AQA A-Level Chemistry: Bonding & Structure

The four substances below each adopt a different type of structure.

Substance	Structure type
Magnesium oxide, MgO	Giant ionic
Iodine, I₂	Simple molecular
Diamond (carbon)	Giant covalent (macromolecular)
Magnesium, Mg	Giant metallic

Describe and explain how the bonding and structure of each substance account for its physical properties. In your answer you should refer to melting point, electrical conductivity (as a solid and, where relevant, when molten or in solution) and hardness or volatility, linking each property to the bonding present.

(6 marks)

A student uses the Pauling electronegativity values below to classify the bonding in four substances. As a rough guide they take an electronegativity difference of $\Delta\chi < 0.4$Δχ<0.4 as essentially pure (non-polar) covalent, $0.4 \leq \Delta\chi < 1.8$0.4≤Δχ<1.8 as polar covalent, and $\Delta\chi \geq 1.8$Δχ≥1.8 as predominantly ionic.

Element	H	C	N	O	Na	Cl
Electronegativity	2.1	2.5	3.0	3.5	0.9	3.0

(a) Calculate the electronegativity difference $\Delta\chi$Δχ for the bonds in each of the following, and classify each bond using the student's scheme: N–Cl, C–H, O–H and Na–Cl. Show your working. (4 marks)

(b) A molecule of HCl is described as having a permanent dipole. State which atom carries the $\delta-$δ− charge and explain why, referring to your value of $\Delta\chi$Δχ for a hydrogen–chlorine bond. (2 marks)

A student is given the five species below and asked to deduce the shape and bond angle of each using electron-pair repulsion theory (VSEPR).

Species	Central atom	Bonding pairs on central atom	Lone pairs on central atom
BF₃	B	3	0
NH₄⁺	N	4	0
NH₃	N	3	1
H₂O	O	2	2
CO₂	C	2 (two double bonds)	0

For each species, deduce its molecular shape and its bond angle, and explain any species whose bond angle is less than the ideal for its number of electron pairs. (5 marks)

Oxygen difluoride, OF₂, is a reactive gas in which a central oxygen atom is bonded to two fluorine atoms by single covalent bonds. Oxygen has 6 electrons in its outer shell. Relevant Pauling electronegativities are O = 3.5 and F = 4.0.

(a) Deduce the shape of an OF₂ molecule and its approximate bond angle, explaining your reasoning. (2 marks)

(b) Predict whether OF₂ has an overall permanent dipole moment. Justify your answer with reference to the polarity of the O–F bonds and the shape of the molecule. (2 marks)

(c) OF₂ (Mᵣ = 54) boils at −145 °C. Suggest, in terms of intermolecular forces, one reason why its boiling point is so low. (1 mark)

Two organic compounds with the same molecular formula C₃H₈O (and therefore the same Mᵣ = 60) have very different boiling points.

Compound	Structure	Boiling point / °C
Propan-1-ol	CH₃CH₂CH₂OH	97
Methoxyethane	CH₃OCH₂CH₃	11

Explain, in terms of the type and strength of the intermolecular forces present, why propan-1-ol has a much higher boiling point than methoxyethane even though they have the same Mᵣ. (4 marks)

The ammonium ion, NH₄⁺, contains a dative covalent (coordinate) bond.

(a) State what is meant by a dative covalent bond. (2 marks)

(b) Using the formation of NH₄⁺ from NH₃ and H⁺ as your example, identify the donor atom and explain why H⁺ is able to accept the bond. (1 mark)