Structure: Models of Bonding and Structure — Free Chemistry SL Practice Questions

Sodium chloride (NaCl) and magnesium oxide (MgO) are both ionic compounds with the same face-centred cubic lattice arrangement. The melting point of NaCl is $801\,^\circ\text{C}$ and that of MgO is $2852\,^\circ\text{C}$. The lattice enthalpies are: NaCl $= -788\,\text{kJ mol}^{-1}$; MgO $= -3795\,\text{kJ mol}^{-1}$. Ionic radii: $\text{Na}^+ = 102\,\text{pm}$, $\text{Cl}^- = 181\,\text{pm}$, $\text{Mg}^{2+} = 72\,\text{pm}$, $\text{O}^{2-} = 140\,\text{pm}$.

Calcium fluoride ($\text{CaF}_2$) is an ionic compound in which $\text{Ca}^{2+}$ ions form a face-centred cubic (FCC) lattice and $\text{F}^-$ ions occupy all tetrahedral holes.

Sodium chloride (NaCl) and caesium chloride (CsCl) are ionic compounds that adopt different lattice structures. In NaCl, each ion is surrounded by six oppositely charged ions (coordination number 6); in CsCl, each ion is surrounded by eight oppositely charged ions (coordination number 8). Ionic radii: $\text{Na}^+ = 102\,\text{pm}$, $\text{Cs}^+ = 167\,\text{pm}$, $\text{Cl}^- = 181\,\text{pm}$. Critical radius ratio ranges: coordination number 4 requires $0.225 \leq \frac{r_+}{r_-} < 0.414$; coordination number 6 requires $0.414 \leq \frac{r_+}{r_-} < 0.732$; coordination number 8 requires $\frac{r_+}{r_-} \geq 0.732$.

Zinc sulfide (ZnS) can crystallize in two different lattice structures: the zinc blende (sphalerite) structure and the wurtzite structure. In both structures, the coordination number of $\text{Zn}^{2+}$ is 4.

Sodium chloride (NaCl) and caesium chloride (CsCl) are both ionic compounds but adopt different crystal lattice structures.

A student investigates the solubility of tetrachloromethane ($\text{CCl}_4$) and trichloromethane ($\text{CHCl}_3$) in water. The student predicts that $\text{CCl}_4$ will be insoluble in water, while $\text{CHCl}_3$ will show some solubility. Electronegativity values: $\chi(\text{H}) = 2.20$, $\chi(\text{C}) = 2.55$, $\chi(\text{Cl}) = 3.16$, $\chi(\text{O}) = 3.44$

Dinitrogen monoxide ($\text{N}_2\text{O}$) is a linear molecule with the arrangement N–N–O. The measured bond lengths are: N–N bond $= 112\,\text{pm}$ and N–O bond $= 119\,\text{pm}$. Electronegativity values: N $= 3.04$, O $= 3.44$. Compare: typical N–N single bond $\approx 145\,\text{pm}$, N=N double bond $\approx 125\,\text{pm}$, N≡N triple bond $\approx 110\,\text{pm}$.

Silicon tetrafluoride, $\text{SiF}_4$, and sulfur tetrafluoride, $\text{SF}_4$, are both covalent molecules containing four fluorine atoms. Their molecular polarities differ significantly. Electronegativity values (Pauling scale): $\text{Si} = 1.90$, $\text{S} = 2.58$, $\text{F} = 3.98$ Boiling points: $\text{SiF}_4 = -86\,^\circ\text{C}$; $\text{SF}_4 = -38\,^\circ\text{C}$

Carbon dioxide ($\text{CO}_2$) and sulfur dioxide ($\text{SO}_2$) are both oxides of non-metals. A student measures their dipole moments: $\text{CO}_2 = 0\,\text{D}$; $\text{SO}_2 = 1.63\,\text{D}$. Electronegativity values: $\text{C} = 2.55$, $\text{S} = 2.58$, $\text{O} = 3.44$

A student compares two simple covalent hydrides: methane ($\text{CH}_4$) and ammonia ($\text{NH}_3$). Their boiling points are $-161.5\,^\circ\text{C}$ and $-33.3\,^\circ\text{C}$ respectively. Electronegativity values: $\text{C} = 2.55$, $\text{H} = 2.20$, $\text{N} = 3.04$

The aerospace alloy Duralumin (approximately $94\%$ aluminium, $4\%$ copper, $1\%$ magnesium, $1\%$ manganese) is stronger than pure aluminium yet maintains good electrical conductivity.

Indium tin oxide (ITO) is a ceramic material used in smartphone touchscreens. It is composed of approximately $90\%$ indium(III) oxide ($\text{In}_2\text{O}_3$) and $10\%$ tin(IV) oxide ($\text{SnO}_2$). When $\text{Sn}^{4+}$ ions substitute for $\text{In}^{3+}$ ions in the indium oxide lattice, the material becomes electrically conductive while remaining transparent.

The electrical conductivity of metals is explained by the metallic bonding model, in which delocalized electrons move freely through a lattice of positive ions. Data for copper at $298\,\text{K}$: - Electrical conductivity: $\sigma = 5.96 \times 10^{7}\,\text{S\,m}^{-1}$ - Conductivity decreases by approximately $0.39$ percent per kelvin increase in temperature - Number density of free electrons: $n = 8.49 \times 10^{28}\,\text{m}^{-3}$ - Electron charge: $e = 1.60 \times 10^{-19}\,\text{C}$ - Effective electron mass: $m_e = 9.11 \times 10^{-31}\,\text{kg}$ $$\sigma = \frac{n e^2 \tau}{m_e}$$

The metallic bonding model explains the high electrical conductivity of metals through the presence of a 'sea' of delocalized electrons. However, not all metals conduct electricity equally well.

A student investigates the properties of three different solid substances at room temperature ($298\,\text{K}$) and standard pressure. - Substance X: Diamond — a giant covalent network solid - Substance Y: Solid iodine ($\text{I}_2$) — a simple molecular solid - Substance Z: Sodium chloride (NaCl) — an ionic lattice The student measures the electrical conductivity of each substance in the solid state and again after melting. Results are shown below. Substance — Conductivity (solid) — Conductivity (liquid) Diamond — Does not conduct Iodine — Does not conduct Sodium chloride — Does not conduct — Conducts

A materials scientist investigates two carbon-based materials for lightweight bicycle frames. Material X is a composite of long, aligned carbon fibres embedded in a polymer matrix. The fibres have strong covalent bonding along their length; only weak intermolecular forces act between the fibres and the polymer. Material Y is a single crystal of pure graphite, consisting of layers of carbon atoms arranged in hexagons. Within each layer, carbon atoms are joined by strong covalent bonds; the layers are held together only by weak London dispersion forces. A tensile strength test (pulling until fracture) gives the following results: Material — Tensile strength / MPa — Fracture surface X — 3500 — clean, straight Y — 25 — jagged, uneven The density of Material X is $1.60\,\text{g cm}^{-3}$ and the density of Material Y is $2.09\,\text{g cm}^{-3}$.

Shape memory alloys (SMAs) such as Nitinol (NiTi) exhibit a reversible martensitic phase transformation. In the low-temperature martensite phase, the alloy deforms easily by twinning (reorientation of crystal domains). On heating, it transforms to the austenite phase and recovers its original shape. A Nitinol wire of original length $10.0\,\text{cm}$ is compressed in the martensite phase to $9.5\,\text{cm}$. On heating to the austenite phase, it recovers its original length, exerting a constant stress of $200\,\text{MPa}$ throughout recovery.

Ultra-high molecular weight polyethylene (UHMWPE) is a polymer used as a bearing surface in artificial hip replacements. A composite material, UHMWPE reinforced with short carbon fibres (CF-UHMWPE), has been developed to improve wear resistance.

Gold nanoparticles exhibit surface plasmon resonance (SPR), in which conduction electrons oscillate collectively in response to incident light, causing strong absorption at a characteristic peak wavelength. For spherical gold nanoparticles suspended in water, the SPR peak shifts to longer wavelengths as particle diameter increases. This property is exploited in lateral flow immunoassays used in medical diagnostics.