AQA Chemistry (8462) 4.2.2.6 · 4.2.3.1–4.2.3.3 · Contrast
Giant covalent structures
A diamond and a pencil lead are both pure carbon. One is the hardest natural material known; the other smears on paper and conducts electricity. What one difference does all that?
Triple Science · Higher tier
Start here
A drill bit and a pencil. Both carbon.
Diamond-tipped drills cut through granite. Graphite, the grey “lead” in a pencil, is so soft it rubs off onto paper and is used to lubricate locks, yet it carries a current well enough to be used as electrodes. Neither melts until it is hotter than 3500 °C.
Commit first
Same element, both very high melting points. What differs?
In a giant covalent structure every atom is joined to its neighbours by strong covalent bonds, and the bonding runs on through the whole solid. There are no separate molecules, so there are no weak forces between molecules to overcome. Melting means breaking huge numbers of strong covalent bonds, which is why these substances have very high melting points. Diamond, graphite and silicon dioxide (sand) are all built this way. The difference between diamond and graphite comes down to one count.
Diamond against graphite
Count the bonds. Then call each property.
0 of 8 called
Diamond · a flat view of a 3D network
Count first. How many covalent bonds does each carbon atom in diamond form?
Bonds per carbon atom
Diamond
—
Graphite
—
Conducts electricity?
Diamond
—
Graphite
—
Hard or soft?
Diamond
—
Graphite
—
Melting point
Diamond
—
Graphite
—
Think again
“Graphite conducts electricity, so graphite must be a metal.”
Spot the flaw
ExplainGraphite conducts, but carbon is a non-metal. Where do its mobile electrons come from?
Graphene is a single layer of graphite, one atom thick. It is very strong and conducts, for the same reasons as graphite’s layers. Fullerenes are molecules of carbon with hollow shapes, built from rings of carbon atoms, usually hexagons with some pentagons. The first found was buckminsterfullerene, C₆₀, a hollow sphere. Carbon nanotubes are cylindrical fullerenes with a very high length-to-diameter ratio. These are core content on every route.
Sort it
Graphene, C₆₀ or nanotube?
Each statement describes one of the three. Decide from the structure.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Compare
- Both structures in every sentence: “diamond has four bonds, whereas graphite has three”.
- Explain
- Tie each property back to the number of bonds.
- Suggest
- Use the structure to give a reasonable use or property.
Key fact
In diamond each carbon forms four covalent bonds; in graphite, three, leaving one delocalised electron per atom and layers held only by weak forces. Every property follows from that count.
Examiner tip
Key note · AQA 4.2.2.6, 4.2.3 (8462)
Giant covalent structures
- Giant covalent structures: all atoms joined by strong covalent bonds throughout. No molecules.
- Very high melting points: many strong covalent bonds must be broken.
- Diamond: each carbon bonds to four others. Very hard, does not conduct.
- Graphite: each carbon bonds to three others in layers of hexagons. One delocalised electron per atom: conducts. Weak forces between layers: soft and slippery.
- Silicon dioxide is giant covalent, like diamond.
- Graphene: one layer of graphite. Fullerenes: hollow carbon molecules such as C₆₀. Nanotubes: cylindrical fullerenes.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask Mr Badmus AI
Losing track of which carbon has three bonds and which has four?
Diamond is drawn as a flat projection of a three-dimensional network in which each carbon sits at the centre of a tetrahedron of four others. Graphite is drawn as three layers seen side-on above one layer seen face-on. Graphene, fullerenes and nanotubes are core content (AQA 5.2.3.3) on every route; they are not Higher-only.