Physical Properties of Group 14 Elements
Overview of Group 14
Group 14 (also called Group IVA or Group IVB) contains Carbon (C), Silicon (Si), Germanium (Ge), Tin (Sn), and Lead (Pb). All have the outer electron configuration ns²np² giving 4 valence electrons.
This group shows the most dramatic transition from non-metal to metal of any group — carbon is a non-metal, silicon and germanium are metalloids (semiconductors), while tin and lead are true metals.
| Element | Symbol | Z | Electron Config | M.p. (°C) | Character | Common OS |
|---|---|---|---|---|---|---|
| Carbon | C | 6 | [He]2s²2p² | 3550 (diamond) | Non-metal | −4, 0, +2, +4 |
| Silicon | Si | 14 | [Ne]3s²3p² | 1414 | Metalloid / semiconductor | +4, −4 |
| Germanium | Ge | 32 | [Ar]3d¹⁰4s²4p² | 938 | Metalloid / semiconductor | +4, +2 |
| Tin | Sn | 50 | [Kr]4d¹⁰5s²5p² | 232 | Metal (soft, silvery) | +2, +4 |
| Lead | Pb | 82 | [Xe]4f¹⁴5d¹⁰6s²6p² | 327 | Soft, dense metal | +2 (stable), +4 |
Trends Down Group 14
- Atomic radius — increases (more electron shells added)
- Ionisation energy — decreases (outer electrons farther from nucleus, better shielded)
- Electronegativity — decreases (C = 2.5, Si = 1.8, Sn = 1.8, Pb = 1.8)
- Melting point — generally decreases from carbon (very high giant covalent) → Pb (327°C metal)
- Metallic character — increases (C = non-metal → Pb = metal)
- Stability of +4 state — decreases; stability of +2 state increases (inert pair effect, see 9.6)
Carbon and Its Allotropes
Diamond
In diamond, each carbon atom is sp³ hybridised and bonded to four other carbon atoms in a regular tetrahedral arrangement, forming a giant covalent (macromolecular) structure. Bond angle = 109.5°.
Properties of Diamond:
- Extremely hard (hardest natural substance) — all 4 bonds must be broken to cleave it
- Very high melting point (~3550°C) — strong covalent bonds throughout
- Does not conduct electricity — all 4 valence electrons used in bonding, no free electrons
- Transparent to visible light — wide band gap
- Good thermal conductor — vibrations transfer efficiently through the rigid lattice
Graphite
In graphite, each carbon atom is sp² hybridised and bonded to three other carbon atoms in a flat hexagonal layer. Bond angle = 120°. The unhybridised p orbital on each carbon overlaps sideways to form a delocalised π system extending across each layer.
Properties of Graphite:
- Conducts electricity — each C uses only 3 of its 4 valence electrons in σ bonds; the 4th forms the π system with delocalised electrons free to carry charge
- Soft and slippery (lubricant) — layers held by weak van der Waals forces; layers slide over each other easily
- High melting point within layers (strong covalent bonds), but cleaves between layers
- Black/dark grey colour — absorbs all visible light (many π transitions)
- Used in pencils, electrodes, lubricants, crucibles
Fullerenes (C₆₀ — Buckminsterfullerene)
Discovered in 1985. C₆₀ consists of 60 carbon atoms arranged in 12 pentagons and 20 hexagons — resembling a football (soccer ball). Each carbon is sp² hybridised and bonded to 3 others. The molecule is approximately spherical with diameter ~0.7 nm.
Properties: Molecular solid (not giant covalent) — held together by van der Waals forces → low melting point. Poor conductor as a solid but can be doped to become superconducting. Used in nanotechnology, drug delivery research, and as lubricants.
Carbon nanotubes are a related form — sheets of graphene (single graphite layer) rolled into cylinders. Extremely high tensile strength (~100× steel). Conduct electricity along the tube axis.
Graphene
A single layer of graphite — one atom thick. It is the thinnest and one of the strongest materials known. Graphene conducts electricity and heat exceptionally well and is transparent (~97.7% of visible light passes through). It is considered a 2D material.
| Allotrope | Structure | Hybridisation | Bonds/C | Conductivity | Melting Point | Key Use |
|---|---|---|---|---|---|---|
| Diamond | Giant covalent 3D | sp³ | 4 | None | ~3550°C | Cutting tools, jewellery |
| Graphite | Layered hexagonal | sp² | 3 (+ π) | Good (along layers) | ~3600°C (sublime) | Electrodes, pencils, lubricants |
| C₆₀ (Fullerene) | Molecular spheres | sp² | 3 | Poor (undoped) | ~600°C (sublimes) | Nanotechnology, research |
| Graphene | Single hexagonal layer | sp² | 3 (+ π) | Excellent | ~3600°C | Electronics, composites |
Silicon Chemistry
Structure and Properties of Silicon
Silicon has the same giant covalent structure as diamond — each Si is sp³ hybridised and bonded to 4 other Si atoms tetrahedrally. However, Si–Si bonds are weaker than C–C bonds (Si is larger, bonds are longer → lower bond energy).
Properties of Silicon:
- Hard, grey, lustrous solid
- Very high melting point (1414°C) — giant covalent structure
- Semiconductor — small band gap; does not conduct at 0 K but conducts at room temperature (thermal excitation promotes electrons across the gap)
- Reacts slowly with dilute acids but dissolves in concentrated alkali: Si + 2NaOH + H₂O → Na₂SiO₃ + 2H₂↑
Silicon Dioxide (Silica, SiO₂)
SiO₂ has a giant covalent structure — each Si is bonded to 4 oxygen atoms (tetrahedral), and each O bridges two Si atoms. There are NO discrete SiO₂ molecules. The formula SiO₂ is an empirical formula representing the ratio Si:O = 1:2.
Compare: CO₂ is a simple molecular compound (small discrete molecules, low m.p.). SiO₂ is a giant covalent compound (no discrete molecules, very high m.p. ~1713°C).
Reactions of SiO₂:
SiO₂ is an acidic oxide. It does NOT react with water (insoluble) or with most acids except HF.
Silicates
Silicates contain the fundamental unit SiO₄⁴⁻ (a tetrahedron). These tetrahedra can link in various ways — single units, chains, rings, sheets, and 3D networks — giving the huge variety of silicate minerals (quartz, feldspar, mica, clay). Na₂SiO₃ (sodium silicate) is soluble in water — used in detergents and as "water glass."
Oxides of Group 14 Elements
| Element | +4 Oxide | Structure | Acid/Base? | +2 Oxide | Acid/Base? |
|---|---|---|---|---|---|
| Carbon (C) | CO₂ | Simple molecular (O=C=O) | Acidic | CO | Neutral |
| Silicon (Si) | SiO₂ | Giant covalent | Acidic | SiO (rare) | — |
| Germanium (Ge) | GeO₂ | Solid | Amphoteric | GeO | Basic |
| Tin (Sn) | SnO₂ | Solid | Amphoteric | SnO | Basic |
| Lead (Pb) | PbO₂ | Solid | Amphoteric | PbO | Basic/amphoteric |
Carbon Oxides in Detail
Carbon dioxide (CO₂): Linear molecule (O=C=O). Carbon is sp hybridised. Acidic oxide:
Carbon monoxide (CO): Colourless, odourless, extremely toxic gas. CO binds to haemoglobin 200× more strongly than O₂ → prevents O₂ transport → tissue death. CO is a neutral oxide — does not react with water. It is a strong reducing agent:
CO has a dative (coordinate) bond structure: :C≡O: — one of the bonds is dative from O to C. CO is isoelectronic with N₂.
Lead Oxides
Lead forms several oxides: PbO (litharge, yellow), PbO₂ (brown), and Pb₃O₄ (red lead, minium — a mixed oxide containing Pb²⁺ and Pb⁴⁺). PbO is predominantly basic, but also shows slight amphoteric character. PbO₂ is a strong oxidising agent used in lead-acid batteries.
Chlorides of Group 14 Elements
| Chloride | Type | State (room T) | Reaction with Water | Notes |
|---|---|---|---|---|
| CCl₄ | Simple covalent | Liquid | Does NOT hydrolyse — no reaction | No d orbitals on C to accept lone pair; too small for nucleophilic attack |
| SiCl₄ | Simple covalent | Liquid | Vigorous hydrolysis — fumes of HCl | Si has empty 3d orbitals — can expand octet; attacks by H₂O |
| GeCl₄ | Covalent | Liquid | Hydrolysis | Similar to SiCl₄ |
| SnCl₂ | Ionic (Sn²⁺) | Solid | Partial hydrolysis → Sn(OH)Cl↓ | Reducing agent; dissolves in HCl |
| SnCl₄ | Covalent | Liquid | Hydrolysis | — |
| PbCl₂ | Ionic (Pb²⁺) | Solid (white) | Sparingly soluble — slightly soluble in hot water | Precipitate test for Pb²⁺ |
| PbCl₄ | Covalent | Yellow oily liquid | Hydrolysis | Unstable — decomposes to PbCl₂ + Cl₂ |
CCl₄ vs SiCl₄ — Key Comparison
This is one of the most important comparisons in A-Level Group 14:
Question: SiCl₄ is added to excess water. Write the equation and explain what is observed.
SiCl₄ reacts vigorously with water. The Cl⁻ is displaced and HCl is produced.
SiCl₄ + 2H₂O → SiO₂ + 4HCl
Observations: White fumes (HCl gas) produced. White solid or gel (SiO₂ / silicic acid Si(OH)₄) forms. The reaction is exothermic.
Contrast: CCl₄ added to water — no visible reaction (CCl₄ is denser than water, sinks to the bottom, no fumes).
+2 and +4 Oxidation States — The Inert Pair Effect
The Inert Pair Effect
Going down Group 14 (and Groups 13, 15), the +2 oxidation state becomes increasingly stable relative to +4. This is called the inert pair effect.
Explanation: The outermost s electrons (ns²) are called the "inert pair." For heavier elements (Sn, Pb), the ns² electrons are in an orbital that has penetrated through many inner electron shells and the relativistic effects increase. The energy required to unpair and use these 2s electrons for bonding is not recovered by the additional bond energies formed. As a result:
- For Carbon and Silicon: +4 is the dominant stable state (both 2p electrons easily promoted)
- For Germanium: +4 dominant, but +2 possible (GeO, GeCl₂ exist)
- For Tin: both +2 and +4 are stable (SnCl₂ and SnCl₄ both common)
- For Lead: +2 is the dominant stable state; Pb(IV) compounds are strong oxidising agents (unstable)
Question: Explain why PbCl₄ is a stronger oxidising agent than SnCl₄.
PbCl₄ contains Pb in the +4 oxidation state. Due to the inert pair effect, Pb(+4) is unstable relative to Pb(+2) — it has a strong tendency to be reduced to Pb²⁺.
PbCl₄ → PbCl₂ + Cl₂ (easily — releases Cl₂ even at room temperature)
Sn(+4) in SnCl₄ is much more stable — the inert pair effect is less pronounced for Sn than for Pb. SnCl₄ does not readily release Cl₂.
Conclusion: PbCl₄ more readily accepts electrons (is reduced) → stronger oxidising agent.
Tin and Lead — Reactions and Compounds
Reactions of Tin and Lead with Acids and Alkalis
Identification of Sn²⁺ and Pb²⁺ Ions
| Ion | Test Reagent | Observation | Equation |
|---|---|---|---|
| Sn²⁺ | NaOH(aq) | White ppt of Sn(OH)₂; dissolves in excess NaOH (amphoteric) | Sn²⁺ + 2OH⁻ → Sn(OH)₂↓; Sn(OH)₂ + 2OH⁻ → [Sn(OH)₄]²⁻ |
| Pb²⁺ | NaOH(aq) | White ppt of Pb(OH)₂; dissolves in excess NaOH | Pb²⁺ + 2OH⁻ → Pb(OH)₂↓; Pb(OH)₂ + 2OH⁻ → [Pb(OH)₄]²⁻ |
| Pb²⁺ | KI(aq) | Bright yellow ppt of PbI₂ | Pb²⁺ + 2I⁻ → PbI₂↓ (yellow) |
| Pb²⁺ | H₂SO₄(aq) | White ppt of PbSO₄ | Pb²⁺ + SO₄²⁻ → PbSO₄↓ (white) |
| Pb²⁺ | Cl⁻ (HCl) | White ppt PbCl₂; dissolves in hot water | Pb²⁺ + 2Cl⁻ → PbCl₂↓ |
Tin as a Reducing Agent (SnCl₂)
Sn²⁺ is easily oxidised to Sn⁴⁺ — making SnCl₂ a useful reducing agent in the laboratory:
Diagonal Relationship: Boron and Silicon
What is the Diagonal Relationship?
Elements in the second row of the periodic table (Period 2) often show similarities with the element to the lower right (diagonal) in Period 3. This happens because charge density (charge/radius) is similar diagonally.
The most important diagonal relationships are: Li–Mg, Be–Al, and B–Si.
Similarities Between Boron (B) and Silicon (Si)
| Property | Boron (B) | Silicon (Si) |
|---|---|---|
| Electronegativity | 2.0 | 1.8 (similar) |
| Structure of element | Giant covalent | Giant covalent |
| Oxide type | B₂O₃ — acidic | SiO₂ — acidic |
| Oxide structure | Giant covalent / glass-like | Giant covalent |
| Chloride hydrolysis | BCl₃ + 3H₂O → B(OH)₃ + 3HCl | SiCl₄ + 4H₂O → Si(OH)₄ + 4HCl |
| Hydrides | B₂H₆ (diborane) — electron deficient | SiH₄ (silane) — forms covalent hydrides |
| Nature of halides | Covalent, hydrolyse in water | Covalent, hydrolyse in water |
| Forms glassy oxides | B₂O₃ — borosilicate glass component | SiO₂ — major glass component |
Uses of Group 14 Elements and Compounds
| Substance | Use | Reason |
|---|---|---|
| Diamond | Cutting, drilling, polishing tools; jewellery | Hardest natural substance; gemstone |
| Graphite | Pencil "lead"; electrodes; lubricant; nuclear moderator | Soft layered structure; conducts; high m.p. |
| Silicon | Semiconductors, transistors, solar cells, microchips | Semiconductor — band gap can be engineered by doping |
| SiO₂ | Glass, optical fibres, silica gel (desiccant) | Transparent, thermally stable, inert |
| Silicones | Waterproofing, lubricants, sealants, breast implants | Chemically inert, flexible polymer backbone (Si–O–Si) |
| SnO₂ | Transparent conducting layer in LCDs; ceramic glazes | Conducts while transparent when doped |
| Pb in PbO₂ | Lead-acid car batteries (cathode) | Strong oxidising agent, reversible electrode reactions |
| Sn (tinplate) | Food cans — tin coating on steel | Corrosion resistant, non-toxic |
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Exercises
-
State the type of structure and bonding in each allotrope of carbon: (a) diamond, (b) graphite, (c) C₆₀ fullerene. For each, predict whether it conducts electricity, giving reasons.
(a) Diamond: giant covalent 3D network; sp³ C; does NOT conduct — all 4 electrons used in bonding, no free electrons. (b) Graphite: giant covalent layered; sp² C; CONDUCTS along layers — delocalised π electrons from unhybridised p orbitals carry charge. (c) C₆₀: simple molecular (discrete molecules); sp² C; does NOT conduct (as pure solid) — no free charge carriers; the π electrons are localised within each molecule.
-
Explain why SiCl₄ is rapidly hydrolysed by water but CCl₄ is not. Include a description of the mechanism.
CCl₄: carbon's 2p orbitals are fully occupied and 3d orbitals are too high in energy to be used. H₂O cannot donate its lone pair to carbon → no reaction. SiCl₄: silicon has empty, accessible 3d orbitals. H₂O acts as a nucleophile, donating a lone pair from oxygen to Si → forms a 5-coordinate intermediate Si(OH)(Cl₄)⁻ → Cl⁻ is displaced → HCl released. This repeats until SiCl₄ + 2H₂O → SiO₂ + 4HCl.
-
Write equations to show that SnO is basic and SnO₂ is amphoteric.
SnO is basic: SnO + H₂SO₄ → SnSO₄ + H₂O (reacts with acid but NOT with alkali). SnO₂ is amphoteric: SnO₂ + 4HCl → SnCl₄ + 2H₂O (reacts with acid); SnO₂ + 2NaOH + 2H₂O → Na₂[Sn(OH)₆] (reacts with alkali to give hexahydroxostannate(IV)).
-
State the oxidation state of carbon in each compound: (a) CO₂ (b) CO (c) CH₄ (d) CCl₄ (e) C₂H₅OH
(a) CO₂: C + 2(−2) = 0 → C = +4. (b) CO: C + (−2) = 0 → C = +2. (c) CH₄: C + 4(+1) = 0 → C = −4. (d) CCl₄: C + 4(−1) = 0 → C = +4. (e) C₂H₅OH: CH₃– carbon = −3 (3H, 1C); CH₂OH carbon = −1 (2H, 1O, 1C). Average = −2; or calculate each individually.
-
Explain the inert pair effect and use it to explain why PbCl₂ is more stable than PbCl₄.
The inert pair effect: in heavy elements (Sn, Pb), the outermost ns² electrons (s electrons of the outermost shell) become increasingly difficult to use in bonding because the energy released by forming additional bonds does not compensate the energy cost of unpairing and promoting these electrons. For lead: Pb has 6s² electrons that remain as a "lone pair" (inert pair) — Pb(+2) leaves these electrons unbonded. Forming PbCl₄ requires using these s electrons (+4 state) — this is energetically unfavourable. Therefore Pb prefers +2, and PbCl₄ readily decomposes: PbCl₄ → PbCl₂ + Cl₂. PbCl₂ is the stable form.
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Describe the similarities between boron and silicon that constitute the diagonal relationship. Give three specific examples with equations.
Similarities: (1) Both elements have giant covalent structures (very high m.p.); both are semiconductors/poor conductors. (2) Both oxides are acidic and giant covalent: B₂O₃ + 6NaOH → 2Na₃BO₃ + 3H₂O; SiO₂ + 2NaOH → Na₂SiO₃ + H₂O. (3) Both chlorides are covalent and hydrolyse in water: BCl₃ + 3H₂O → B(OH)₃ + 3HCl; SiCl₄ + 2H₂O → SiO₂ + 4HCl. (4) Both form polymeric, glass-like oxides used in glass manufacture (borosilicate glass = SiO₂ + B₂O₃).
Multiple Choice Quiz — 25 Questions
Unit 9: Group 14 Elements and Compounds
25 Questions · Select one answer eachWhat is the outer electron configuration of all Group 14 elements?
Which allotrope of carbon has a giant covalent structure and does NOT conduct electricity?
Why does graphite conduct electricity but diamond does not?
The lubricating property of graphite is due to:
Which statement correctly describes the structure of SiO₂?
SiCl₄ is rapidly hydrolysed by water but CCl₄ is not. What is the main reason?
What are the products when SiCl₄ reacts with excess water?
The inert pair effect explains why, going down Group 14:
Which of the following is the most stable oxidation state for lead in aqueous solution?
Carbon monoxide (CO) is toxic because:
What is the oxidation state of carbon in CO?
Which Group 14 element is used as a semiconductor in computer chips?
The reaction Fe₂O₃ + 3CO → 2Fe + 3CO₂ occurs in the blast furnace. In this reaction, CO acts as:
Which allotrope of carbon has a simple molecular structure?
SiO₂ reacts with molten sodium oxide (Na₂O) to give:
A white precipitate forms when HCl(aq) is added to Pb(NO₃)₂(aq). The precipitate is:
Which of the following correctly states the hybridisation and bond angle in diamond?
The diagonal relationship exists between B and Si because they have similar:
Which property of graphite makes it suitable for use as an electrode?
SnCl₂ acts as a reducing agent in many reactions. What happens to tin during reduction by SnCl₂?
What is observed when NaOH(aq) is added to Pb²⁺(aq) first in small amounts and then in excess?
Which acid reacts with SiO₂ (silica)?
Carbon dioxide (CO₂) has a much lower melting point than silicon dioxide (SiO₂) because:
PbCl₄ decomposes readily at room temperature. The products are:
Which statement about carbon monoxide (CO) is correct?
Unit Test — 50 Marks
Section A — Short Answer
30 marksCompare the structures of diamond and graphite. For each: state the hybridisation, number of bonds per carbon, type of structure, and whether it conducts electricity. [4]
Explain, with reference to orbital theory, why SiCl₄ is hydrolysed by water but CCl₄ is not. Write an equation for the reaction of SiCl₄ with water. [4]
Describe the inert pair effect. Use it to explain the trend in stability of the +2 oxidation state from carbon to lead. Give one specific example for tin and one for lead. [5]
Carbon monoxide and carbon dioxide are both oxides of carbon. For each: (a) state the oxidation state of carbon (b) classify the oxide as acidic, basic, or neutral. Write a relevant equation for each classification. [4]
Describe three similarities in chemistry between boron and silicon that illustrate the diagonal relationship. For each similarity, write a relevant equation. [4] (1 mark for each similarity + equation, 1 mark for explaining the cause)
Identify the precipitate formed in each reaction involving Pb²⁺ ions: (a) Pb²⁺ + NaOH(aq) [small amount] (b) result of (a) + excess NaOH (c) Pb²⁺ + KI(aq) (d) Pb²⁺ + H₂SO₄(aq) (e) Pb²⁺ + HCl(aq), then warm. State the colour and write an equation for each. [5]
Draw and describe the structure of C₆₀ (Buckminsterfullerene). Compare its physical properties to diamond and graphite, explaining the differences. [4]
Section B — Extended Answer
20 marks(a) Compare the physical and chemical properties of SiO₂ and CO₂. Use your knowledge of structure and bonding to explain the differences. [6]
(b) Write equations for three reactions of SiO₂ and three reactions of CO₂. [4]
(a) Describe the trend in metallic character across Group 14. Relate this to the changes in atomic structure going from carbon to lead. [4]
(b) Explain the chemistry of tin and lead with respect to their oxidation states. Why is SnCl₂ a reducing agent and PbO₂ an oxidising agent? [3]
(c) Silicon is described as a semiconductor. Explain what this means, how it differs from conductors and insulators, and why it is used in solar cells. [3]