Answers · Giant structures

Giant structures — final check key

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

1. GK1 · Final check

Model answer. (a) In graphite each carbon atom forms three strong covalent bonds within a flat layer of hexagons, and the layers are held together only by weak forces. The layers slide over each other easily, so the graphite powder lets the links of the chain slide past each other. (b) In diamond each carbon atom forms four covalent bonds in a giant three-dimensional network. Scratching diamond would mean breaking many strong covalent bonds, so diamond is harder than the tile: the wheel wears the tile away and is hardly worn itself. That is why diamond is used in cutting tools. (c) The formula SiO₂ gives the ratio 1 silicon : 2 oxygen, so there are 2 × 2.0 × 10¹⁹ = 4.0 × 10¹⁹ oxygen atoms. Each silicon atom forms 4 bonds, each to an oxygen atom, so there are 4 × 2.0 × 10¹⁹ = 8.0 × 10¹⁹ silicon–oxygen bonds. Check from the oxygen side: each oxygen atom has 2 bonds, and 2 × 4.0 × 10¹⁹ = 8.0 × 10¹⁹. (d) Giant covalent structures such as diamond and silicon(IV) oxide also have very high melting points, so a high melting point does not show that a solid is ionic. An ionic compound conducts when molten, because its ions are then free to move. This solid does not conduct even when molten, so it contains no ions. It is hard and melts very high, so it is not made of small molecules either: it has a giant covalent structure.

A full answer includes

(a) I wrote that each carbon atom in graphite forms 3 covalent bonds within a flat layer.
(a) I wrote that the layers are held together only by weak forces.
(a) I wrote that the layers slide over each other, so the links of the chain slide easily.
(a) I did not write that graphite’s covalent bonds are weak.
(b) I wrote that each carbon atom in diamond forms 4 covalent bonds in a giant three-dimensional network.
(b) I wrote that scratching diamond would mean breaking many strong covalent bonds.
(b) I wrote that diamond is harder than the tile, so the tile is worn away, not the diamond.
(b) I did not write that diamond is hard because of strong intermolecular forces.
(c) Method: I used the ratio 1 silicon : 2 oxygen and multiplied 2.0 × 10¹⁹ by 2.
(c) Answer: 4.0 × 10¹⁹ oxygen atoms.
(c) Method: each silicon atom forms 4 bonds to oxygen atoms, so I multiplied 2.0 × 10¹⁹ by 4.
(c) Answer: 8.0 × 10¹⁹ silicon–oxygen bonds.
(c) I did not give 8.0 × 10¹⁹ oxygen atoms: each oxygen atom is shared by two silicon atoms.
(d) I wrote that an ionic compound conducts when molten, because its ions are then free to move.
(d) I wrote that this solid does not conduct when molten, so it contains no ions.
(d) I wrote that a high melting point shows only that many strong bonds must break, not which kind.
(d) I deduced a giant covalent structure, like diamond and silicon(IV) oxide.
(d) I did not deduce small molecules: they would melt at a low temperature.

Also accept

(a) 3 bonds per atom in flat layers; weak forces between the layers, so they slide. (b) 4 bonds per atom in a 3D network; many strong bonds to break, so it is harder than the tile. (c) 4.0 × 10¹⁹ oxygen atoms; 8.0 × 10¹⁹ Si–O bonds. (d) An ionic compound conducts when molten; this one does not, so it has no ions: giant covalent.

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.
“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.
“There are 8.0 × 10¹⁹ oxygen atoms, four for each silicon atom.” — Each silicon atom has four oxygen neighbours, but each oxygen atom is shared by two silicon atoms. So there are 2 oxygen atoms for each silicon atom.
“A high melting point shows that a solid is ionic.” — Giant covalent solids such as diamond also have very high melting points. What shows ions is conducting when molten, and this solid does not.

2. GK2 · Final check

Model answer. (a) Silver is a giant metallic lattice of positive silver ions surrounded by a sea of delocalised electrons; metallic bonding is the electrostatic attraction between the ions and the electrons. (b) In solid silver, the delocalised electrons are free to move through the lattice, so they carry a current. Solid sodium chloride is made of Na⁺ and Cl⁻ ions, which are charged but held in fixed places in the lattice, and it has no delocalised electrons. So it has no charged particles free to move, and it does not conduct until it is melted. (c) Silver is ductile. When the rod is pulled, the layers of silver ions slide over each other; the sea of delocalised electrons still surrounds and attracts every ion in its new place, so the rod becomes a wire instead of snapping. Metallic bonding is strong, not weak.

A full answer includes

(a) I wrote positive silver ions in a giant metallic lattice.
(a) I wrote a sea of delocalised electrons.
(a) I wrote that metallic bonding is the electrostatic attraction between the ions and the electrons.
(a) I did not write that a metal contains negative ions.
(b) I wrote that the delocalised electrons in silver are free to move, so they carry a current.
(b) I wrote that sodium chloride is made of ions held in fixed places in its lattice.
(b) I wrote that sodium chloride has no delocalised electrons.
(b) I concluded that solid sodium chloride has no charged particles free to move.
(b) I did not write that the positive silver ions carry the current.
(b) I did not write that sodium chloride has no charged particles: its ions are charged but cannot move.
(c) I named it: ductile (ductility).
(c) I wrote that the layers of silver ions slide over each other when the rod is pulled.
(c) I wrote that the electron sea still attracts the ions in their new places, so the rod does not snap.
(c) I did not write that silver stretches because metallic bonding is weak.

Also accept

(a) Positive ions and delocalised electrons attract. (b) Silver: delocalised electrons move; sodium chloride: ions held in place, no delocalised electrons. (c) Ductile: layers of ions slide, the electron sea still holds them.

Watch for

“A metal is held together by positive ions and negative ions.” — A metal has no negative ions. The negative charge is the sea of delocalised electrons.
“Metals conduct because their positive ions move.” — In a solid metal the ions stay in their places in the lattice. The delocalised electrons carry the current.
“Solid sodium chloride does not conduct because it has no charged particles.” — It is made of charged ions, Na⁺ and Cl⁻. In the solid they are held in fixed places, so they cannot move and carry a current.
“Metals bend because metallic bonding is weak.” — Metallic bonding is strong: most metals have high melting points. The layers slide because the electron sea holds the ions wherever they move to.