Chemical Bonding

1. VSEPR Theory and Geometry

Valence Shell Electron Pair Repulsion theory explains why molecules have specific shapes. Repulsion order: $lp-lp > lp-bp > bp-bp$.

• $sp^3$ with 1 lone pair: Pyramidal (e.g., $NH_3$).
• $sp^3$ with 2 lone pairs: Bent/V-shaped (e.g., $H_2O$).

Interactive diagram: open this page in a browser to use it.

2. Molecular Orbital Theory (MOT)

Explains bonding in homonuclear diatomic molecules. For $B_2, C_2, N_2$, the $\pi_{2p_x} = \pi_{2p_y}$ orbitals are lower in energy than $\sigma_{2p_z}$. For $O_2$ and $F_2$, the order is reversed.

3. Hybridisation and Shape

• $sp$: linear, 180° ($BeCl_2$, $CO_2$).
• $sp^2$: trigonal planar, 120° ($BF_3$).
• $sp^3$: tetrahedral, 109.5° ($CH_4$); with lone pairs, pyramidal $NH_3$ (107°) and bent $H_2O$ (104.5°).
• $sp^3d$: trigonal bipyramidal ($PCl_5$); with lone pairs, see-saw ($SF_4$), T-shaped ($ClF_3$), linear ($XeF_2$).
• $sp^3d^2$: octahedral ($SF_6$); square pyramidal ($BrF_5$), square planar ($XeF_4$).

Shortcut: steric number = σ-bonds + lone pairs on the central atom.

4. Dipole Moment

$\mu = q \times d$ (debye). Symmetric molecules ($CO_2$, $BF_3$, $CH_4$, $CCl_4$) have zero dipole moment even though their bonds are polar. $NH_3$ (1.47 D) has a larger dipole moment than $NF_3$ (0.24 D): in $NH_3$ the lone-pair moment adds to the bond moments, while in $NF_3$ it opposes them.

5. Fajans' Rules

Covalent character in an ionic bond increases with a small, highly charged cation, a large anion, and a cation with a non-noble-gas (pseudo-noble) configuration. So LiI is more covalent than LiF, and $AlCl_3$ is more covalent than NaCl.

6. Hydrogen Bonding

Intermolecular H-bonding raises boiling points (HF, $H_2O$, $NH_3$). Intramolecular H-bonding (o-nitrophenol) lowers boiling point and water solubility compared with the para isomer, which bonds intermolecularly.

7. Bond Parameters

Bond order rises → bond length falls → bond enthalpy rises. Examples: $O_2^{2-}$ (1) < $O_2^-$ (1.5) < $O_2$ (2) < $O_2^+$ (2.5). $N_2$ has bond order 3, and $N_2^+$ is weaker (2.5).

Practice questions

1. The shape of XeF₄ is:

   1. Tetrahedral
   2. Square planar
   3. See-saw
   4. Square pyramidal
   Answer

   B. Xe has 4 bond pairs and 2 lone pairs (sp³d²); the lone pairs sit opposite each other, giving square planar.

2. Which molecule has zero dipole moment?

   1. NH₃
   2. H₂O
   3. CO₂
   4. SO₂
   Answer

   C. CO₂ is linear, so the two C=O bond dipoles cancel.

3. The bond order of O₂⁺ is:

   1. 1.5
   2. 2
   3. 2.5
   4. 3
   Answer

   C. O₂ has bond order 2; removing an antibonding electron raises it to 2.5.

4. Which molecule is paramagnetic?

   1. N₂
   2. O₂
   3. F₂
   4. C₂
   Answer

   B. MOT places two unpaired electrons in the degenerate π^*2p orbitals of O₂.

5. o-Nitrophenol is more volatile than p-nitrophenol because of:

   1. Intermolecular H-bonding in the ortho isomer
   2. Intramolecular H-bonding in the ortho isomer
   3. Higher molar mass of the ortho isomer
   4. Resonance in the para isomer
   Answer

   B. Intramolecular H-bonding in o-nitrophenol prevents association between molecules, lowering the boiling point.