Answers · Giant structures

A sea of electrons — answer key

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

1. GQ10 · Practice

Model answer. In a metal, the atoms lose their outer electrons and become positive ions in a giant lattice. The lost electrons are delocalised: they move through the whole lattice. Metallic bonding is the electrostatic attraction between the positive ions and this sea of delocalised electrons.

A full answer includes

I chose: positive ions attracted to a sea of delocalised electrons.
I did not choose negative ions: a metal has none.
I did not choose shared pairs: those are covalent bonds.
I did not choose weak forces between layers: metallic bonding is strong.

Also accept

Positive ions attracted to a sea of delocalised electrons.
The attraction between positive ions and a sea of delocalised electrons.

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.
“Metal atoms share pairs of electrons in covalent bonds.” — The electrons are not shared between two particular atoms. They are delocalised: they move through the whole lattice.
“Metallic bonding is the weak force between layers of metal atoms.” — Metallic bonding is strong, and it acts in all directions, not only between layers. Weak forces between layers are what graphite has.

2. GQ11 · Practice

Model answer. The corrected sentences: (1) Each potassium atom loses its one outer electron. (2) So each atom becomes a positive ion, K⁺. (3) The ions are held together in a giant metallic lattice: there are no separate molecules. (4) The lattice holds together because of the electrostatic attraction between the ions and the sea of electrons they lost.

A full answer includes

1: I gave loses (or gives up).
2: I gave positive.
3: I gave metallic.
4: I gave attraction.
I did not give negative in 2: losing a negative electron leaves a positive ion.
I did not give ionic in 3: a metal has no negative ions.

Also accept

1 loses; 2 positive; 3 metallic; 4 attraction.

Watch for

“Metal atoms gain electrons.” — Metal atoms have few outer electrons, and they lose them. A potassium atom loses its single outer electron.
“The metal atoms become negative ions.” — They lose electrons, and losing negative electrons leaves a positive ion.
“A metal is made of molecules.” — A metal has no separate molecules. Its ions are held in one giant metallic lattice that goes on through the whole piece.
“The lattice is ionic, because it contains ions.” — An ionic lattice has positive and negative ions. A metal has only positive ions, held by the sea of electrons: a metallic lattice.
“The ions and the sea of electrons repel each other.” — They have opposite charges, so they attract. That attraction is the metallic bond.

3. GQ12 · Practice

Model answer. A current is a flow of charge, so a solid conducts only if it contains charged particles that are free to move. Positive ions in a sea of delocalised electrons are a metal: the delocalised electrons move through the lattice and carry the current. Ions held in fixed places are a solid ionic compound: they are charged, but they cannot move. Atoms with all their outer electrons in covalent bonds, like diamond, and small molecules have no charged particles free to move.

A full answer includes

I chose: positive ions surrounded by a sea of delocalised electrons.
I used the rule: a solid conducts only if it has charged particles that are free to move.
I did not choose the ions held in fixed places: they are charged but cannot move.
I did not choose the atoms or the small molecules: they have no charged particles free to move.

Also accept

Positive ions surrounded by a sea of delocalised electrons.

Watch for

“Positive and negative ions held in fixed places in a lattice.” — The ions are charged, but in the solid they are held in place, so they cannot carry a current. This solid would conduct only when molten.
“Atoms whose outer electrons are all used in covalent bonds.” — Every outer electron is in a bond, so no electron is free to move, and the atoms carry no charge. This is a solid like diamond.
“Small molecules whose electrons stay inside each molecule.” — The molecules carry no charge, and their electrons cannot leave them, so nothing charged moves through the solid.

4. GQ13 · Practice

Model answer. Row D. Hammering a metal into a new shape without breaking is malleability; ductile means drawn out into a wire. Iron is a giant lattice of positive ions in layers, in a sea of delocalised electrons. Under the hammer, the layers of ions slide over each other. In their new places, the ions are still surrounded by the sea of electrons and still attracted to it, so the iron holds together in its new shape. The bonding is not weak: the glowing iron is still far below its melting point.

A full answer includes

I chose row D.
I checked the property: hammered into a new shape without breaking is malleable.
I checked the explanation: the layers of ions slide, and the electron sea still attracts them in their new places.
I did not choose ductile: that means drawn out into a wire.
I did not choose weak bonding: metallic bonding is strong, and the iron is far below its melting point.

Also accept

Row D

Watch for

“Row A.” — Ductile means it can be drawn out into a wire. Being hammered into a new shape is a different property.
“Row B.” — Metallic bonding is strong: the glowing iron is far below its melting point, so its ions do not flow like a liquid. Ask what happens to the layers of ions, and what still holds them.
“Row C.” — Neither column is right. Being hammered into shape is not ductility, and metallic bonding is strong: the iron is far below its melting point.

5. GQ14 · Practice

Model answer. X is a metal: positive ions in layers, in a sea of delocalised electrons. Conducting as a solid needs charged particles that move in the solid; in a metal these are the delocalised electrons. An ionic solid conducts only when molten, and small molecules carry no charge, so neither fits. Layers held only by weak forces, as in graphite, conduct, but they separate when they are pulled. In a metal, the layers of ions slide while the electron sea still attracts them in their new places, so the metal stretches into a wire in one piece.

A full answer includes

I chose: positive ions in layers, in a sea of delocalised electrons.
I used the first clue: X has charged particles that move even in the solid.
I used the second clue: X can change shape in one piece, so its layers stay held after they move.
I did not choose ions free to move when molten: an ionic solid does not conduct as a solid.
I did not choose layers held by weak forces: they would come apart instead of stretching into a wire.

Also accept

Positive ions in layers, in a sea of delocalised electrons.

Watch for

“Positive and negative ions that are free to move when molten.” — An ionic solid does not conduct as a solid, because its ions are held in place, and it shatters when it is hammered or pulled. X conducts as a solid.
“Layers of atoms held to each other only by weak forces.” — Layers held only by weak forces, as in graphite, separate and flake off when they are pulled. X stretches into a wire in one piece, so something strong still holds its layers.
“Small molecules held to each other only by weak forces.” — Molecules carry no charge, and their electrons stay inside them, so a molecular solid does not conduct. X conducts.

6. GQ15 · Practice

Model answer. (a) Tin has metallic bonding: positive tin ions in layers in a giant lattice, attracted to a sea of delocalised electrons. When the strip is bent, layers of ions slide over each other; the sea of electrons still surrounds and attracts every ion, so the tin changes shape without breaking. (b) In silicon, each covalent bond joins two particular atoms in a fixed direction. The atoms cannot slide to new places without breaking bonds, so bending breaks them and the wafer snaps.

A full answer includes

(a) I wrote that tin has positive ions arranged in layers.
(a) I wrote that the ions are surrounded by a sea of delocalised electrons.
(a) I wrote that the layers of ions slide over each other when the strip is bent.
(a) I wrote that the electron sea still attracts every ion after the layers slide.
(a) I did not write that tin bends because metallic bonding is weak.
(b) I wrote that each covalent bond in silicon joins two particular atoms in a fixed direction.
(b) I wrote that the atoms cannot slide to new places without breaking bonds.
(b) I wrote that the bonds break, so the wafer snaps.
(b) I did not write that silicon snaps because its bonds are weak.

Also accept

(a) Layers of positive ions slide; the electron sea still attracts them. (b) Covalent bonds join fixed atoms; they break instead of sliding.

Watch for

“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.
“Silicon snaps because its covalent bonds are weak.” — Its covalent bonds are strong: silicon is hard and melts at about 1400 °C. It snaps because bonds that join fixed atoms break instead of sliding.

7. GQ16 · Practice

Model answer. (a) A Group I atom has 1 outer electron, so each lithium atom gives 1 electron and becomes Li⁺. (b) A Group II atom has 2 outer electrons, so each calcium atom gives 2 and becomes Ca²⁺. (c) 400 ions × 2 electrons = 800 delocalised electrons. Their total negative charge balances the 800 positive charges on the ions, so the metal has no overall charge.

Checked automatically

(a) Electrons from each lithium atom: 1 (exact value)
- Common wrong answer 2 : Lithium is in Group I: 1 outer electron.
(b) Electrons from each calcium atom: 2 (exact value)
- Common wrong answer 1 : Calcium is in Group II: 2 outer electrons.
(c) Delocalised electrons in the sea: 800 (exact value)
- Common wrong answer 400 : Each calcium atom gave 2 electrons, not 1.
- Common wrong answer 2 : That is per atom; the piece has 400 ions.
- Common wrong answer 200 : You divided by 2. Each ion gave 2 electrons, so there are more electrons than ions.

A full answer includes

(a) I deduced 1.
(b) I deduced 2.
(c) Method: I multiplied 400 ions by 2 electrons from each atom.
(c) Answer: 800 electrons.
(c) I did not give 400: each calcium atom gave 2 electrons, not 1.

Also accept

(a) 1 (b) 2 (c) 800

Watch for

“Each calcium atom gives 1 electron, like lithium.” — Calcium is in Group II: it has 2 outer electrons, and gives both.
“The sea has one electron for each ion.” — Each Ca²⁺ ion gave 2 electrons, so there are 2 electrons for each ion.

8. GQ17 · Practice

Model answer. Copper: the delocalised electrons move through the lattice; the positive ions stay in their places. Graphite: it has no ions; the fourth outer electron of each carbon atom is delocalised and moves along the layers. Molten sodium chloride: it has no delocalised electrons; melting frees the Na⁺ and Cl⁻ ions, which move and carry the current. Row C.

A full answer includes

I chose row C.
I checked copper: its delocalised electrons move; its ions stay in place.
I checked graphite: it has no ions; its delocalised electrons move.
I checked molten sodium chloride: its ions move; it has no delocalised electrons.
I did not choose a row where the copper ions carry the current.

Also accept

Row C

Watch for

“Row A.” — Graphite is pure carbon joined by covalent bonds: it has no ions. Its fourth outer electrons are delocalised, and they carry the current.
“Row B.” — In a metal the positive ions stay in place and the delocalised electrons move. Sodium chloride has no delocalised electrons.
“Row D.” — Sodium chloride has no delocalised electrons. Melting frees its ions, and they move and carry the current.

9. GT2 · Apply

Model answer. (a) The charges balance: 600 electrons ÷ 300 ions = 2 electrons from each atom. So each ion is M²⁺, and M has 2 outer electrons: Group II. (b) In lead, the layers of positive ions slide over each other, but the sea of delocalised electrons still surrounds and attracts every ion in its new place, so the metal holds together and spreads into a sheet. In graphite, the layers are held together only by weak forces. When they are pushed, nothing strong holds one layer to the next, so the layers separate and the graphite crumbles into flakes. The covalent bonds inside each layer are strong and do not break.

A full answer includes

(a) Method: I divided the 600 electrons by the 300 ions: 2 electrons from each atom.
(a) Answer: each ion is 2+ (M²⁺).
(a) I deduced that M is in Group II.
(a) I did not give the charge as 2−: metal ions are positive.
(b) I wrote that in lead the layers of positive ions slide over each other.
(b) I wrote that the sea of delocalised electrons still attracts the ions in their new places.
(b) I wrote that the layers of graphite are held together only by weak forces.
(b) I wrote that when hammered, graphite’s layers separate and flake off.
(b) I did not write that graphite crumbles because its covalent bonds are weak.

Also accept

(a) 2+, Group II. (b) Lead: ions slide and the electron sea still holds them; graphite: only weak forces between layers, so the layers separate into flakes.

Watch for

“The ions of M have a charge of 2−.” — Metal atoms lose electrons to the sea, so their ions are positive: M²⁺.
“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.
“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.