Giant Covalent Structures

Diamond and graphite are both pure carbon, yet one cuts rock and the other lubricates machinery, and explaining that contrast is a banker question on Paper 4, typically 4-6 marks, occasionally the full 6-mark extended response. This is Supplement material, Extended candidates only. The marks go to candidates who describe bonding numbers precisely: four bonds per carbon in diamond, three in graphite, and what the spare electron does.

What “giant covalent” means (Supplement)

A giant covalent (macromolecular) structure is a lattice of atoms in which every atom is joined to its neighbours by strong covalent bonds, repeating throughout the whole crystal. There are no separate molecules: the entire structure is one network. Melting one means breaking the strong covalent bonds themselves, not weak forces between molecules as in simple molecules, so melting points are very high (diamond sublimes/melts above 3,500 °C).

Diamond, graphite and silicon(IV) oxide compared

	Diamond	Graphite	Silicon(IV) oxide, SiO2
Bonds per atom	Each C bonded to 4 others, tetrahedral	Each C bonded to 3 others in flat hexagonal layers	Each Si to 4 O; each O to 2 Si, tetrahedral
Spare electrons	None (all 4 used)	1 per atom, delocalised along layers	None
Hardness	Very hard	Soft, slippery (layers slide)	Hard
Conducts electricity?	No	Yes, along the layers	No
Melting point	Very high	Very high	Very high
Uses	Cutting tools, drill bits, jewellery	Electrodes, lubricant, pencil “lead”	Sand/glass; abrasives

Diamond: every carbon uses all four outer electrons in covalent bonds to four neighbours, building a rigid tetrahedral network. That accounts for its hardness (use: cutting and grinding tools) and its inability to conduct: no free electrons.

Graphite: each carbon bonds to three others, forming layers of hexagons. The fourth electron per atom is delocalised between the layers and free to move, so graphite conducts electricity, which is why it serves as inert electrodes in electrolysis (topic 4 leans on this fact). The covalent bonds within layers are strong, but the forces between layers are weak, so layers slide over each other: graphite is soft and works as a lubricant.

Silicon(IV) oxide has a diamond-like structure with oxygen bridges: each silicon atom is covalently bonded to four oxygen atoms, and each oxygen to two silicons. It is hard, very high melting, and a non-conductor. Sand and quartz are the everyday forms.

A dot-and-cross approach does not scale to giant structures; questions instead use ball-and-stick diagrams. Be ready to count bonds from a diagram and to say “the structure continues in all directions”. Examiners accept that phrase as recognition that the lattice is giant.

Worked exam question

Diamond and graphite are both forms of carbon. (a) Explain why diamond is used on the tips of cutting tools. (2) (b) Explain why graphite conducts electricity but diamond does not. (3) (c) State why both have high melting points. (1)

Model answer: (a) Each carbon atom is bonded covalently to four others in a rigid (tetrahedral) lattice (1), making diamond very hard (1). (b) In graphite each carbon bonds to only three others (1), so one electron per atom is delocalised/free to move (1) and carries charge along the layers; in diamond all four outer electrons are held in covalent bonds, so there are no free electrons (1). (c) A large amount of energy is needed to break the strong covalent bonds throughout the giant structure (1).

Mark-by-mark: (a) requires the structural cause, then the property; “diamond is hard” alone repeats common knowledge for 1 mark at best. (b) the three marks map onto three-bonds, delocalised-electron, diamond-contrast; omit the diamond half and the third mark goes. (c) must say covalent bonds break; “strong bonds” unspecified can be ambiguous.

The mistakes that cost marks

1. Explaining graphite’s conduction with “free ions” or its softness with “weak covalent bonds between layers”. The mobile particles are delocalised electrons; the interlayer forces are weak forces, not covalent bonds.
2. Saying diamond is hard “because it is a strong substance”, which is circular. Name the four covalent bonds per atom and the rigid lattice.
3. Letting molecule vocabulary leak in: “diamond molecules”, “SiO2 molecules”. Giant structures have no molecules; SiO2 is an empirical ratio, not a molecular formula.
4. Forgetting silicon(IV) oxide entirely. It appears on papers precisely because students revise only the two carbons; learn the 4-and-2 bonding pattern.

How examiners want it phrased

Typical student wording	Accepted mark-scheme wording
”Diamond is the hardest thing ever"	"Each carbon is covalently bonded to four others in a rigid giant lattice, so diamond is very hard"
"Graphite has loose electrons"	"One electron per carbon atom is delocalised and free to move, carrying charge"
"The layers are barely connected"	"Weak forces between layers allow the layers to slide over each other"
"It takes forever to melt"	"Strong covalent bonds throughout the structure require a large amount of energy to break”

The Malaysia note

Extended candidates in Malaysian international schools meet giant covalent structures late in the bonding unit, and the May/June timetable squeezes it. Teachers report it is the bonding subtopic most likely to be self-studied. It also pairs naturally with metallic bonding in compare-the-structures questions, so revise the two together. Unsure whether the Extended route is even right for your child? That is a standard conversation in our free 1-hour trial, with the Core/Extended decision made on evidence, not guesswork.

Test yourself

All three are Supplement level. Try them from memory, then click to check.

Q1 (2 marks). Explain why graphite is used as a lubricant.

Show answer

• The forces (of attraction) between the layers are weak [1] • So the layers can slide over each other, making graphite soft/slippery [1]

Q2 (3 marks). Describe the structure of silicon(IV) oxide and explain why it has a very high melting point.

Show answer

• Each silicon atom is covalently bonded to four oxygen atoms, and each oxygen atom to two silicon atoms [1] • It is a giant covalent (macromolecular) structure: the network continues in all directions [1] • A large amount of energy is needed to break the strong covalent bonds throughout the structure [1]

Q3 (2 marks). Explain why graphite is suitable for use as electrodes in electrolysis.

Show answer

• Each carbon atom bonds to only three others, so one electron per atom is delocalised [1] • These delocalised electrons are free to move and carry charge, so graphite conducts electricity [1]