The Carbon Group: Occurrence, Structures and Physical Properties – Trends in Chemical Properties of Group 14 Elements and Their Compounds (Senior 4 Chemistry) | Ganzaa

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 1 of 5

The Carbon Group: Occurrence, Structures and Physical Properties

25 min

From pencil "lead" to tin ore

Rwanda is one of Africa's producers of tin ore: in 2019 its mines reported 3,727 tonnes of cassiterite, tin(IV) oxide, much of it from the Rutongo mines in Rulindo District. The tin from such ore coats the steel of food cans and joins the parts of every circuit board. Tin belongs to Group 14 together with carbon, silicon, germanium and lead, and this group shows the change from non-metal to metal more clearly than any other: at the top, carbon forms the hardest natural substance (diamond) and a soft, slippery solid (graphite); in the middle, silicon and germanium are the semiconductors of electronics; at the bottom, tin and lead are soft, low-melting metals. This lesson explains that change.

Definition

Group 14 (the carbon group): carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb) and the artificial element flerovium (Fl). Each atom has the outer configuration $ns^2\,np^2$ . The common oxidation states are +4 and, increasingly down the group, +2. Example: $\mathrm{Si}$ is $[\mathrm{Ne}]\,3s^2\,3p^2$ and is +4 in $\mathrm{SiO_2}$ ; lead is +2 in $\mathrm{PbO}$ .

Allotropes: different structural forms of the same element in the same physical state. Example: diamond, graphite and the fullerene $\mathrm{C_{60}}$ are allotropes of carbon; white tin and grey tin are allotropes of tin.

Semiconductor: a solid whose electrical conductivity lies between that of a metal and an insulator, and increases as the temperature rises. Example: pure silicon and germanium.

Where they occur

Table 9.1: Sources of the Group 14 elements

Element	Main sources	Formula
Carbon	coal, graphite and diamond; carbonate rocks such as limestone; carbon dioxide in air; methane in natural gas	C; $\mathrm{CaCO_3}$ ; $\mathrm{CO_2}$ ; $\mathrm{CH_4}$
Silicon	sand and quartz; silicate minerals and clays. Silicon is the second most abundant element in the crust (28.2% by mass), after oxygen	$\mathrm{SiO_2}$ ; silicates
Germanium	a by-product of zinc refining and of coal ash	no common ore
Tin	cassiterite	$\mathrm{SnO_2}$
Lead	galena	PbS

Tin and lead are extracted by reduction with carbon. Cassiterite is heated with coke: $\mathrm{SnO_2 + 2C \rightarrow Sn + 2CO}$ . Galena is first roasted in air to the oxide, which is then reduced: $\mathrm{2PbS + 3O_2 \rightarrow 2PbO + 2SO_2}$ , then $\mathrm{PbO + C \rightarrow Pb + CO}$ . Silicon for steel-making is made in an electric arc furnace, $\mathrm{SiO_2 + 2C \rightarrow Si + 2CO}$ , and then purified much further for electronics.

Physical properties down the group

Table 9.2: The Group 14 elements

Property	C	Si	Ge	Sn	Pb
Atomic number	6	14	32	50	82
Outer configuration	$2s^2\,2p^2$	$3s^2\,3p^2$	$3d^{10}\,4s^2\,4p^2$	$4d^{10}\,5s^2\,5p^2$	$4f^{14}\,5d^{10}\,6s^2\,6p^2$
Covalent radius / pm	75	116	121	140	144
First ionisation energy / kJ mol⁻¹	1086.5	786.5	762	708.6	715.6
Electronegativity (Pauling)	2.55	1.90	2.01	1.96	2.33*
Melting point / °C	sublimes, about 3600	1414	938	232	327
Density / g cm⁻³	2.27 (graphite); 3.51 (diamond)	2.33	5.32	7.29	11.34
Structure	giant covalent (diamond, graphite)	giant covalent (diamond type)	giant covalent (diamond type)	metallic (white tin)	metallic
Electrical behaviour	diamond insulator; graphite conducts	semiconductor	semiconductor	metal	metal

*The Pauling value for lead in its +4 state; for lead(II) the value is 1.87.

Why the character changes

Non-metal to metal. Down the group the atoms get bigger and the outer electrons are held less tightly, so they are shared less in localised covalent bonds and more in a delocalised "sea". Carbon is a non-metal, silicon and germanium are metalloids, tin and lead are metals.
Melting points fall (Figure 9.1). Carbon, silicon and germanium form giant covalent lattices in which every atom is joined to four others; melting breaks strong covalent bonds, and the bonds weaken as the atoms grow (C–C stronger than Si–Si stronger than Ge–Ge). Tin and lead are metals with weaker bonding between large atoms. Lead melts a little higher than tin because the structures of the two metals differ.
Ionisation energy falls steeply from carbon to silicon, then only slowly (Figure 9.2). As in Group 13, the poorly shielding 3d electrons hold germanium's outer electrons almost as tightly as silicon's, and the 4f electrons make lead's first ionisation energy (715.6) slightly higher than tin's (708.6).
Conductivity: diamond is an insulator because all four outer electrons of every atom are fixed in C–C bonds. In graphite each carbon uses three electrons for bonds within its layer, and the fourth is delocalised across the layer, so graphite conducts along the layers. Silicon and germanium are semiconductors: a few bonding electrons gain enough energy to move, more as the temperature rises.

Figure 9.1: Melting points of the Group 14 elements. *Carbon (graphite) does not melt at atmospheric pressure; it sublimes at about 3600 °C.

Figure 9.2: First ionisation energies of the Group 14 elements.

Two forms of tin

Ordinary white tin is a shiny, soft metal. Below 13.2 °C it can slowly change into grey tin, a brittle, powdery solid with the same diamond-type structure as germanium. The change is slow, and it starts at points on the surface that spread like a disease, so it was called "tin pest". It is why tin objects left for long periods in very cold buildings can crumble. Grey tin is less dense, so the metal expands and falls apart as it changes.

Worked example 1: how much tin is in tin ore?

(a) Calculate the percentage by mass of tin in pure cassiterite, $\mathrm{SnO_2}$ . $A_{r}$ : Sn 118.71, O 16.00. (b) Rwanda reported 3,727 t of cassiterite concentrate in 2019. If the concentrate were 70% $\mathrm{SnO_2}$ by mass (an illustrative figure), what mass of tin could it give?

Solution. (a) $M(\mathrm{SnO_2}) = 118.71 + 2(16.00) = 150.71\ \mathrm{g\,mol^{-1}} \qquad \%\,\mathrm{Sn} = \frac{118.71}{150.71} \times 100 = 78.77\%$ (b) Mass of $\mathrm{SnO_2}$ $= 0.70 \times 3727 = 2609\ \mathrm{t}$ ; mass of tin $= 0.7877 \times 2609 = 2055\ \mathrm{t}$ , about 2100 t to two significant figures (the 70% has only two).

Worked example 2: explaining a property from structure

Diamond and graphite are both pure carbon. Explain why (a) diamond is much harder than graphite, (b) graphite conducts electricity but diamond does not, and (c) both have very high sublimation temperatures.

Solution.

(a) In diamond every atom is joined by strong covalent bonds to four others in all three directions, so no plane of the crystal can slide. In graphite the layers are held to each other only by weak London forces (they are 335 pm apart, compared with 142 pm between bonded atoms in a layer), so the layers slide over each other.
(b) In graphite each carbon atom forms three bonds and its fourth outer electron is delocalised over the layer, so it can move when a voltage is applied. In diamond all four outer electrons are in localised bonds.
(c) Both are giant covalent structures: to turn either into vapour, strong C–C covalent bonds must be broken, not just the forces between the layers.

Warning

Common mistakes. (1) Saying graphite is soft because its covalent bonds are weak: the bonds within a layer are stronger than those in diamond; only the forces between layers are weak. (2) Calling silicon a metal because it is shiny: it is a brittle semiconductor, a metalloid. (3) Expecting first ionisation energy to fall all the way down the group: lead's is slightly higher than tin's. (4) Thinking "pencil lead" contains lead: it is graphite mixed with clay.

Safety. Lead and its compounds are toxic, and the World Health Organization states that no level of exposure to lead is known to be without harmful effects; young children are the most at risk. Handle lead samples with gloves or wash your hands at once afterwards, never eat or drink in the laboratory, and never heat lead or its compounds outside a fume cupboard.

Tip

Group 14 in one line: a non-metal (C), two metalloids (Si, Ge) and two metals (Sn, Pb). Melting points collapse where giant covalent structures give way to metallic bonding, and the d and f electrons that shield poorly make germanium and lead hold their electrons more tightly than you would expect.

Professor Dave Explains: Periodic Table Part 5, the Carbon Group (11 min). Watch for the allotropes of carbon, why silicon and germanium are semiconductors, and how tin and lead differ from the elements above them.

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Practice

Write the full electron configurations of silicon and of germanium, and the outer configuration shared by Group 14.
Name the main ore of tin and of lead, and write an equation for the reduction of each ore (or its roasted product) by carbon.
Classify each Group 14 element as non-metal, metalloid or metal, and give one physical property that supports each classification.
Explain why silicon melts at 1414 °C but tin melts at 232 °C.
Use Table 9.2 to describe the trend in first ionisation energy down Group 14, and explain the one irregularity.
Calculate the percentage by mass of lead in galena, PbS. $A_{r}$ : Pb 207.2, S 32.07.
A tin organ pipe kept in an unheated church through a very cold winter became covered in grey, crumbling patches. Explain what happened.

Show answersHide answers

Si: $1s^2\,2s^2\,2p^6\,3s^2\,3p^2$ ; Ge: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^{10}\,4s^2\,4p^2$ ; $ns^2\,np^2$ .
Cassiterite, $\mathrm{SnO_2 + 2C \rightarrow Sn + 2CO}$ ; galena, roasted to PbO, then $\mathrm{PbO + C \rightarrow Pb + CO}$ .
C non-metal (diamond is an insulator; graphite is brittle and soft); Si and Ge metalloids (shiny but brittle semiconductors); Sn and Pb metals (malleable, good conductors, metallic bonding).
Silicon is a giant covalent lattice: melting breaks many strong Si–Si bonds. Tin is a metal with relatively weak metallic bonding between large atoms.
It decreases from C (1086.5) to Sn (708.6) because the outer electrons are further from the nucleus and better shielded; the fall from Si to Ge is small because 3d electrons shield poorly. Lead (715.6) is slightly higher than tin because the 4f and 5d electrons shield poorly (and the 6s electrons are pulled in).
$M(\mathrm{PbS}) = 207.2 + 32.07 = 239.27$ ; % Pb $= \frac{207.2}{239.27} \times 100 = 86.6\%$ .
Below 13.2 °C white tin slowly turns into grey tin (tin pest), a brittle, powdery allotrope with a diamond-type structure and lower density, so the metal swells and crumbles where the change has started.

Quick check

Graphite conducts electricity but diamond does not. Why?

Key points

Group 14 atoms are $ns^2\,np^2$ ; the oxidation states are +4 and +2, with +2 more important lower down.
Carbon (coal, graphite, diamond, carbonates), silicon (silica, silicates; 28.2% of the crust), tin (cassiterite, $\mathrm{SnO_2}$ , mined in Rwanda) and lead (galena, PbS) are the main sources; tin and lead are extracted by reduction with carbon.
Down the group the elements change from non-metal (C) through metalloids (Si, Ge) to metals (Sn, Pb); melting points fall sharply where giant covalent structures give way to metallic bonding.
Diamond (four bonds per atom, insulator, very hard) and graphite (layers, delocalised electrons, conducts, soft) are allotropes of carbon; white tin changes slowly to brittle grey tin below 13.2 °C.

Sources and credits

Image: Cassiterite4.jpg by Didier Descouens, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Image: Diamond and graphite.jpg by User:Itub, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Image: Polycrystalline-germanium.jpg by Jurii, via Wikimedia Commons, licensed under CC BY 3.0.
Rwanda's cassiterite output and the Rutongo mines: The Mineral Industry of Rwanda, 2019, U.S. Geological Survey Minerals Yearbook (public domain).
Data: ionisation energies from Wikipedia's ionisation energies data page; covalent radii from the atomic radii data page; crustal abundance from Abundance of elements in Earth's crust; lead's electronegativities from Lead; the tin transition from Tin; graphite bond and layer distances from Graphite; all CC BY-SA 4.0, adapted by Ganzaa. Melting points, densities and electronegativities from the PubChem Periodic Table, National Library of Medicine.
Lead and health: Lead poisoning and health, World Health Organization fact sheet.
Video: Periodic Table Part 5: Carbon Group (C, Si, Ge, Sn, Pb, Fl), Professor Dave Explains, embedded from YouTube.
The text, charts, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1From pencil "lead" to tin ore

2Where they occur

3Physical properties down the group

4Why the character changes

5Two forms of tin

From pencil "lead" to tin ore

Where they occur

Physical properties down the group

Why the character changes

Two forms of tin