JEE Chemistry Chemical Bonding and Molecular Structure Notes Set 04

Molecular Orbital Theory

In MO theory the atomic orbitals combine to give bonding orbitals (lower energy, electron density between the nuclei) and antibonding orbitals (higher energy, a node between the nuclei). Electrons fill orbitals by the Aufbau principle, Pauli exclusion and Hund's rule. Bond order = (number of bonding electrons - number of antibonding electrons) / 2. Higher bond order means a shorter and stronger bond. A species with unpaired electrons is paramagnetic, and one with all electrons paired is diamagnetic.

1. Order of energy for O2 and F2: sigma 2s, sigma* 2s, sigma 2p, pi 2p, pi* 2p, sigma* 2p. For B2, C2 and N2 (up to 14 electrons), s-p mixing makes pi 2p lie below sigma 2p.
2. Quick rule: 14 electron diatomics (N2, CO, CN-, NO+, C2 2-) have bond order 3 and are diamagnetic.

Bond length order for oxygen species: O2 2+ (shortest, bond order 3) < O2+ < O2 < O2- < O2 2-. Bond energy follows bond order, so N2 > O2 > O2-. F2 has an unusually low bond energy because of lone pair repulsion between small atoms.

Intermolecular Forces

1. Ion-dipole: Na+ in water. Dipole-dipole: acetone and acetonitrile. London (dispersion) forces: all molecules, rising with the number of electrons and polarisability (noble gases, halogens, hydrocarbons).
2. Boiling points rise with intermolecular attraction: Xe is above Kr, above Ne, above He. Branching lowers the boiling point of isomers because of the smaller surface area.
3. Hydrogen bonding needs H attached to N, O or F. Strength: F-H...F is above O-H...O is above N-H...N is above Cl-H...Cl.
4. Intermolecular H-bonding raises boiling point, viscosity and enthalpy of vaporisation. Examples: water, HF, alcohols, carboxylic acids and the base pairs in DNA. Alcohols boil higher than isomeric ethers.
5. Intramolecular H-bonding (chelation) occurs in o-nitrophenol and salicylaldehyde. It lowers solubility in water, lowers the boiling point and makes the compound steam volatile, unlike its para isomer.
6. Ice has an open cage structure held by hydrogen bonds, so ice is less dense than water, and water has maximum density at 4 C.
7. Each water molecule can form up to four hydrogen bonds: two donated and two accepted.

Metallic Bond and Solid Types

The electron sea model describes a metal as a lattice of positive kernels in a sea of delocalised valence electrons. More electrons in the sea and smaller kernels give stronger bonding and hardness, so iron is much harder than sodium. Network solids (diamond, SiO2, SiC) melt by breaking covalent bonds. Molecular solids (dry ice, ice, I2) change state by overcoming only intermolecular forces.

Worked Examples

1. Bond order of N2 is (10 - 4) / 2 = 3, of N2+ is (9 - 4) / 2 = 2.5, so N2+ has a longer, weaker bond.
2. Is O2 2- paramagnetic? It has 18 electrons, with all antibonding orbitals filled, so it is diamagnetic.
3. Why is o-nitrophenol less soluble in water than p-nitrophenol? It has intramolecular hydrogen bonding, so fewer OH groups are free to hydrogen bond with water.