Silicon, silicates, silicones and zeolites

Sand, glass and chips

Beach sand, window glass, a computer chip, a rubbery kitchen spatula and the pebbles in a water softener are all based on silicon. Silicon is the second most abundant element in the Earth's crust, at about 28 per cent by mass. It never occurs free. It is always joined to oxygen, in silica and the silicates.

Silicon

Preparation. Silica is reduced with carbon in an electric furnace: $\text{SiO}_{2} + 2\text{C} \to \text{Si} + 2\text{CO}$. This gives impure silicon. Silicon for chips must be extremely pure. It is made by reducing the volatile chloride with hydrogen: $\text{SiCl}_{4} + 2\text{H}_{2} \to \text{Si} + 4\text{HCl}$. It is then purified by zone refining. A heater moves along a rod and sweeps the impurities along with the molten zone.

Properties. It is a hard, grey solid with a metallic lustre and a high melting point. It is a semiconductor. A little boron added to it gives a p-type semiconductor, and a little phosphorus gives an n-type one.

It does not react with acids that are not oxidising. It dissolves in hot alkali: $\text{Si} + 2\text{NaOH} + \text{H}_{2}\text{O} \to \text{Na}_{2}\text{SiO}_{3} + 2\text{H}_{2}$. It burns in oxygen at a high temperature: $\text{Si} + \text{O}_{2} \to \text{SiO}_{2}$.

Silica

Carbon dioxide is a gas of small molecules. Silica, $\text{SiO}_{2}$, is a hard solid that melts above 1900 K. The difference is the bonding. The $p\pi\text{-}p\pi$ double bond of Si=O is weak, so silicon makes four single bonds to four oxygen atoms. Each silicon is at the centre of a tetrahedron, and each oxygen joins two silicons. The result is a giant network.

Silica occurs as quartz, cristobalite and tridymite. It is unreactive, but it is attacked by hydrofluoric acid: $\text{SiO}_{2} + 4\text{HF} \to \text{SiF}_{4} + 2\text{H}_{2}\text{O}$, and by hot concentrated alkali: $\text{SiO}_{2} + 2\text{NaOH} \to \text{Na}_{2}\text{SiO}_{3} + \text{H}_{2}\text{O}$. Melting sand with sodium carbonate and calcium carbonate gives glass: $\text{Na}_{2}\text{CO}_{3} + \text{CaCO}_{3} + 6\text{SiO}_{2} \to \text{Na}_{2}\text{O}\cdot \text{CaO}\cdot 6\text{SiO}_{2} + 2\text{CO}_{2}$.

Silicates

In every silicate the basic unit is the tetrahedral $\text{SiO}_{4}^{4-}$ ion. Silicon is in the centre and the four oxygens are at the corners. The units can join by sharing corner oxygen atoms. They never share an edge or a face. The more corners are shared, the more complex the structure.

• Orthosilicate: 0 shared corners. The anion is $\text{SiO}_{4}^{4-}$.
• Pyrosilicate: 1 shared corner. The anion is $\text{Si}_{2}\text{O}_{7}^{6-}$.
• Ring and single chain: 2 shared corners. The anion is $(\text{SiO}_{3})\text{n}^{2}\text{n}\text{-}$.
• Double chain: 2 or 3 shared corners, 2.5 on average. The anion is $(\text{Si}_{4}\text{O}_{11})\text{n}^{6}\text{n}\text{-}$.
• Sheet: 3 shared corners. The anion is $(\text{Si}_{2}\text{O}_{5})\text{n}^{2}\text{n}\text{-}$.
• Three-dimensional framework: 4 shared corners. The compound is $\text{SiO}_{2}$.

Each tetrahedron has four oxygens. A shared corner is counted for half in each tetrahedron. So a tetrahedron that shares $s$ corners has $4 - s/2$ oxygen atoms of its own, and the charge on each silicon is $4 - s$. A chain with $s = 2$ has 3 oxygens for each silicon and the charge $2-$. A sheet with $s = 3$ has $2.5$ oxygens and the charge $1-$. This is why the sheet is written $(\text{Si}_{2}\text{O}_{5})\text{n}^{2}\text{n}\text{-}$.

Some examples: zircon $\text{ZrSiO}_{4}$ and olivine $\text{Mg}_{2}\text{SiO}_{4}$ are orthosilicates. Beryl $\text{Be}_{3}\text{Al}_{2}\text{Si}_{6}\text{O}_{18}$ has rings of six tetrahedra. Spodumene $\text{LiAl}(\text{SiO}_{3})_{2}$ has single chains. Tremolite, one of the amphiboles, has double chains. Talc, mica and clay minerals have sheets. The sheets slide, which is why talc feels soft and mica splits into thin plates.

Silicones

Silicones are polymers with a backbone of alternate silicon and oxygen atoms, $\text{-}\text{Si}\text{-}\text{O}\text{-}\text{Si}\text{-}\text{O}\text{-}$, and organic groups on the silicon. They are made in steps.

1. Methyl chloride is passed over hot silicon with a copper catalyst at about 573 K: $\text{Si} + 2\text{CH}_{3}\text{Cl} \to (\text{CH}_{3})_{2}\text{SiCl}_{2}$.
2. The dichloride is hydrolysed. Water replaces the chlorine by hydroxyl: $(\text{CH}_{3})_{2}\text{SiCl}_{2} + 2\text{H}_{2}\text{O} \to (\text{CH}_{3})_{2}\text{Si}(\text{OH})_{2} + 2\text{HCl}$.
3. The diol molecules join and lose water: $\text{n}(\text{CH}_{3})_{2}\text{Si}(\text{OH})_{2} \to [\text{-}(\text{CH}_{3})_{2}\text{Si}\text{-}\text{O}\text{-}]\text{n} + \text{nH}_{2}\text{O}$. This is a chain.

Adding a little $(\text{CH}_{3})_{3}\text{SiCl}$ puts a methyl cap on the end of the chain and controls the length. Adding some $\text{CH}_{3}\text{SiCl}_{3}$ makes links between chains and gives a harder material. The silicones are not damaged by heat or water, they repel water, they do not conduct electricity, and they are not poisonous. They are used in sealants, greases, rubber, waterproofing and medical implants.

Zeolites

A zeolite is a three-dimensional aluminosilicate. Its framework has $\text{SiO}_{4}$ and $\text{AlO}_{4}$ tetrahedra joined at the corners, as in silica. Each aluminium atom, with the charge $3+$, takes the place of a silicon atom with $4+$, so the framework has a $1-$ charge for each aluminium. Sodium, potassium or calcium ions in the pores balance that charge. These ions are not tied to the framework, so they can be exchanged. Two aluminium tetrahedra are never next to each other in the framework. This is called Löwenstein's rule.

The framework has cages and channels of a fixed size, about $0.3$ to $1\ \text{nm}$ wide. A zeolite acts as a molecular sieve: it lets in the small molecules and keeps out the large ones.

• Water softening. The sodium ions of the zeolite are exchanged for the calcium and magnesium ions of hard water.
• Drying. The pores hold water or other small molecules.
• Catalysis. The zeolite ZSM-5 turns methanol into hydrocarbons of petrol, and is used in the cracking of petroleum.

The zeolite of the picture has the formula $\text{Na}_{12}[(\text{AlO}_{2})_{12}(\text{SiO}_{2})_{12}]\cdot 27\text{H}_{2}\text{O}$. It has 12 aluminium atoms and 12 silicon atoms, so the ratio of Si to Al is 1.

Practise with new numbers

Try it in Python

The program uses the number of shared corners to find the oxygen, the charge and the type of each silicate family. It also finds the metal ions needed to balance the charge.

Common mistakes

• Saying that silica is a molecule like CO$_2$. It is a giant network.
• Writing the silicate unit as $\text{SiO}_{3}$. It is $\text{SiO}_{4}$. The formula $\text{SiO}_{3}^{2-}$ belongs to a chain, where two corners are shared.
• Forgetting that a shared oxygen counts half. That is why the rule gives $4 - s/2$.
• Putting aluminium and silicon in the same place in a zeolite. Aluminium replaces some silicon and adds a negative charge.
• Thinking that silicones are carbon compounds. The backbone is Si-O.

Remember these

• Silica: a network of SiO$_4$ tetrahedra. Silicates: tetrahedra sharing 0, 1, 2, 2.5, 3 or 4 corners.
• A tetrahedron sharing $s$ corners has $4 - s/2$ oxygens and the charge $4 - s$ on each silicon.
• Silicones: $(\text{CH}_{3})_{2}\text{SiCl}_{2}$ from $\text{CH}_{3}\text{Cl}$ and Si with copper at 573 K, then hydrolysis and condensation. $(\text{CH}_{3})_{3}\text{SiCl}$ ends a chain.
• Zeolites: aluminosilicate frameworks with exchangeable ions, used for softening water and as sieves and catalysts.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many oxygen atoms does a silicon atom have to itself, in a sheet silicate?

2. Why is a chain of silicone not hydrolysed by water?

3. What happens to the sodium ions of a zeolite in hard water?

4. Which reagent attacks silica, and what does it make?

References

• OpenStax, Chemistry 2e, Chapter 18 (Representative Metals, Metalloids and Nonmetals), the section on the structure and properties of the metalloids, silicon.
• NCERT Chemistry for Class 11, the chapter The p-Block Elements (older edition), the sections on silicon, silicones, silicates and zeolites.
• Greenwood and Earnshaw, Chemistry of the Elements, the chapter on silicon.