Concept 04 / 09The p-Block Elements
Group 14 Elements: Trends, Catenation and Allotropes of Carbon
General characteristics of carbon, silicon, germanium, tin and lead: oxidation states and inert pair effect, catenation and multiple bonding, anomalous carbon, and the structures and properties of diamond, graphite, fullerenes and other forms of carbon.
Group 14: carbon and silicon (non-metals), germanium (metalloid), tin and lead (metals). Configuration .
Trends. Atomic radius increases down the group (with only a small increase from Si to Ge and from Sn to Pb, because of poorly shielding d and f electrons). Ionisation enthalpies are higher than group 13 and generally decrease down the group. Electronegativity is fairly constant from Si to Pb (about to ).
Oxidation states. and . The inert pair effect makes more stable down the group: is a reducing agent (tends to ), is a reducing agent, while is a strong oxidising agent () and is the stable state. In the state, the group forms mainly covalent compounds; Si and heavier elements can expand their covalency beyond using d orbitals (, ), but carbon cannot. Hence is not hydrolysed, while is: (water attacks the vacant d orbital of Si).
Catenation. The ability to form chains and rings with itself. It depends on the strength of the E-E bond: C-C , Si-Si , Ge-Ge , Sn-Sn . Carbon catenates far more than any other element, the basis of organic chemistry; lead shows none.
Anomalous carbon. Small size, high electronegativity, no d orbitals: maximum covalency ; forms strong - multiple bonds with itself and with O and N (, , ); heavier members do not, so is a gas of discrete molecules while is a network solid.
Allotropes of carbon (see figure).
Diamond: each C is and tetrahedrally bonded to four others (C-C ) in a rigid 3D network. Hardest natural substance, very high melting point, electrical insulator (no free electrons), excellent heat conductor. Used in cutting tools and jewellery.
Graphite: layers of carbon in hexagonal rings (C-C ), with one delocalised electron per atom; layers held by weak van der Waals forces, apart. Soft and slippery (lubricant), good electrical conductor along the layers. Graphite is the thermodynamically most stable form ( of diamond is ); diamond is made from graphite at very high pressure and temperature.
Fullerenes: discovered in 1985; (buckminsterfullerene) is a soccer-ball molecule of five-membered and six-membered rings, all carbon. It is the only pure molecular form of carbon, soluble in organic solvents.
Other forms: graphene (a single graphite layer), carbon nanotubes, and microcrystalline forms such as coke, charcoal and carbon black (impure graphite).
Key Takeaways:
- Stability of increases down the group: stable, oxidant, reductant.
- Catenation: C >> Si > Ge ≈ Sn; follows E-E bond strength.
- does not hydrolyse (no d orbitals); does.
- Diamond insulator; graphite conductor; has pentagons and hexagons.
- Pitfall: graphite, not diamond, is the standard state of carbon.
Brain-Teaser 1: Why is a strong oxidising agent while is not?
Solution: The inert pair effect makes the stable state for lead, so readily gains two electrons (as in ). For tin, is the more stable state, so has little tendency to be reduced.
Brain-Teaser 2: Graphite conducts electricity but diamond does not. Explain from hybridisation.
Solution: In diamond, all four valence electrons of each carbon are used in localised bonds, leaving no mobile electrons. In graphite, each carbon uses three electrons in bonds; the fourth occupies a p orbital perpendicular to the layer, and these overlap into a delocalised system that allows electrons to move along the layers.
Brain-Teaser 3: How many carbon-carbon single and double bonds are in one Kekule structure of ?
Solution: Each carbon has three neighbours, so the number of C-C bonds . Each carbon has one double bond, so there are double bonds and single bonds.
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