Chemical Bonding

Q1. (4 marks)

• Definition (1): Formal charge = (valence electrons) − (non-bonding electrons) − ½(bonding electrons).
• Central O (double + single bond, 1 lone pair): FC = $6 - 2 - \frac{1}{2}(6) = 6-2-3 = +1$ (1)
• Double-bonded terminal O (2 lone pairs): FC = $6 - 4 - \frac{1}{2}(4) = 6-4-2 = 0$ (1)
• Single-bonded terminal O (3 lone pairs): FC = $6 - 6 - \frac{1}{2}(2) = 6-6-1 = -1$ (1)
• Net charge = $+1+0-1 = 0$ ✓

Q2. (4 marks)

• Octet rule: atoms tend to gain, lose or share electrons to achieve 8 electrons in valence shell (noble gas config). (1)
• (a) Incomplete octet: $BF_3$ (B has 6 e⁻) / $BeCl_2$ (1)
• (b) Expanded octet: $SF_6$ / $PCl_5$ (1)
• (c) Odd-electron: $NO$ / $NO_2$ / $ClO_2$ (1)

Q3. (4 marks)

• Fajan's rules — covalent character increases with: (2, ½ each)
  • small cation size, large anion size, high charge on ions, cation with pseudo-noble-gas (d-electron) configuration.
• Anion size: $Cl^- < Br^- < I^-$; larger anion → more polarizable → more covalent. (1)
• Order of covalent character: $LiCl < LiBr < LiI$. (1)

Q4. (5 marks)

• Total electrons in $O_2$ = 16. (1)
• Config: \sigma_{1s}^2\,\sigma^*_{1s}^2\,\sigma_{2s}^2\,\sigma^*_{2s}^2\,\sigma_{2p_z}^2\,\pi_{2p_x}^2\pi_{2p_y}^2\,\pi^*_{2p_x}^1\pi^*_{2p_y}^1 (2)
• Bond order = ½(bonding − antibonding) = ½(10 − 6) = 2 (1)
• Two unpaired electrons in $\pi^*$ orbitals → paramagnetic. (1)

Q5. (4 marks, 1 each)

• (a) $NH_3$: pyramidal, ~107° (sp³, 1 lp)
• (b) $H_2O$: bent, ~104.5° (sp³, 2 lp)
• (c) $BF_3$: trigonal planar, 120°
• (d) $SF_6$: octahedral, 90°

Q6. (4 marks)

• $\mu = q \cdot d$ (1)
• $q = 0.16 \times 1.6\times10^{-19} = 2.56\times10^{-20}\ \text{C}$ (1)
• $\mu = 2.56\times10^{-20} \times 1.27\times10^{-10} = 3.2512\times10^{-30}\ \text{C·m}$ (1)
• In Debye: $\mu = \frac{3.2512\times10^{-30}}{3.33\times10^{-30}} \approx 0.976\ D$ (1)

Q7. (3 marks)

• $CO_2$ is linear; the two C=O bond dipoles are equal and opposite → vector sum = 0 → non-polar. (1.5)
• $H_2O$ is bent (~104.5°); the two O–H bond dipoles do not cancel → net dipole → polar. (1.5)

Q8. (4 marks, 1 each)

• (a) $BeCl_2$: sp
• (b) $CH_4$: sp³
• (c) $PCl_5$: sp³d
• (d) $SF_6$: sp³d²

Q9. (3 marks)

• Water molecules form strong intermolecular hydrogen bonds (O highly electronegative, small). (1.5)
• $H_2S$ has only weak van der Waals/dipole forces (S too large & less electronegative to H-bond). More energy needed to break H-bonds in water → higher boiling point. (1.5)

Q10. (5 marks — definition 2, any 3 differences 3)

Award 1 per correct distinction (max 3) + 2 for correct overlap descriptions.

[
  {"claim":"Ozone formal charges sum to zero (+1, 0, -1)","code":"central=6-2-Rational(6,2); dbl=6-4-Rational(4,2); sgl=6-6-Rational(2,2); result=(central==1 and dbl==0 and sgl==-1 and central+dbl+sgl==0)"},
  {"claim":"O2 bond order = 2","code":"bo=Rational(10-6,2); result=(bo==2)"},
  {"claim":"Dipole moment approx 0.976 D","code":"q=0.16*1.6e-19; d=1.27e-10; mu=q*d; D=mu/3.33e-30; result=abs(D-0.976)<0.01"},
  {"claim":"O2 total valence electron count is 16","code":"result=(8+8==16)"}
]