Trends in Chemical Properties of Group 14 Elements and Their Compounds

Lessons

Senior 4 · Chemistry

Trends in Chemical Properties of Group 14 Elements and Their Compounds

5 lessons

Lesson 5 of 5

Uses of Group 14 Elements, and Diagonal Relationships

25 min

From a phone to a fire extinguisher

Open a mobile phone and you find Group 14 everywhere: a silicon chip at its heart, tin-based solder holding every component to the circuit board, and glass made from silicon dioxide over the screen. The same group gives the graphite in your pencil, the carbon dioxide in fire extinguishers and fizzy drinks, and the lead in the battery that starts a minibus. Each use depends on a property you have met in this unit. The second half of the lesson looks back across Units 6 to 9 at a pattern that links the first members of Groups 1, 2 and 13 to the elements diagonally below them.

Definition

Alloy: a mixture of a metal with one or more other elements, made to improve its properties. Example: bronze (copper and tin) is harder than either metal; solder (mainly tin) melts at a low temperature.

Doping: adding a tiny, controlled amount of another element to a semiconductor to change its conductivity. Example: silicon doped with boron or phosphorus in solar cells and computer chips.

Uses of the Group 14 elements and their compounds

Table 9.5: Uses and the properties behind them

Element or compound	Use	Property that makes it suitable
Diamond	cutting tools, drill tips, jewellery	hardest natural substance (rigid 3-D covalent network); sparkles because it bends light strongly
Graphite	pencils, electrodes, dry lubricant	layers slide and rub off; conducts electricity; very high sublimation temperature and unreactive
Coke (carbon)	reducing agent in smelting tin, iron and lead	takes oxygen from metal oxides at high temperature: $\mathrm{SnO_2 + 2C \rightarrow Sn + 2CO}$
Activated charcoal	water filters, gas masks	enormous surface area adsorbs impurities
Carbon dioxide	fire extinguishers; fizzy drinks; dry ice for cooling	denser than air and does not support burning; dissolves under pressure; solid sublimes at −78.5 °C
Silicon	computer chips, solar cells	semiconductor whose conductivity can be controlled by doping
Silicon dioxide and silicates	glass, cement, bricks, ceramics	hard, high-melting giant structures; glass is transparent
Silicones	sealants, waterproofing, heat-resistant rubber	chains of Si–O with organic groups: water-repellent and stable to heat
Germanium dioxide	core of optical fibres	raises the refractive index of silica glass, keeping light inside the core
Tin	coating steel food cans; solder; bronze	resists corrosion and is non-toxic; low melting point (232 °C); hardens copper
Lead and lead(IV) oxide	lead–acid batteries	Pb and $\mathrm{PbO_2}$ react reversibly with sulphuric acid, so the battery can be recharged
Lead	shielding against X-rays and gamma rays	very dense (11.3 g cm⁻³)

A use that ended: leaded petrol

For most of the twentieth century, tetraethyllead was added to petrol to stop engines "knocking". The lead from exhausts spread through air, soil and dust, and harmed the brains of children around the world. Country by country it was banned, and in July 2021, when Algeria stopped selling it, leaded petrol for cars was phased out everywhere. Lead-free solder has replaced most lead-tin solder for the same reason: the World Health Organization notes that no level of lead exposure is known to be without harmful effects.

Worked example 1: a lead–acid battery

When a lead–acid battery delivers current, both plates turn into lead(II) sulphate: $\mathrm{Pb(s) + PbO_2(s) + 2H_2SO_4(aq) \rightarrow 2PbSO_4(s) + 2H_2O(l)}$ (a) Give the oxidation number of lead in each substance and say which is oxidised and which reduced. (b) Use the inert pair effect to explain why the reaction happens.

Solution. (a) Pb: 0; $\mathrm{PbO_2}$ : +4; $\mathrm{PbSO_4}$ : +2. Lead metal is oxidised (0 to +2); lead(IV) oxide is reduced (+4 to +2). (b) +2 is the most stable oxidation state of lead, so both the metal and the +4 oxide move towards it; the energy released drives the current. Charging the battery forces the reaction backwards.

Diagonal relationships across Groups 1, 2, 13 and 14

You have now met three pairs of elements that resemble each other although they are in different groups: lithium and magnesium (Unit 6), beryllium and aluminium (Unit 7) and boron and silicon (Unit 8). In each pair the second element is one period down and one group to the right of the first (Figure 9.9).

Figure 9.9: The diagonal relationships.

Why? Going across a period, the charge on the ion rises and its size falls, so polarising power and electronegativity increase. Going down a group, ions get bigger and electronegativity falls. A diagonal step does both, so the effects partly cancel. Beryllium (1.57) and aluminium (1.61), and boron (2.04) and silicon (1.90), have almost equal electronegativities. For lithium and magnesium the match is in size: $\mathrm{Li^+}$ (76 pm) is almost the same size as $\mathrm{Mg^{2+}}$ (72 pm) and much smaller than $\mathrm{Na^+}$ (102 pm), so it polarises anions far more than the other Group 1 ions. The explanation is approximate, but the chemical evidence is clear.

Table 9.6: Evidence for the diagonal relationships

Pair	Shared behaviour	How the rest of the first element's group differs
Li and Mg	burn in air to the normal oxide ( $\mathrm{Li_2O}$ , MgO); combine with nitrogen ( $\mathrm{Li_3N}$ , $\mathrm{Mg_3N_2}$ ); carbonates decompose on heating; nitrates give $\mathrm{NO_2}$ and $\mathrm{O_2}$ ; carbonate, fluoride and phosphate only slightly soluble	Na to Cs form peroxides or superoxides, no nitrides; their carbonates are stable; their nitrates give only $\mathrm{O_2}$ and a nitrite
Be and Al	amphoteric oxides and hydroxides; covalent chlorides that are Lewis acids and form bridged structures ( $\mathrm{(BeCl_2)_n}$ , $\mathrm{Al_2Cl_6}$ ); made passive by concentrated nitric acid; carbides give methane with water	Mg to Ba have basic oxides and ionic chlorides
B and Si	metalloid semiconductors; acidic, glass-forming oxides; chlorides fully hydrolysed; many volatile hydrides; very hard carbides	Al to Tl are metals with amphoteric or basic oxides

\mathrm{4LiNO_3 \rightarrow 2Li_2O + 4NO_2 + O_2} \qquad \text{but} \qquad \mathrm{2NaNO_3 \rightarrow 2NaNO_2 + O_2}

\mathrm{Be_2C + 4H_2O \rightarrow 2Be(OH)_2 + CH_4} \qquad \mathrm{Al_4C_3 + 12H_2O \rightarrow 4Al(OH)_3 + 3CH_4}

The relationship fades after Group 14: carbon and phosphorus are not much alike.

Worked example 2: using the diagonal relationship to predict

Lithium burns in nitrogen. Predict the product, write the equation, and predict what happens when the product is added to water. Use magnesium as your guide.

Solution. Magnesium forms $\mathrm{Mg_3N_2}$ , which reacts with water to give the hydroxide and ammonia. By analogy, lithium forms lithium nitride, and nitrogen is $\mathrm{N^{3-}}$ , so three $\mathrm{Li^+}$ are needed: $\mathrm{6Li(s) + N_2(g) \rightarrow 2Li_3N(s)}$ $\mathrm{Li_3N(s) + 3H_2O(l) \rightarrow 3LiOH(aq) + NH_3(g)}$ The ammonia can be detected by its smell and because it turns damp red litmus paper blue. Sodium does not form a nitride in this way: the diagonal neighbour is a better guide than the group neighbour.

Warning

Common mistakes. (1) Listing a use without the property that explains it: the property earns the mark. (2) Saying graphite is used in electrodes "because it is soft": the reasons are conductivity, a very high sublimation point and low reactivity. (3) Pairing the wrong elements: the diagonal goes down and to the right (Li–Mg, not Li–Be or Na–Be). (4) Claiming diagonal neighbours have equal charge densities: the similarity is real but approximate.

Safety. Car batteries contain concentrated sulphuric acid and lead compounds: never open one, and wear gloves and eye protection when handling a damaged battery. Old batteries must be recycled through approved collectors, not broken open for their lead, because lead dust is toxic. Solder only in a ventilated space, and wash your hands afterwards.

Tip

For any "use" question, write one sentence in the form "X is used for Y because it is Z". For diagonal relationships, remember the down-and-right step and one fact per pair: Li₃N like Mg₃N₂; Be(OH)₂ amphoteric like Al(OH)₃; B₂O₃ glassy and acidic like SiO₂.

Khan Academy India: Diagonal relationship (7 min). Watch how the video explains the similarity of diagonal neighbours by the opposing trends across a period and down a group, using lithium and magnesium as the main example.

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Practice

For each use, name a Group 14 element or compound and the property that makes it suitable: (a) cutting glass; (b) electrodes for electrolysis; (c) fire extinguishers; (d) food cans; (e) solar cells.
Explain why tin is used to coat steel cans but zinc (which protects steel even better) is not used for food cans.
Why did many countries ban tetraethyllead in petrol?
State what is meant by a diagonal relationship and name the three pairs found in Groups 1, 2, 13 and 14.
Give two reactions in which lithium resembles magnesium rather than sodium, with equations.
Beryllium carbide reacts with water. Predict the products by comparison with aluminium carbide and write the equation.
Calculate the volume of methane, at room temperature and pressure (24.0 dm³ mol⁻¹), formed when 1.44 g of aluminium carbide, $\mathrm{Al_4C_3}$ , reacts with excess water. $A_{r}$ : Al 26.98, C 12.01.
Evaluate this statement: "Lithium is more like magnesium than like sodium."

Show answersHide answers

(a) diamond: hardest substance; (b) graphite: conducts, high sublimation temperature, unreactive; (c) carbon dioxide: denser than air, does not support burning; (d) tin: resists corrosion, non-toxic; (e) silicon: semiconductor that can be doped.
Zinc compounds can dissolve into acidic foods and are less suitable in contact with food; tin is non-toxic and resists attack by food acids.
Lead from exhaust fumes is toxic, especially harming children's brain development; no level of exposure is known to be safe.
A similarity between a Period 2 element and the Period 3 element one group to its right: Li–Mg, Be–Al, B–Si.
Any two: $\mathrm{6Li + N_2 \rightarrow 2Li_3N}$ (like $\mathrm{3Mg + N_2 \rightarrow Mg_3N_2}$ ); $\mathrm{Li_2CO_3 \rightarrow Li_2O + CO_2}$ on heating (like $\mathrm{MgCO_3}$ ); $\mathrm{4LiNO_3 \rightarrow 2Li_2O + 4NO_2 + O_2}$ (like $\mathrm{Mg(NO_3)_2}$ ); $\mathrm{4Li + O_2 \rightarrow 2Li_2O}$ , the normal oxide.
Beryllium hydroxide and methane: $\mathrm{Be_2C + 4H_2O \rightarrow 2Be(OH)_2 + CH_4}$ .
$M(\mathrm{Al_4C_3}) = 4(26.98) + 3(12.01) = 143.95$ ; $n = \frac{1.44}{143.95} = 0.0100$ mol; methane $= 3 \times 0.0100 = 0.0300$ mol; $V = 0.0300 \times 24.0 = 0.720\ \mathrm{dm^3}$ .
Largely true for its compounds: like magnesium, lithium forms a nitride and a normal oxide, and its carbonate and nitrate decompose on heating, all unlike sodium. But lithium is still a Group 1 metal: it forms only a 1+ ion, its hydroxide is a soluble strong base, and it reacts with water like the other alkali metals. So "more like magnesium" holds for the properties that depend on polarising power, not for those that depend on the number of outer electrons.

Quick check

Match each use to the Group 14 material chosen for it.

Tap one on the left, then its partner on the right.

Key points

Each use follows a property: diamond (hard), graphite (conducts, layers slide), coke (reducing agent), $\mathrm{CO_2}$ (dense, non-flammable), silicon (dopable semiconductor), silica (glass), tin (corrosion-resistant, low-melting), lead (dense; Pb/ $\mathrm{PbO_2}$ in batteries).
Lead is toxic; leaded petrol for cars was phased out worldwide in 2021, and lead-free solder has replaced most lead-tin solder.
Diagonal relationships: Li–Mg, Be–Al, B–Si. Across a period polarising power and electronegativity rise; down a group they fall; a diagonal step roughly cancels the two.
Evidence: $\mathrm{Li_3N}$ like $\mathrm{Mg_3N_2}$ ; amphoteric $\mathrm{Be(OH)_2}$ like $\mathrm{Al(OH)_3}$ ; acidic glassy $\mathrm{B_2O_3}$ like $\mathrm{SiO_2}$ .

1From a phone to a fire extinguisher

2Uses of the Group 14 elements and their compounds

3A use that ended: leaded petrol

4Diagonal relationships across Groups 1, 2, 13 and 14

From a phone to a fire extinguisher

Uses of the Group 14 elements and their compounds

A use that ended: leaded petrol

Diagonal relationships across Groups 1, 2, 13 and 14