplete octet of the central atom
• Odd-electron molecules
• The expanded octetFrom the Kössel and Lewis treatment of theformation of an ionic bond, it follows that theformation of ionic compounds wouldprimarily depend upon:
• The ease of formation of the positive andnegative ions from the respective neutralatoms;
• The arrangement of the positive andnegative ions in the solid, that is, thelattice of the crystalline compound.

The Lattice Enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into gaseous constituent ions. For example, the lattice enthalpy of NaCl is 788 kJ mol–1. This means that 788 kJ of energy is required to separate one mole of solid NaCl into one mole of Na+  (g) and one mole of Cl– (g) to an infinite distance.Bond length is defined as the equilibriumdistance between the nuclei of two bondedatoms in a molecule. Bond lengths aremeasured by spectroscopic, X-ray diffractionand electron-diffraction techniques . The covalent radius is measuredapproximately as the radius of an atom’score which is in contact with the core ofan adjacent atom in a bonded situation.  The vander Waals radius represents the overall sizeof the atom which includes its valence shellin a nonbonded situation.  Bond Angle is defined as the angle between the orbitalscontaining bonding electron pairs around thecentral atom in a molecule/complex ion. Bondangle is expressed in degree which can beexperimentally determined by spectroscopicmethods. It gives some idea regarding thedistribution of orbitals around the centralatom in a molecule/complex ion and hence ithelps us in determining its shape. Forexample H–O–H bond angle in water can berepresented as under:

Bond Enthalpy It is defined as the amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state. The unit of bond enthalpy is kJ mol^–1. For example, the H–H bond enthalpy in hydrogen molecule is 435.8 kJ mol^–1 . H₂ (g) → H(g) + H(g); ∆_aH  = 435.8 kJ mol^–1. Bond OrderIn the Lewis description of covalent bond,the Bond Order is given by the number ofbonds between the two atoms in amolecule. The bond order, for example in H₂(with a single shared electron pair), in O₂(with two shared electron pairs) and in N₂(with three shared electron pairs) is 1,2,3respectively. A general correlation useful forunderstanding the stablities of moleculesis that: with increase in bond order, bondenthalpy increases and bond lengthdecreases. The concept of resonance was introducedto deal with the type of difficulty experiencedin the depiction of accurate structures ofmolecules like O₃. According to the conceptof resonance, whenever a single Lewis structure cannot describe a moleculeaccurately, a number of structures with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons are taken as the canonical structures of the hybrid which describes the molecule accurately.

Thus for O₃, the two structures shown above constitute the canonical structures or resonance structures and their hybrid i.e., theIII structure represents the structure of O₃more accurately. This is also called resonance hybrid. Resonance is represented by a double headed arrow. In general, it may be stated that

• Resonance stabilizes the molecule as the energy of the resonance hybrid is lessthan the energy of any single cannonical structure; and,
• Resonance averages the bond characteristics as a whole. Thus the energy of theO₃resonancehybrid is lower than either of the two cannonical froms I and II.

1. Which of the following techniques used to measure bond length?

1. Spectroscopic techniques
2. X-ray diffraction
3. Electron-diffraction techniques
4. All the above

2. The unit of bond enthalpy is …

1. kJ mol^–1
2. Cal mol^-1
3. Cal mol
4.  kJ mol

3. With increase in bond order, bond enthalpy… and bond length ….

1. Decreases, decreases
2. Increases, decreases
3. Increases, increases
4. Decreases, increases

4. The …. is measured approximately as the radius of an atom’s core which is in contact with the core of an adjacent atom in a bonded situation.

1. Ionic radius
2. Metallic radius
3. Covalent radius
4. None of above

5. … is given by the number of bonds between the two atoms in a molecule.

1. Bond Order
2. Bond size
3. Bond enthalpy
4. Bond angle