Why Carbon Forms Long Chains but Silicon Forms Rock

Carbon forms one of the hardest natural substances and one of the softest; silicon, just below it, builds sand, rocks, glass and the chips inside every phone. Group 14 shows how a change in atomic size can transform an element's chemistry.

This lesson covers the allotropes of carbon, silicon, silicates and silicones, and the contrast between the chemistry of carbon and silicon.

Carbon exists as crystalline allotropes — diamond, graphite and fullerenes — and as amorphous forms such as charcoal and coke, and their very different properties come from how the carbon atoms are bonded.

Diamond:

- Each carbon is $\mathrm{s}{\mathrm{p}}^3$ hybridised and bonded to four others in a rigid three-dimensional network, with C-C bonds 154 pm long
- Extremely hard, with a very high melting point
- A poor conductor of electricity, because every valence electron is held in a bond
- Uses: cutting and grinding tools, and jewellery

Graphite:

- Each carbon is $\mathrm{s}{\mathrm{p}}^2$ hybridised and bonded to three others in flat hexagonal layers, with C-C bonds 141.5 pm long
- The layers are 340 pm apart and held by weak forces, so they slide over one another
- The fourth electron of each carbon is delocalised, so graphite conducts electricity along its layers
- Uses: lubricant, electrodes and pencil cores

Fullerenes:

- Cage-like molecules; the best known, ${\mathrm{C}}_{60}$, is shaped like a football, with 20 hexagons and 12 pentagons
- Each carbon is $\mathrm{s}{\mathrm{p}}^2$ hybridised, and fullerenes are made of discrete molecules rather than an endless network

Other forms. Graphene is a single layer of graphite, carbon nanotubes are rolled-up graphene sheets, and charcoal, coke and carbon black are amorphous forms.

An everyday example. Water purifiers in Indian homes often contain activated charcoal, whose enormous internal surface traps chlorine and odour-causing molecules.

The substance. Graphite, not diamond, is the more stable allotrope under ordinary conditions — diamond survives only because its conversion to graphite is extremely slow.

**Silicon is a hard, semiconducting element found in nature as silica and silicates; silicates are built from $\mathrm{Si}{\mathrm{O}}_4^{4-}$ tetrahedra, and silicones are synthetic polymers with a silicon-oxygen backbone and organic side groups.

Silicon and silica:**

- Silicon is obtained by reducing silica with carbon, $\mathrm{Si}{\mathrm{O}}_2+2C\to \mathrm{Si}+2CO$, and purified further for use as a semiconductor in chips and solar cells
- Silica, $\mathrm{Si}{\mathrm{O}}_2$, found as quartz and sand, is a giant network of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra
- Silica is attacked by hydrofluoric acid, $\mathrm{Si}{\mathrm{O}}_2+4HF\to \mathrm{Si}{\mathrm{F}}_4+2{\mathrm{H}}_2\mathrm{O}$, which is used to etch glass

Silicates:

- The basic unit is the $\mathrm{Si}{\mathrm{O}}_4^{4-}$ tetrahedron, with silicon at the centre and oxygen at each corner
- Tetrahedra share oxygen atoms to form chains, sheets as in mica, or three-dimensional networks as in feldspars and zeolites
- Zeolites act as catalysts in petroleum refining and as water softeners
- Glass and cement are man-made silicates

**Silicones, $({\mathrm{R}}_2\mathrm{SiO}{)}_{\mathrm{n}}$:**

- Made by hydrolysing dialkyldichlorosilanes: $(\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{SiC}{\mathrm{l}}_2+2{\mathrm{H}}_2\mathrm{O}\to (\mathrm{C}{\mathrm{H}}_3{)}_2\mathrm{Si}(\mathrm{OH}{)}_2+2HCl$
- The silanol molecules then condense, losing water, into long Si-O-Si chains
- Adding $(\mathrm{C}{\mathrm{H}}_3{)}_3\mathrm{SiCl}$ caps the chain ends, while $\mathrm{C}{\mathrm{H}}_3\mathrm{SiC}{\mathrm{l}}_3$ creates cross-links
- Properties: water-repellent, stable to heat, good electrical insulators, chemically inert and non-toxic
- Uses: sealants, greases, waterproof fabrics, electrical insulation and surgical implants

An everyday example. The silicone sealant around a bathroom tap or kitchen sink stays flexible and waterproof for a long time, while rooftop solar panels across India use purified silicon to turn sunlight into electricity.

The substance. Silicones are part organic and part inorganic — the organic groups make them water-repellent, while the inorganic Si-O backbone makes them heat stable.

Carbon is small, forms strong C-C bonds and multiple bonds, and has no d orbitals, while silicon is larger, forms weaker Si-Si bonds, avoids multiple bonds and can use d orbitals — so carbon builds chains of molecules and silicon builds network solids.

Catenation:

- The C-C bond enthalpy is 348 kJ mol${}^{-1}$, but Si-Si is only 297 kJ mol${}^{-1}$
- Carbon forms long chains and rings, the basis of organic chemistry; silicon forms only short chains

Multiple bonds:

- Small carbon atoms overlap sideways to form $p\pi$-$p\pi$ bonds, giving double and triple bonds
- Larger silicon atoms overlap poorly sideways, so silicon prefers single Si-O bonds
- That is why $\mathrm{C}{\mathrm{O}}_2$ is a gas of separate molecules, while $\mathrm{Si}{\mathrm{O}}_2$ is a hard network solid

Use of d orbitals:

- Carbon has no d orbitals, so its maximum covalency is 4
- Silicon has vacant 3d orbitals and can reach a covalency of 6, as in $[\mathrm{Si}{\mathrm{F}}_6{]}^{2-}$, while no $[\mathrm{C}{\mathrm{F}}_6{]}^{2-}$ ion exists
- $\mathrm{CC}{\mathrm{l}}_4$ is not hydrolysed by water, but $\mathrm{SiC}{\mathrm{l}}_4$ reacts readily, because a lone pair on water can attack the vacant d orbital of silicon:

$$\mathrm{SiC}{\mathrm{l}}_4+4{\mathrm{H}}_2\mathrm{O}\to \mathrm{Si}(\mathrm{OH}{)}_4+4HCl$$

Other differences. Carbon is more electronegative (2.5) than silicon (1.8), and carbon is a non-metal while silicon is a metalloid.

An everyday example. Carbon dioxide bubbling out of soda water and the sand on a Goa beach are oxides of neighbouring Group 14 elements — one a gas at room temperature, the other a solid that melts only above the melting point of iron.

The substance. Si-O bonds are much stronger than Si-Si bonds — which is why silicon in nature is locked into silica and silicates rather than chains of silicon atoms.