Answers · Ions and ionic bonding

Salt and the bulb — answer key

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

1. BQ1 · Practice

Model answer. An ionic compound conducts electricity when molten or aqueous, but not when solid. So the molten lithium chloride and the solution light the bulb. The crystals and the powder are both solid: grinding makes smaller grains, but the ions inside each grain are still held in place, so the bulb stays dark.

A full answer includes

I chose the lithium chloride heated until molten.
I chose the lithium chloride dissolved in water.
I did not choose the dry powder: ground salt is still a solid.

Also accept

Molten lithium chloride; lithium chloride dissolved in water.

Watch for

“Dry crystals of lithium chloride light the bulb.” — The crystals are made of ions, but in a solid the ions are held in place. They cannot move to carry the charge.
“The dry powder lights the bulb, because it is broken up.” — A powder is still tiny solid grains. The ions inside each grain are still held in place.
“The solution conducts because water conducts.” — Pure water leaves the bulb dark, and molten salt conducts with no water at all. The moving ions carry the current.

2. BQ2 · Practice

Model answer. A potassium atom loses its single outer electron, leaving 2,8,8, a full outer shell. It still has 19 protons but now has 18 electrons, so its charge is 19 − 18 = +1: K⁺, a positive ion, a cation. Losing a negative electron makes the particle positive, not negative.

A full answer includes

I chose: it loses 1 electron and becomes K⁺, arranged 2,8,8.
I checked the charge: 19 protons − 18 electrons = 1+.
I did not choose K⁻: losing a negative electron leaves the ion positive.

Also accept

It loses one electron to form K⁺, 2,8,8.

Watch for

“It loses 1 electron and becomes K⁻.” — An electron is negative. Losing one leaves more protons than electrons, so the ion is positive: K⁺.
“It gains 1 proton and becomes K⁺.” — The nucleus does not change in a reaction. K⁺ is positive because it has one electron fewer than protons.
“It gains 7 electrons to fill its outer shell.” — Losing the 1 outer electron is far easier than gaining 7. Metal atoms lose electrons.

3. BQ3 · Practice

Model answer. An ionic bond is a strong electrostatic attraction between oppositely charged ions. The electron transfer comes first and makes the ions; the attraction between the ions is the bond. It is strong, which is why ionic compounds have high melting points. A shared pair of electrons is a covalent bond, not an ionic one.

A full answer includes

I chose: a strong electrostatic attraction between oppositely charged ions.
I did not choose the electron transfer: it makes the ions, and the bond comes after.
I did not choose a shared pair: nothing is shared in an ionic bond.

Also accept

A strong electrostatic attraction between oppositely charged ions.

Watch for

“A weak electrostatic attraction between oppositely charged ions.” — The attraction is strong: that is why ionic compounds need a lot of energy to melt.
“A pair of electrons shared between two oppositely charged ions.” — Ions do not share electrons. A shared pair is a covalent bond; in an ionic compound the electron has moved over completely.
“The electron transfer that turns two atoms into oppositely charged ions.” — The electron transfer makes the ions. The bond is the strong electrostatic attraction between the oppositely charged ions afterwards.

4. BQ4 · Practice

Options as numbered on the pupil sheet: 1 — Cation; 2 — Anion; 3 — Atom with no charge.

Model answer. The charge is protons − electrons. 13 − 10 = +3, a cation (Al³⁺). 9 − 10 = −1, an anion (F⁻). 10 − 10 = 0, an atom with no charge (neon). 20 − 18 = +2, a cation (Ca²⁺). 8 − 10 = −2, an anion (O²⁻). Cations are positive ions and anions are negative ions.

A full answer includes

I matched 13 protons, 10 electrons to cation (3+).
I matched 9 protons, 10 electrons to anion (1−).
I matched 10 protons, 10 electrons to atom with no charge.
I matched 20 protons, 18 electrons to cation (2+).
I matched 8 protons, 10 electrons to anion (2−).
I did not call a particle with extra electrons a cation.

Also accept

Cation; anion; atom; cation; anion.

Watch for

“A cation has extra electrons.” — A cation is positive: it has fewer electrons than protons. An anion has extra electrons.
“Every particle with 10 electrons is an ion.” — Count the protons too. With 10 protons and 10 electrons the charges cancel, so it is an atom.

5. BQ5 · Practice

Model answer. (a) The charge is protons − electrons = 16 − 18 = −2, so Z is 2−: a negative ion, an anion. (b) An atom has as many electrons as protons, so the atom had 16 electrons and gained 2 to reach 18. The element with 16 protons is sulfur, so Z is a sulfide ion, S²⁻. (c) In an ionic compound the metal atom transfers electrons to the non-metal atom; nothing is shared. The ionic bond is the strong electrostatic attraction between the oppositely charged ions that form. A shared pair of electrons is a covalent bond.

A full answer includes

(a) I wrote the charge: 16 − 18 = 2−.
(a) I wrote that Z is an anion.
(b) I wrote that the atom gained 2 electrons.
(b) I identified the element as sulfur, from its 16 protons.
(c) I wrote that in an ionic compound electrons are transferred, not shared.
(c) I wrote that the ionic bond is the strong electrostatic attraction between the oppositely charged ions.
I did not write that the atom lost or gained protons.

Also accept

(a) 2−, an anion. (b) It gained 2 electrons; sulfur. (c) The electrons are transferred, not shared; the bond is the electrostatic attraction between the oppositely charged ions.

Watch for

“Z is a cation, because 18 is more than 16.” — More electrons than protons gives a negative charge, so Z is an anion.
“The atom lost 2 protons to become Z.” — Protons stay in the nucleus during a reaction. Only the number of electrons changes.
“An ionic bond is a shared pair of electrons.” — A shared pair is a covalent bond. In an ionic compound the electrons are transferred, and the bond is the attraction between the ions.

6. BQ6 · Practice

Model answer. (a) X. (b) Ionic compounds have high melting points, and X melts at 770 °C. They conduct when molten but not when solid, and X does exactly that. W conducts as a solid too, which an ionic compound does not; it is a metal. Y melts far too low. Z has a high melting point but does not conduct even when molten.

A full answer includes

(a) I chose X.
(b) I gave its high melting point, 770 °C.
(b) I gave that it conducts when molten but not as a solid.
I did not choose W: an ionic solid does not conduct, but W does.

Also accept

(a) X. (b) High melting point; conducts when molten, not when solid.

Watch for

“W is ionic, because it conducts.” — An ionic solid does not conduct. W conducts as a solid, so it is a metal.
“Y is ionic, because it does not conduct as a solid.” — Y melts at only 80 °C and does not conduct when molten either. An ionic compound has a high melting point and conducts when molten.
“Z is ionic, because it has the highest melting point.” — A high melting point alone is not enough. An ionic compound conducts when molten, and Z does not.

7. BQ7 · Practice

Model answer. Magnesium oxide has a giant lattice of ions, like sodium chloride. Along each row of the diagram the ions go Mg²⁺, O²⁻, Mg²⁺, O²⁻, and the same is true down each column, so no ion has a neighbour of its own charge in a row or a column. Along a diagonal the ions are all of one kind, so they do not take turns there. The ions at the edges are cut off: the drawing is a window into the crystal, and a real crystal holds far more ions, all in the same pattern. There are no pairs: each ion is attracted to all the oppositely charged ions around it, four of them in this layer alone.

A full answer includes

I chose: along each row of ions, positive and negative ions take turns.
I chose: the same pattern carries on far past the edges of the diagram.
I did not choose the diagonals: along each diagonal the ions are all of one kind.
I did not choose one partner: each ion is attracted to all the oppositely charged ions around it.

Also accept

Along each row of ions, positive and negative ions take turns; the same pattern carries on far past the edges of the diagram.

Watch for

“Along each diagonal of the diagram, positive and negative ions take turns.” — Follow a diagonal ion by ion: every ion on it is of one kind. The ions take turns along the rows and the columns.
“Each magnesium ion is held by just one oxide ion, its partner.” — Each ion is attracted to all the oppositely charged ions around it, four of them in this layer alone. There are no pairs.
“The crystal is only as big as the drawing.” — The ions at the edges are cut off: the drawing is a window into the crystal. The same pattern carries on past its edges, as one giant lattice.

8. BQ8 · Practice

Model answer. Solid and molten lead(II) bromide both contain Pb²⁺ and Br⁻ ions. In the solid each ion is locked in its place in the lattice and cannot travel to a rod, so the solid does not conduct electricity. When it melts, the lattice breaks down and the ions are free to move, so they carry the charge and the bulb lights. Melting does not make the ions, free any electrons or change the size of the ions.

A full answer includes

I chose: in the solid the ions are held in place; in the melt they are free to move.
I did not choose free electrons: in a molten ionic compound the ions carry the charge.

Also accept

In the solid the ions are held in place; in the melt they are free to move.

Watch for

“In the solid there are no ions; in the melt the atoms have become ions.” — The ions are there in the solid too. They are held in fixed positions, so they cannot move and carry the charge.
“In the melt, free electrons carry the charge.” — In a molten ionic compound the moving particles are the ions, not electrons.
“In the melt the ions are smaller, so they can slip past each other.” — The ions are the same size in the solid and the melt. What changes is that the lattice breaks down, so the ions can move.

9. BQ9 · Practice

Model answer. (a) Calcium oxide has a giant lattice: a regular arrangement of alternating positive Ca²⁺ ions and negative O²⁻ ions that repeats in every direction. (b) Each ion is held by strong electrostatic attractions to all the oppositely charged ions around it, in every direction. To melt the solid, very many of these strong attractions must be overcome, which needs a lot of energy, so the melting point is high. There are no molecules, so there are no weak forces between molecules to break.

A full answer includes

(a) I wrote "giant lattice".
(a) I wrote that the Ca²⁺ and O²⁻ ions alternate in a regular arrangement.
(b) I wrote that each ion is held by strong electrostatic attractions to the oppositely charged ions around it.
(b) I wrote that many of these attractions must be overcome to melt the solid.
(b) I wrote that this needs a lot of energy.
I did not write about molecules or intermolecular forces: calcium oxide has none.

Also accept

(a) A giant lattice of alternating Ca²⁺ and O²⁻ ions. (b) Many strong attractions between oppositely charged ions must be overcome, which needs a lot of energy.

Watch for

“It melts high because strong bonds inside each CaO molecule must break.” — There are no molecules in an ionic compound: the whole lattice is held together by attractions between ions.
“Calcium oxide is a lattice of separate Ca–O pairs.” — Each ion is attracted to all the oppositely charged ions around it, not to one partner.

10. BT1 · Apply

Model answer. (a) A Group II atom has 2 outer electrons. The strontium atom loses both, so it forms Sr²⁺, a cation with a full outer shell. (b) Pure water on its own left the bulb dark, so water alone does not conduct. The solution conducts because it contains Sr²⁺ and Cl⁻ ions that are free to move and carry the charge. (c) In the crystal each strontium ion is surrounded by many chloride ions, and each chloride ion by strontium ions. The ionic bonds are the strong electrostatic attractions between all these oppositely charged neighbours, in every direction. It does not matter which chloride ions received the electrons. (d) In the dry crystals the ions are held in fixed positions in the lattice, so they cannot move and carry the charge. In the solution the ions are free to move, so they carry the current.

A full answer includes

(a) I predicted the charge 2+: Sr²⁺.
(a) I wrote that the strontium atom loses its 2 outer electrons.
(b) I used the result that pure water alone leaves the bulb dark.
(b) I wrote that the moving ions from the strontium chloride carry the charge.
(c) I wrote that each strontium ion is surrounded by many chloride ions in the lattice.
(c) I wrote that it is attracted to all of them, whichever ions took its electrons.
(d) I wrote that in the crystals the ions are held in fixed positions.
(d) I wrote that in the solution the ions are free to move and carry the charge.
I did not write that the dry crystals contain no ions.

Also accept

(a) Sr²⁺: the atom loses its 2 outer electrons. (b) Pure water alone does not conduct; the moving ions do. (c) Each strontium ion is attracted to all the chloride ions around it. (d) Ions fixed in the crystals, free to move in the solution.

Watch for

“A strontium ion is 1+, like a sodium ion.” — Strontium is in Group II: its atom loses 2 outer electrons, so its ion is 2+.
“The solution conducts because water conducts.” — Pure water leaves the bulb dark, and molten salt conducts with no water at all. The moving ions carry the current.
“Each ion is bonded only to the ion that took or gave its electrons.” — The bond is the attraction between oppositely charged ions, and each ion attracts all the oppositely charged ions around it.
“Solid salt has no charged particles; melting or dissolving makes the ions.” — The ions are there in the solid too. They are held in fixed positions, so they cannot move and carry the charge.