Answers · Giant structures

Giant structures — return questions key

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

1. GR1 · Return

Model answer. (a) In graphite each carbon atom forms three covalent bonds to other carbon atoms in a flat layer. Its fourth outer electron is not in a bond: it is delocalised and moves along the layers, so these electrons carry the current through the brushes. (b) In diamond each carbon atom forms four covalent bonds in a three-dimensional network. Scratching diamond would mean breaking many strong covalent bonds in all directions, so diamond is harder than tooth and wears the tooth away instead of being worn. (c) Silicon(IV) oxide is a giant covalent structure, like diamond: each silicon atom is bonded to four oxygen atoms and each oxygen atom to two silicon atoms. Its atoms share electrons in covalent bonds, so it contains no ions. Every outer electron stays in a bond or on its own atom, so none is delocalised. With no charged particles free to move, no current can flow through the tube.

A full answer includes

(a) I wrote that each carbon atom in graphite forms 3 covalent bonds in a flat layer.
(a) I wrote that the fourth outer electron of each atom is delocalised.
(a) I wrote that these electrons move along the layers and carry the current.
(a) I did not write that graphite conducts because it contains ions.
(b) I wrote that each carbon atom in diamond forms 4 covalent bonds in a three-dimensional network.
(b) I wrote that scratching diamond would mean breaking many strong covalent bonds in all directions.
(b) I wrote that diamond is harder than tooth, so it wears the tooth away instead.
(b) I did not write that diamond is hard because of strong intermolecular forces.
(c) I wrote that silicon(IV) oxide is a giant covalent structure, like diamond.
(c) I wrote that it contains no ions.
(c) I wrote that every outer electron stays in a covalent bond or on its own atom, so none is delocalised.
(c) I concluded that no charged particles are free to move, so no current can flow.
(c) I did not write that it is made of SiO₂ molecules.

Also accept

(a) 3 bonds in layers; the fourth electron is delocalised and moves. (b) 4 bonds in a 3D network; many strong bonds in all directions to break. (c) Giant covalent, no ions, no delocalised electrons: no charged particles 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 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 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.
“Silicon(IV) oxide does not conduct because its ions are held in fixed places.” — It has no ions: silicon and oxygen atoms share electrons in covalent bonds. It does not conduct because every electron stays in a bond or on its own atom.

2. GR2 · Return

Model answer. (a) Each aluminium atom loses its three outer electrons, so each ion is Al³⁺. The piece has 150 ions × 3 electrons = 450 delocalised electrons, whose charge balances the ions. (b) The pupil is incorrect. When aluminium melts, its ions can move past each other, but they do not take their electrons back: the liquid still contains positive Al³⁺ ions and a sea of delocalised electrons. The delocalised electrons can still move through the liquid, so they carry the current, just as they do in the solid wire. A metal does not need its fixed lattice to conduct.

A full answer includes

(a) I deduced 3+ (Al³⁺): each atom gives its 3 outer electrons to the sea.
(a) Method: I multiplied 150 ions by 3 electrons from each atom.
(a) Answer: 450 delocalised electrons.
(b) I wrote that molten aluminium still contains positive ions and delocalised electrons.
(b) I wrote that the ions do not take back their electrons when the metal melts.
(b) I wrote that the delocalised electrons can still move through the liquid.
(b) I wrote that these moving electrons carry the current, as in the solid wire.
(b) I did not write that the current in molten aluminium is carried by its ions.

Also accept

(a) Al³⁺; 450 electrons. (b) Molten aluminium still has its delocalised electrons, and they move and carry the current.

Watch for

“The sea has one electron for each aluminium ion.” — Each aluminium atom gives all 3 of its outer electrons, so there are 3 electrons for each Al³⁺ ion.
“Molten aluminium conducts because its ions move, as in molten sodium chloride.” — Its ions can move in the liquid, but the current is carried by the delocalised electrons, as it is in the solid metal.
“A metal needs its fixed lattice to conduct.” — The delocalised electrons carry the current, not the lattice. Melting lets the ions move past each other, but the sea of electrons is still there, so the liquid still conducts.