Edexcel A-Level Chemistry: Bonding & Structure

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

Substance	Structure type
Calcium fluoride, CaF₂	Giant ionic
Sulfur, S₈	Simple molecular
Silicon(IV) oxide, SiO₂	Giant covalent (macromolecular)
Copper, Cu	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 temperature, 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 classifies the bonding in four substances from Pauling electronegativity values. As a 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 < 1.7$0.4≤Δχ<1.7 as polar covalent, and $\Delta\chi \geq 1.7$Δχ≥1.7 as predominantly ionic.

Element	H	Si	P	S	Cl	K
Electronegativity	2.1	1.8	2.1	2.5	3.0	0.8

(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: P–H, S–Cl, Si–Cl and K–Cl. Show your working. (4 marks)

(b) A molecule of HBr has a permanent dipole. Given that bromine has an electronegativity of 2.8, state which atom carries the $\delta-$δ− charge and explain why, referring to a calculated value of $\Delta\chi$Δχ for the hydrogen–bromine bond. (2 marks)

A student is given the five species below and asked to deduce the shape and bond angle(s) of each using electron-pair repulsion theory (VSEPR). Several involve a central atom with an expanded octet.

Species	Central atom	Bonding pairs (electron domains from bonds)	Lone pairs on central atom
PF₅	P	5	0
SF₆	S	6	0
SF₄	S	4	1
ClF₃	Cl	3	2
SO₂	S	2 (two double bonds)	1

For each species, deduce its molecular shape and its bond angle(s), and explain, for SF₄ and ClF₃, why a trigonal-bipyramidal arrangement of five electron domains leads to the shape you give. (5 marks)

Dichlorine monoxide, Cl₂O, is a reactive gas in which a central oxygen atom is bonded to two chlorine atoms by single covalent bonds. Oxygen has 6 electrons in its outer shell. Relevant Pauling electronegativities are O = 3.5 and Cl = 3.0.

(a) Deduce the shape of a Cl₂O molecule and its approximate bond angle, explaining your reasoning. (2 marks)

(b) Predict whether Cl₂O has an overall permanent dipole moment. Justify your answer with reference to the polarity of the O–Cl bonds and the shape of the molecule, and state clearly which atom carries the $\delta-$δ− charge. (2 marks)

(c) Cl₂O (Mᵣ = 87) boils at about 2 °C. Suggest, in terms of intermolecular forces, one reason why its boiling temperature is so low. (1 mark)

The boiling temperatures of the four hydrogen halides are shown below.

Hydrogen halide	HF	HCl	HBr	HI
Boiling temperature / °C	−20	−85	−67	−35

Explain, in terms of intermolecular forces, (a) why the boiling temperature increases steadily from HCl to HBr to HI, and (b) why HF, the first member of the series, does not fit this trend and instead has an unexpectedly high boiling temperature. (4 marks)

Aluminium is a typical metal that is widely used because it conducts electricity well and can be hammered and drawn into shapes.

(a) Describe the metallic bonding present in solid aluminium. (2 marks)

(b) Using your description, explain why aluminium is malleable (it can be hammered into shape without breaking). (1 mark)