Silicon(IV) Oxide (SiO2)

Silicon(IV) oxide, also called silica, is found in sand and quartz. It is a giant covalent structure (sometimes called macromolecular). Its properties come from a strong, 3D network of covalent bonds.

Structure

In $\text{SiO}_2$, atoms are joined in a huge repeating network, not as small molecules.

• Each silicon (Si) atom is covalently bonded to four oxygen (O) atoms in a tetrahedral shape.
• Each oxygen atom is bonded to two silicon atoms.
• This creates a 3D “climbing frame” of strong Si–O bonds throughout the solid.

Properties explained by the structure

• Very high melting point: many strong covalent bonds must be broken, which needs a lot of energy.
• Hard and brittle: the rigid network makes it hard; when stressed, layers cannot slide, so it shatters.
• Insoluble in water and solvents: there are no small molecules to pull apart; the network is too strong.
• Does not conduct electricity: no free electrons or ions to carry charge.

Comparison with diamond

• Both are giant covalent solids with each central atom bonded to four others in a tetrahedral arrangement.
• Both are very hard, have very high melting points, are insoluble, and do not conduct electricity.
• Difference: diamond is pure carbon; silica is silicon and oxygen in a 4–2 bonding pattern (Si to 4 O; each O to 2 Si).

Silicon(IV) oxide as an acidic oxide

Silicon is a non‑metal, so $\text{SiO}_2$ is an acidic oxide. It reacts with bases to form salts.

Example (formation of slag in the blast furnace):

$$\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3$$

It can also react with sodium hydroxide:

$$\text{SiO}_2 + 2\,\text{NaOH} \rightarrow \text{Na}_2\text{SiO}_3 + \text{H}_2\text{O}$$

Uses and real-world links

• Glass and windows: high melting point and transparency.
• Optical fibres and lenses: forms very pure, clear glass.
• Abrasives and sandpaper: hardness makes it good for grinding.

Common misconceptions

• $\text{SiO}_2$ is not made of small molecules like $\text{CO}_2$; the formula shows the ratio of atoms in a giant network.
• Silicon(IV) oxide is different from silicon (the element) and from silicones (polymers used in sealants).