Silicon

Silicon (Si) is a metalloid with a giant covalent structure similar to diamond. It is vital in the semiconductor industry and appears in bonding and structure questions at IGCSE.

Where it appears in 0620

Silicon is a bonding-and-structure element more than a reactions element:

• Giant covalent structures: silicon and silicon(IV) oxide sit alongside diamond and graphite as the macromolecular examples — high melting point, hard, non-conducting.
• Extraction of metals: silicon(IV) oxide (sand) is the acidic impurity removed as slag in the iron blast furnace.
• Oxides: SiO2 is classified as an acidic oxide.

Structure comparisons (Si vs SiO2 vs diamond vs a simple molecule) are the questions examiners return to again and again.

Position in the periodic table

• Symbol: Si
• Atomic number: 14
• Group 14 (IV), Period 3
• Electron configuration: 2, 8, 4
• Metalloid (properties between metals and non-metals)

Physical properties

Property	Detail
Appearance	Grey, shiny, metallic lustre
Melting point	1414 C (very high)
Conductivity	Semiconductor (conducts under certain conditions)
Hardness	Hard and brittle
Structure	Giant covalent

Giant covalent structure

Like diamond, silicon has a giant covalent structure:

• Each Si atom is bonded to four other Si atoms
• Tetrahedral arrangement
• Every bond is a strong covalent bond

This explains:

• Very high melting point: many strong covalent bonds must be broken
• Hard: rigid 3D network
• Insoluble: bonds too strong to be disrupted by solvents
• Does not conduct as a pure crystalline solid at room temperature (it conducts as a semiconductor when doped or heated)

Semiconductor properties

Pure silicon is a poor conductor. Adding tiny amounts of other elements (such as boron or phosphorus) — called “doping” — turns it into a semiconductor. This underpins:

• Computer chips and processors
• Solar cells
• Transistors and integrated circuits

Silicon dioxide (SiO2)

Silicon dioxide also has a giant covalent structure:

• Each Si bonded to 4 O atoms; each O bonded to 2 Si atoms
• Very high melting point (~1710 C)
• Hard solid
• Found naturally as sand, quartz, and flint

SiO2 behaves as an acidic oxide: SiO2(s) + 2NaOH(aq) -> Na2SiO3(aq) + H2O(l)

In the blast furnace it is removed as molten slag: CaO(s) + SiO2(s) -> CaSiO3(l)

Comparison with carbon allotropes

Feature	Silicon	Diamond	Graphite
Bonds per atom	4	4	3
Structure	Giant covalent 3D	Giant covalent 3D	Giant covalent layers
Melting point	Very high	Very high	Very high
Conductivity	Semiconductor	None	Good (along layers)
Hardness	Hard	Very hard	Soft

Uses

Use	Reason
Semiconductor chips	Semiconductor properties when doped
Solar panels	Converts light to electricity
Glass (as SiO2)	Transparent, high melting point
Ceramics (as SiO2)	Heat-resistant

Key facts at a glance

Fact	Detail
Symbol	Si
Proton number	14
Group / Period	14 (IV) / 3
Classification	Metalloid
Structure	Giant covalent (4 bonds per atom, tetrahedral)
Key compound	SiO2 — sand, quartz, glass

Common exam mistakes

• Confusing silicon with silicon dioxide. Silicon (Si) is the grey semiconductor element; silicon dioxide (SiO2) is the compound found as sand. They are not interchangeable.
• Saying silicon conducts “because it has delocalised electrons”. It does not have free electrons the way graphite does. Pure silicon is a poor conductor at room temperature and only behaves as a semiconductor.
• Treating SiO2 as a simple molecule. SiO2 is giant covalent (macromolecular), which is why its melting point is so high — unlike molecular CO2, which is a gas.
• Muddling the bonding in Si and SiO2. In pure silicon each atom bonds to four silicon atoms; in SiO2 each silicon bonds to four oxygen atoms.

Exam-style questions

Silicon has a melting point of 1414 C. Explain, in terms of its structure and bonding, why the melting point is so high. (3 marks)

Mark scheme

• silicon has a giant covalent (macromolecular) structure [1]
• each silicon atom is joined to four others by strong covalent bonds [1]
• a large amount of energy is needed to break the many strong covalent bonds [1]

Examiner note: “strong bonds” alone is not enough — credit needs the number of bonds (a giant network) to explain the very high value.

Carbon dioxide is a gas at room temperature but silicon(IV) oxide melts at over 1700 C. Both contain covalent bonds. Explain the difference. (3 marks)

Mark scheme

• carbon dioxide is a simple molecule / has a simple molecular structure [1]
• so only weak intermolecular forces are overcome when it melts or boils, needing little energy [1]
• silicon(IV) oxide is giant covalent, so many strong covalent bonds must be broken, needing much more energy [1]

Examiner note: the wrong answer here is “CO2 has weak covalent bonds” — the bonds are strong; it is the forces between molecules that are weak.