JEE Chemistry Chemical Bonding and Molecular Structure Notes Set 02

Why Atoms Bond

Atoms join to reach a lower energy and, in most main group cases, an outer shell of eight electrons (the octet). The three classical bond types are ionic, covalent and coordinate. Metallic bonding, hydrogen bonding and van der Waals forces are described later in these notes.

Ionic Bond

1. It forms by complete transfer of electrons, giving cations and anions held by electrostatic attraction.
2. Favoured by low ionisation energy of the metal, high electron affinity of the non-metal and a high lattice energy. Lattice energy rises with ionic charge and falls with ionic size, so MgO is far above NaCl.
3. Properties: non-directional, high melting and boiling points, brittle, soluble in polar solvents, conduct only when molten or dissolved (not as solids).
4. Inert pair effect: in heavy p block elements the ns2 pair resists bonding, so the lower oxidation state becomes stable (Pb2+, Bi3+, Tl+). Ions with an (18 + 2) configuration come from this.
5. Fajans' rules: covalent character increases with a small, highly charged cation, a large, highly charged anion, and a cation with an 18-electron core rather than a noble gas core. Example: LiI is more covalent than LiF, and SnCl4 is more covalent than SnCl2.
6. Hydration: water is a good solvent for ionic solids because hydration energy compensates the lattice energy and a high dielectric constant weakens the attraction between ions.

Covalent Bond

1. Formed by sharing electron pairs through overlap of orbitals. It is directional, so molecules have definite shapes. Compounds usually have low melting points, except network solids such as diamond, quartz and silicon carbide.
2. A sigma bond is head-on overlap along the bond axis and permits rotation. A pi bond is sideways overlap of parallel p orbitals and does not permit rotation. A single bond is one sigma, a double bond is one sigma plus one pi, and a triple bond is one sigma plus two pi.
3. Order of overlap strength: s-s is weakest, then p-p, then s-p is the most effective.
4. Covalency can exceed the number of unpaired electrons only if empty orbitals of the same shell are available. Phosphorus makes PCl5 using the 3d orbitals, but nitrogen cannot make NCl5.
5. Octet exceptions: electron-deficient (BF3, BeCl2, AlCl3), expanded octet (PCl5, SF6, IF7) and odd-electron (NO, NO2).

Coordinate Bond and Lewis Structures

A coordinate bond forms when one atom donates both electrons of the pair and the other atom has a vacant orbital. Examples: NH3 to BF3, the bonds in H3O+ and NH4+, and the third bond in CO. Complex ions such as [Fe(CN)6]4- are held by coordinate bonds.

1. Lewis structures show the connectivity. The least electronegative atom (not hydrogen) is usually central.
2. Formal charge = valence electrons - non-bonded electrons - number of bonds. In ozone the formal charges on the three oxygens are 0, +1 and -1.
3. In oxyacids the acidic hydrogens are bonded to oxygen as O-H, except H3PO3 (two OH, one P-H) and H3PO2 (one OH, two P-H).

Resonance

When one Lewis structure cannot describe a species, the real structure is a hybrid of two or more contributing forms with the same arrangement of atoms and the same number of paired electrons. Resonance lowers the energy (resonance stabilisation) and equalises bond lengths, for example the two C-O bonds in HCOO-, or the three C-O bonds in CO3 2- (bond order 1.33). More equivalent contributing structures mean a more stable hybrid.

Dipole Moment

1. Dipole moment is charge multiplied by distance, mu = q x d. Unit: 1 D = 3.33 x 10^-30 C m = 10^-18 esu cm. It is a vector.
2. Percentage ionic character = (observed dipole moment / moment for full charge separation) x 100.
3. Zero dipole moment: symmetrical shapes such as CO2, BF3, CH4, CCl4, SF6, XeF4, trans-1,2-dichloroethene, p-dichlorobenzene and hexachlorobenzene.
4. Net dipole: H2O, NH3, SO2, cis-1,2-dichloroethene, o- and m-dichlorobenzene. NH3 (1.47 D) is far more polar than NF3 (0.23 D) because in NF3 the lone pair moment opposes the bond moments.

Worked Examples

1. HCl has a dipole moment of 1.03 D and a bond length of 1.275 A. For 100 percent ionic character, mu = 4.8 x 1.275 = 6.12 D. Ionic character = 1.03 / 6.12 = 17 percent.
2. Sigma and pi bonds in benzene: 6 C-C sigma + 6 C-H sigma = 12 sigma, and 3 pi, so the ratio is 4.
3. Which is more ionic, PbCl2 or PbCl4? PbCl2, because Pb2+ is bigger and less highly charged than Pb4+.