Giant Covalent Structures

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 1.49 — Why giant covalent structures are solids with high melting and boiling points
• 1.50 — How the structures of diamond, graphite and C60 fullerene influence their properties

Specification section 1(f) Ionic bonding — also taught in Simple Molecular Substances, Metallic Bonding.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Not every covalent substance is made of small molecules. In a GIANT COVALENT structure the covalent bonds continue throughout the whole piece of material, so that a diamond is not a collection of molecules but effectively one enormous molecule. Diamond, graphite and silicon dioxide are the examples on this specification.

The consequence for melting point is immediate, and it is the mirror image of the last page. To melt a giant covalent structure you must break the COVALENT BONDS themselves, because there is nothing else holding it together — no weak forces between molecules, since there are no separate molecules. Covalent bonds are very strong and there are enormous numbers of them, so this takes a colossal amount of energy. Diamond melts above 3500 °C.

Think of it like a scaffold welded at every joint

A stack of scaffolding poles laid loose in a yard can be pulled apart by hand, however strong each pole is. Weld every joint and the same poles become a single rigid frame that cannot be taken apart without cutting steel. Nothing about the poles changed — only whether the joins are strong. Molecular substances are the loose stack and giant covalent structures are the welded frame, and the difference in melting point between methane and diamond is entirely the difference between separating things and cutting them.

Now statement 1.50, which is the interesting part: diamond and graphite are both pure carbon, made of identical atoms joined by identical covalent bonds, and they could hardly be more different. Diamond is the hardest natural substance and does not conduct; graphite is soft enough to write with and conducts electricity well. The whole difference is STRUCTURE.

Diamond — four bonds, and what follows from that

1. Each carbon atom forms FOUR covalent bonds to four other carbons.
2. This produces a rigid three-dimensional network extending throughout the crystal, with no weak direction anywhere.
3. Melting requires breaking many strong covalent bonds, so the melting point is extremely high — above 3500 °C.
4. Because the network is rigid in every direction, diamond is extremely HARD, which is why it is used in cutting tools and drill bits.
5. All four outer electrons of every carbon are used in bonding, so there are NO FREE ELECTRONS — and diamond does not conduct electricity.

Graphite — three bonds, and one electron left over

1. Each carbon atom forms only THREE covalent bonds, producing flat sheets of hexagons.
2. The sheets are held to one another only by WEAK forces, with no covalent bonds between them.
3. The sheets can therefore SLIDE over each other, which makes graphite soft and slippery — hence pencils and its use as a lubricant.
4. Carbon has four outer electrons and only three are used in bonding, so each atom has ONE DELOCALISED ELECTRON free to move through the sheet.
5. Those free electrons make graphite a good CONDUCTOR of electricity, which is why it is used for electrodes.
6. Within each sheet the bonding is still strong covalent, so graphite's melting point is also very high.

That comparison is the most efficient thing on this page, because every single property follows from one number: four bonds or three. Four uses up all the electrons and locks the structure rigid — hard, non-conducting. Three leaves an electron over and leaves the sheets free — soft, conducting. A learner who holds that one contrast does not need to memorise six properties.

C60 FULLERENE is the third form and it does not belong with the other two, which is the point of including it. It consists of separate MOLECULES of sixty carbon atoms arranged in a hollow ball of hexagons and pentagons, like a football. Because the molecules are separate, C60 is a SIMPLE MOLECULAR substance despite being made of carbon — so the weak forces between its molecules give it a much lower melting point than diamond or graphite, and it is soft and slippery for the same reason. It was discovered in 1985, and its discoverers received the Nobel Prize in 1996.

Deciding which structure a substance has, and predicting from it

Substance Z is a solid at room temperature, melts at 1610 °C, and does not conduct electricity in the solid or the molten state. What structure does it have?

1. A very high melting point means either ionic or giant covalent — the weak forces of a molecular substance could never require that much energy.
2. It does not conduct when MOLTEN. An ionic compound would conduct once molten, because its ions become free to move.
3. So it is not ionic. It must be giant covalent, in which the atoms are neutral and all electrons are used in bonding.
4. That also explains the lack of conduction in the solid: no ions and no free electrons in either state.
5. Z is in fact silicon dioxide, which melts at 1610 °C.

Answer: Giant covalent — and the deciding evidence was that it does not conduct when MOLTEN.

The molten test in that example is worth keeping, because it is how the two high-melting structures are told apart. Both ionic and giant covalent substances melt high. Only the ionic one conducts once it has melted, because only it contains ions. Graphite is the honourable exception on the covalent side, and it conducts for a completely different reason — delocalised electrons rather than mobile ions.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Giant covalent structure

A structure in which covalent bonds continue throughout the whole piece of material, so there are no separate molecules. Melting requires breaking covalent bonds themselves.

Statement 1.50 — the three forms of carbon

The high melting point of a giant covalent structure

1. The covalent bonds continue throughout the whole structure, so there are no separate molecules.
2. Melting therefore requires strong COVALENT BONDS to be broken, not weak intermolecular forces.
3. There are enormous numbers of these bonds throughout the structure.
4. A very large amount of energy is therefore required.
5. So the melting and boiling points are very high, and all such substances are solids at room temperature.

Diamond and graphite — everything from four bonds against three

• DIAMOND: 4 bonds per carbon → rigid 3-D network → HARD, and no electrons left over → does NOT conduct
• GRAPHITE: 3 bonds per carbon → flat sheets with weak forces between → SOFT and slippery
• GRAPHITE: one electron per atom left over → DELOCALISED → CONDUCTS electricity
• Both have very high melting points, because both require covalent bonds to be broken
• Same element, same kind of bond — every difference comes from the number of bonds per atom

Telling ionic from giant covalent

1. Both have high melting points, so the melting point alone cannot distinguish them.
2. Melt the substance and test whether it conducts.
3. IONIC conducts when molten, because its ions become free to move.
4. GIANT COVALENT does not conduct when molten, because it has no ions and no free electrons.
5. The exception is graphite, which conducts through delocalised electrons rather than ions.

Model answer [4 marks]

Explain why diamond has a very high melting point and does not conduct electricity. [4]

In diamond each carbon atom is joined by strong covalent bonds to four others, forming a giant covalent structure that extends throughout the crystal. Melting it requires many of these strong covalent bonds to be broken, which needs a very large amount of energy, so the melting point is extremely high. Each carbon atom uses all four of its outer electrons in bonding, so there are no delocalised electrons and no ions. Since there are no charged particles free to move, diamond does not conduct electricity.

Model answer [4 marks]

Explain why graphite is soft and conducts electricity, although it is made of the same atoms as diamond. [4]

In graphite each carbon atom forms only three covalent bonds, producing flat sheets of hexagons. The sheets are held together only by weak forces with no covalent bonds between them, so the sheets can slide over one another, which makes graphite soft and slippery. Because each carbon uses only three of its four outer electrons in bonding, one electron per atom is delocalised and free to move through the sheet. These delocalised electrons are charged particles that can move, so graphite conducts electricity.

Not this: Graphite is soft because its covalent bonds are weaker than diamond's.

This: The bonds are the same strong carbon–carbon covalent bonds in both. Graphite is soft because the SHEETS are held to one another only by weak forces and can slide — nothing inside a sheet is being broken when a pencil writes.

Mark-losing trap. Melting a giant covalent structure BREAKS COVALENT BONDS — there are no weak forces to overcome instead.

Mark-losing trap. Graphite conducts because of DELOCALISED ELECTRONS, not ions. It has no ions at all.

Mark-losing trap. C60 is SIMPLE MOLECULAR, not giant covalent — which is why its melting point is low.

Mark-losing trap. Both ionic and giant covalent melt high. Only the IONIC one conducts when molten.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — How many covalent bonds does each carbon atom form in diamond?
2. Grade 7 · Explain [3 marks] — Explain why diamond has a very high melting point.
3. Grade 8 · Explain [3 marks] — Graphite conducts electricity but diamond does not, although both are pure carbon. Explain why.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of why graphite is soft and conducts electricity, while diamond is hard and does not.
5. 9+ · Analyse [6 marks] — A student is told that C60 fullerene is made of carbon atoms each bonded to three others, like graphite. They predict that C60 will therefore have a very high melting point and will conduct electricity well. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who insisted that properties follow from structure and place, and the one who burnt a diamond to find out what it was. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Dmitri Mendeleev — the one who insisted that properties follow from structure and place

Diamond and graphite are the sharpest challenge to the idea that an element has fixed properties — same atoms, same bonds, utterly different substances. Mendeleev built the periodic table on the principle that properties are systematic rather than arbitrary, and he trusted that principle so far that he left gaps for undiscovered elements and predicted their densities and melting points in advance. He is the right person to ask what an element's properties actually belong to, and how far you can predict a substance you have never seen from the way its atoms are arranged.

• “How can the same element give two substances this different?”
• “What does an element's properties actually depend on?”
• “How did you dare to predict the properties of elements nobody had found?”
• “Where would you have placed carbon's two forms in your table?”
• “What made you trust a pattern over the measurements of the day?”

Humphry Davy — the one who burnt a diamond to find out what it was

That diamond and graphite are the same element is stated flatly on this page and was once an extraordinary claim, and Davy is among those who settled it experimentally. Using a large burning lens to focus sunlight, he burned a diamond in oxygen and showed that the only product was carbon dioxide — the same product as burning charcoal, and in the same proportion. There is no more direct way to demonstrate that the hardest substance known and the black stuff in a grate are one element. He is also the right person for graphite's use as an electrode, since his own electrolysis work depended on carbon that conducts.

• “How did you show a diamond is made of carbon?”
• “What did you use to get a diamond hot enough to burn?”
• “Why was that result so surprising at the time?”
• “Why is carbon useful as an electrode?”
• “What did it feel like to destroy a diamond for an experiment?”

Carry on through the course

• Previous lesson: Simple Molecular Substances
• Next lesson: Metallic Bonding
• The Three States of Matter — What the particles are doing in a solid, a liquid and a gas — and what really changes when ice melts.
• Pure Substances, Mixtures and How to Separate Them — Why a pure substance melts at one temperature, and which separation technique to reach for when.
• Inside the Atom — Protons, neutrons and electrons — where they are, what they weigh, what they carry, and how isotopes give an element a decimal mass.
• Electronic Configurations and the Periodic Table — Why the table has the shape it has: the group number IS the outer-shell electron count, and everything follows from that.
• Solubility and Solubility Curves — Why a solution can be full, why hot water holds more, and how one curve tells you exactly how many crystals you will get back.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification