Answers · Giant structures

Diamond and graphite — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. GQ1 · Practice

Model answer. Each carbon atom in diamond forms four covalent bonds, one to each of four other carbon atoms. The four bonds point to the corners of a tetrahedron, in three dimensions, so diamond has no flat layers. Every atom is bonded the same way through the whole crystal. Atoms at the edge of the diagram show only two or three solid bonds, with dashed stubs where the network goes on.

A full answer includes

I chose four, one to each of four other carbon atoms.
I counted on an atom in the middle, whose bonds are all solid lines.
I did not count an atom at the edge: its dashed stubs are bonds to atoms that are not drawn.
I did not choose four in one flat layer: the bonds of diamond point in three dimensions.

Also accept

Four, one to each of four other carbon atoms.

Watch for

“Count the bonds of an atom at the edge of the diagram.” — An edge atom has dashed stubs where the network goes on. Count the bonds of an atom in the middle.
“Two, one to each of two other carbon atoms.” — Some atoms at the edge of the diagram show only two solid bonds, but they also have dashed stubs. An atom in the middle, with all its neighbours drawn, has four.
“Three, one to each of three other carbon atoms.” — Some edge atoms show three solid bonds and a dashed stub. Three bonds per atom, in flat layers, is graphite; in diamond each atom uses all four outer electrons in four bonds.
“Four, all to carbon atoms in one flat layer.” — Diamond has no layers. Its four bonds point to the corners of a tetrahedron, so the network goes on in all three directions.

2. GQ2 · Practice

Options as numbered on the pupil sheet: 1 — Diamond; 2 — Graphite; 3 — Both.

Model answer. (1) Four bonds per carbon atom: diamond. (2) Three bonds per carbon atom: graphite. (3) Layers of hexagons held by weak forces: graphite. (4) A giant covalent structure: both; neither has separate molecules. (5) Delocalised electrons that can move: graphite, from the fourth outer electron of each atom. (6) A three-dimensional network of strong bonds: diamond.

A full answer includes

I matched 1, four bonds per atom, to diamond.
I matched 2, three bonds per atom, to graphite.
I matched 3, layers held by weak forces, to graphite.
I matched 4, a giant covalent structure, to both.
I matched 5, delocalised electrons, to graphite.
I matched 6, a three-dimensional network, to diamond.
I did not match 4 to diamond only: graphite has no separate molecules either.

Also accept

1 diamond; 2 graphite; 3 graphite; 4 both; 5 graphite; 6 diamond.

Watch for

“Only diamond is a giant covalent structure.” — Graphite is too: covalent bonds run through each whole layer, and there are no separate molecules.
“Diamond has delocalised electrons too.” — Diamond uses all four outer electrons of each atom in bonds, so it has none.

3. GQ3 · Practice

Model answer. In graphite each carbon atom has three strong covalent bonds within a flat layer, and the layers are held together only by weak forces. So the layers slide over each other easily, and the teeth of the zip slide on them.

A full answer includes

I chose: the layers are held together by weak forces, so they slide over each other.
I did not choose weak covalent bonds: the bonds inside each layer are strong.
I did not choose delocalised electrons: they explain conducting, not slipping.

Also accept

The layers are held by weak forces, so they slide over each other.

Watch for

“Graphite is soft because its covalent bonds are weak.” — The covalent bonds inside each layer are strong. The forces between the layers are weak, so the layers slide over each other.
“Graphite is slippery because of its delocalised electrons.” — The delocalised electrons explain why graphite conducts. It is slippery because its layers slide over each other.
“Graphite is made of small round molecules that roll like ball bearings.” — Graphite has no separate molecules. Each layer is one giant covalent structure, and whole layers slide.

4. GQ4 · Practice

Model answer. (a) Graphite contains no ions: it is made only of carbon atoms joined by covalent bonds. Each carbon atom uses three of its four outer electrons in bonds within a layer. The fourth electron is delocalised and moves along the layers, and these moving electrons carry the current. (b) Diamond has no separate molecules, so there are no intermolecular forces in it. Each carbon atom uses all four outer electrons in four strong covalent bonds, so no electrons are free to move, and diamond does not conduct.

A full answer includes

(a) I wrote that graphite has no ions: only carbon atoms joined by covalent bonds.
(a) I wrote that each carbon atom in graphite uses 3 of its 4 outer electrons in bonds.
(a) I wrote that the fourth electron is delocalised and moves along the layers, carrying the current.
(b) I wrote that diamond has no separate molecules, so it has no intermolecular forces.
(b) I wrote that each carbon atom in diamond uses all 4 outer electrons in its 4 covalent bonds.
(b) I wrote that no electrons are free to move, so diamond does not conduct.
I did not write that graphite conducts because its layers slide: sliding explains why it is slippery.

Also accept

(a) Graphite has no ions; the delocalised fourth electron of each carbon atom moves along the layers. (b) Diamond has no molecules or intermolecular forces; all 4 outer electrons are in bonds, none free to move.

Watch for

“Graphite conducts because it contains ions.” — Graphite has no ions. Its delocalised electrons, one from each carbon atom, carry the current.
“Diamond has strong intermolecular forces.” — Diamond is one giant network with no separate molecules, so it has no intermolecular forces. Its atoms are held by covalent bonds.
“Diamond conducts, because every carbon atom has outer electrons.” — All four outer electrons of each carbon atom are used in its four bonds, so none is free to move.

5. GQ5 · Practice

Options as numbered on the pupil sheet: 1 — Diamond: its delocalised electrons conduct electricity; 2 — Diamond: very hard, with strong covalent bonds in all directions; 3 — Graphite: very hard, because each atom has four bonds; 4 — Graphite: its delocalised electrons conduct electricity; 5 — Graphite: its layers slide over each other.

Model answer. (1) A wire that slices marble needs a coating harder than the stone: diamond, with strong covalent bonds in all directions. (2) A dry lubricant needs something slippery: graphite, whose layers slide over each other. (3) An electrode must conduct: graphite, whose delocalised electrons carry the current. Diamond has no delocalised electrons, and graphite is soft, so the other two reasons are wrong.

A full answer includes

I matched 1, the wire that slices marble, to a: diamond is very hard.
I matched 2, the hinge lubricant, to b: graphite’s layers slide.
I matched 3, the electrode, to c: graphite’s delocalised electrons conduct.
I did not use d: diamond has no delocalised electrons.
I did not use e: graphite has three bonds per atom and is soft.

Also accept

1 a; 2 b; 3 c.

Watch for

“Diamond’s delocalised electrons let it conduct.” — Diamond uses all four outer electrons of each atom in bonds, so it has no delocalised electrons and does not conduct.
“Graphite is very hard because each atom has four bonds.” — In graphite each atom has three bonds, and its layers slide apart under force, so graphite is soft. Four bonds in all directions is diamond.
“Diamond makes a good electrode because it is so strong.” — An electrode must conduct. Diamond has no electrons free to move, so it cannot.

6. GQ6 · Practice

Model answer. Both are pure carbon, both are giant covalent structures, and both contain strong covalent bonds between carbon atoms (in graphite, within each layer; the forces between its layers are weak). Only in diamond does each atom form four bonds (graphite: three). Only graphite conducts. Neither is made of small separate molecules.

A full answer includes

I chose: they are made only of carbon atoms.
I chose: they are giant covalent structures.
I chose: they contain strong covalent bonds between carbon atoms.
I did not choose four bonds per atom: in graphite each atom forms three.
I did not choose conducts electricity: only graphite conducts.

Also accept

Only carbon atoms; giant covalent structures; strong covalent bonds between carbon atoms.

Watch for

“Both conduct, because both are carbon.” — Only graphite conducts: only graphite has delocalised electrons.
“Each carbon atom forms four bonds in both.” — In graphite each atom forms three bonds; its fourth outer electron is delocalised.
“Diamond and graphite are made of small separate molecules.” — Neither has separate molecules: each is a giant covalent structure, one network of bonded atoms.

7. GQ7 · Practice

Model answer. (a) Each silicon atom forms four covalent bonds, each to an oxygen atom: 4. (b) Each oxygen atom forms two covalent bonds, each to a silicon atom: 2. Every oxygen atom is shared between two silicon atoms, so there are 4 ÷ 2 = 2 oxygen atoms for every silicon atom, which is why the formula is SiO₂.

Checked automatically

(a) Oxygen atoms bonded to each silicon atom: 4 (exact value)
- Common wrong answer 2 : That is the ratio in the formula. Count the bonds round one silicon atom in the middle: each goes to an oxygen atom.
(b) Silicon atoms bonded to each oxygen atom: 2 (exact value)
- Common wrong answer 1 : An oxygen atom between two silicon atoms has a bond to each of them.
- Common wrong answer 4 : That is the number for silicon. An oxygen atom has only two bonds.

A full answer includes

(a) I deduced 4 oxygen atoms round each silicon atom.
(b) I deduced 2 silicon atoms round each oxygen atom.
I counted on atoms in the middle, whose bonds are all solid lines.
I did not give 2 for (a) just because the formula is SiO₂.

Also accept

(a) 4 (b) 2

Watch for

“Each silicon atom is bonded to two oxygen atoms, as the formula SiO₂ suggests.” — Each silicon atom has four bonds, each to an oxygen atom. Each oxygen atom is shared by two silicon atoms, so the ratio is still 1 silicon to 2 oxygen.
“Count the bonds of an atom at the edge of the diagram.” — An edge atom has dashed stubs where the network goes on. Count the bonds of an atom in the middle.

8. GQ8 · Practice

Model answer. Diamond and silicon(IV) oxide are both giant covalent structures: every atom is held by strong covalent bonds pointing in all three directions. Scratching or melting either one means breaking many strong covalent bonds, which needs a lot of energy.

A full answer includes

I chose: both are giant covalent structures, with strong covalent bonds in all directions.
I did not choose intermolecular forces: neither has separate molecules.
I did not choose ions: both are made of non-metal atoms joined by covalent bonds.

Also accept

Both are giant covalent structures with strong covalent bonds in all directions.

Watch for

“Diamond is hard because of strong intermolecular forces.” — Diamond has no separate molecules, so there are no intermolecular forces. It is hard because strong covalent bonds run through the whole network.
“Silicon(IV) oxide contains carbon, like diamond.” — It contains silicon and oxygen only. What the two share is their structure, not an element.
“Diamond and silicon(IV) oxide are ionic lattices.” — Both are made of non-metal atoms that share electrons in covalent bonds. Neither contains ions, which is also why neither conducts when molten.

9. GQ9 · Practice

Model answer. (a) Y. It melts at 1710 °C, it is very hard (it scratches glass), and it doesn’t conduct, solid or molten. W melts too low, so it is made of small molecules; X conducts and bends, so it is a metal; Z conducts when molten, so it is ionic. (b) Silicon(IV) oxide is a giant covalent structure: each silicon atom is bonded to four oxygen atoms, and each oxygen atom to two silicon atoms, in a network that goes on in all three directions. It has no separate molecules: SiO₂ only gives the ratio of atoms, 1 silicon to 2 oxygen.

A full answer includes

(a) I identified Y.
(a) I gave two pieces of evidence: of the next three lines, I tick the two I gave.
(a) Evidence: a very high melting point, 1710 °C.
(a) Evidence: very hard, it scratches glass.
(a) Evidence: it does not conduct, as a solid or when molten.
(a) I did not choose Z: it conducts when molten, so it is ionic.
(b) I wrote that each silicon atom is covalently bonded to 4 oxygen atoms.
(b) I wrote that each oxygen atom is bonded to 2 silicon atoms.
(b) I wrote that the atoms form a giant three-dimensional network.
(b) I did not write that it is made of SiO₂ molecules.

Also accept

(a) Y, with two of: very high melting point; very hard; no conduction, solid or molten. (b) Si bonded to 4 O, O bonded to 2 Si, giant 3D network.

Watch for

“Z, because it has a high melting point too.” — Z conducts when molten, so it contains ions: it is ionic, not covalent.
“Silicon(IV) oxide is made of small SiO₂ molecules, like carbon dioxide.” — It has no separate molecules: one giant network. SiO₂ gives the ratio of atoms, 1 silicon to 2 oxygen.

10. GQ18 · Practice

Model answer. Row B. Diamond and silicon(IV) oxide are both giant covalent structures: atoms held by strong covalent bonds in a network that goes on in three dimensions. Scratching or melting either one means breaking many strong bonds, so both are very hard and have very high melting points. Neither contains ions, and every outer electron stays in a bond or on its own atom, so no charged particles are free to move: neither conducts. Row A is wrong because small molecules would melt at a low temperature, row C because neither solid contains ions, and row D because neither has delocalised electrons.

A full answer includes

I chose row B.
I checked the particles: atoms, with no separate molecules and no ions.
I checked the bonding: strong covalent bonds in a giant three-dimensional network.
I checked why it does not conduct: no ions, and no electrons free to move.
I did not choose row D: neither solid has delocalised electrons.

Also accept

Row B

Watch for

“Row A.” — Neither solid has separate molecules. Small molecules held by weak forces would be soft and would melt at a low temperature.
“Row C.” — Both solids are made of non-metal atoms that share electrons in covalent bonds, so neither contains ions. The last column is true of a solid ionic compound, not of these two.
“Row D.” — Neither solid has delocalised electrons: every outer electron stays in a bond or on its own atom. Delocalised electrons would move and carry a current, as they do in graphite.

11. GT1 · Apply

Model answer. (a) Like diamond: every atom forms four covalent bonds, in a network that goes on in three dimensions, with no flat layers. All four outer electrons of each atom are used in bonds, so there are no delocalised electrons. That fits its poor conductivity: graphite conducts well because of its delocalised electrons, and silicon carbide does not. (b) To scratch or dent silicon carbide, many strong covalent bonds pointing in all directions would have to break, so it is very hard. (c) Each silicon atom has four carbon neighbours, but each carbon atom also has four silicon neighbours. Counted from either side, every bond joins one silicon atom to one carbon atom, so the numbers of the two atoms are equal: 1 : 1, SiC. (d) All three are giant covalent structures: three-dimensional networks of strong covalent bonds in all directions, with no separate molecules. That is why all three are very hard and have very high melting points.

A full answer includes

(a) I chose diamond.
(a) I wrote that each atom forms 4 covalent bonds, as in diamond.
(a) I wrote that the bonds form a network in three dimensions, not flat layers.
(a) I wrote that every outer electron is used in a bond, so there are no delocalised electrons.
(a) I used the poor conductivity as evidence: graphite conducts well because of its delocalised electrons.
(a) I did not choose graphite just because silicon carbide contains carbon.
(b) I wrote that scratching or denting it would mean breaking many strong covalent bonds.
(b) I wrote that these bonds run in all directions, so it is very hard.
(c) I wrote that each carbon atom is also bonded to 4 silicon atoms.
(c) I concluded that there are equal numbers of silicon and carbon atoms: 1 : 1, so SiC.
(d) I stated that all three are giant covalent structures.
(d) I stated that all three have strong covalent bonds in all directions.
(d) I did not write that they have strong intermolecular forces.

Also accept

(a) Diamond: 4 bonds per atom in 3D, no layers; all outer electrons in bonds, none delocalised, so it hardly conducts. (b) Many strong covalent bonds in all directions to break: very hard. (c) Each C also bonded to 4 Si, so 1 Si : 1 C. (d) Giant covalent; strong covalent bonds in all directions.

Watch for

“Like graphite, because it contains carbon.” — The bonding decides, not the element: four bonds per atom in three dimensions is like diamond.
“The formula should be SiC₄, because each silicon atom has four carbon neighbours.” — Each carbon atom has four silicon neighbours too. Counted from both sides, the numbers of silicon and carbon atoms are equal: SiC.
“Diamond is hard because of strong intermolecular forces.” — Diamond has no separate molecules, so there are no intermolecular forces. It is hard because strong covalent bonds run through the whole network.