Answers · Ions and ionic bonding

Ions and ionic bonding — final check key

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

1. BK1 · Final check

Model answer. (a) P. It has a high melting point (711 °C), does not conduct as a solid, and conducts when molten and in solution. R conducts as a solid as well as when molten, which an ionic compound never does: R is a metal (zinc). Q melts low and never conducts (glucose). (b) 12 − 10 = 2, so one ion is 2+; 35 − 36 = −1, so the other is 1−. The 2+ ion (Mg²⁺) forms when a metal atom (magnesium) loses its 2 outer electrons; the 1− ion (Br⁻) forms when a non-metal atom (bromine) gains 1 electron. P is magnesium bromide. (c) A giant lattice: a regular arrangement of alternating positive and negative ions, held together by ionic bonds, the strong electrostatic attractions between oppositely charged ions. (d) Melting means overcoming very many strong attractions in every direction, which needs a lot of energy, so the melting point is high. In the solid the ions are held in fixed positions, so they cannot carry the charge. When the solid is molten or dissolved, the ions are free to move and carry the current.

A full answer includes

(a) I identified P.
(a) I wrote that R also conducts as a solid.
(a) I wrote that an ionic compound does not conduct as a solid.
(b) I wrote the charge 12 − 10 = 2+.
(b) I wrote the charge 35 − 36 = 1−.
(b) I wrote that the 2+ ion forms when a metal atom loses 2 electrons.
(b) I wrote that the 1− ion forms when a non-metal atom gains 1 electron.
(c) I wrote "giant lattice".
(c) I wrote that the positive and negative ions alternate in a regular arrangement.
(c) I wrote: strong electrostatic attraction between oppositely charged ions.
(d) I wrote that many strong attractions between the ions must be overcome to melt it.
(d) I wrote that this needs a lot of energy.
(d) I wrote that in the solid the ions are held in fixed positions.
(d) I wrote that when molten or dissolved the ions are free to move and carry the charge.
I did not write that electrons carry the current in the melt or the solution.

Also accept

(a) P; R conducts as a solid, so it is not ionic. (b) 2+ (lost 2 electrons) and 1− (gained 1). (c) Giant lattice of alternating ions; strong electrostatic attraction. (d) Strong attractions need much energy; ions fixed in the solid, free to move when molten or dissolved.

Watch for

“R is ionic, because it conducts when molten.” — R conducts as a solid too. An ionic solid does not conduct, so R is a metal.
“Q is ionic, because it dissolves in water.” — Its solution does not conduct and it melts at only 146 °C, so it is not made of ions.
“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.
“In the melt, electrons carry the current.” — In a molten ionic compound the moving particles are the ions, not electrons.

2. BK2 · Final check

Model answer. (a) A Group I atom has 1 outer electron. The rubidium atom loses it and forms Rb⁺; the chlorine atom (Group VII) gains it and forms Cl⁻. One electron lost and one gained, so the ions combine 1 : 1: RbCl. In the diagram the chloride ion’s outer shell holds chlorine’s own 7 electrons as dots and rubidium’s electron as 1 cross, in square brackets with charge −. (b) Each magnesium atom loses its 2 outer electrons: Mg²⁺. Each nitrogen atom has 5 outer electrons and gains 3 to make 8: N³⁻. The electrons lost must equal the electrons gained: 3 magnesium atoms lose 3 × 2 = 6, and 2 nitrogen atoms gain 2 × 3 = 6. So the ratio is 3 Mg²⁺ to 2 N³⁻: Mg₃N₂. Each nitride ion’s outer shell has 5 dots and 3 crosses.

A full answer includes

(a) I wrote that the rubidium atom loses its 1 outer electron: Rb⁺.
(a) I wrote that the chlorine atom gains that electron: Cl⁻.
(a) I gave the ratio 1 : 1.
(a) I described the chloride ion’s outer shell: 7 dots and 1 cross.
(b) I wrote Mg²⁺ for the magnesium ion.
(b) I wrote N³⁻ for the nitride ion.
(b) I wrote the ratio 3 : 2, magnesium ions to nitride ions.
(b) I checked the electrons lost: 3 × 2 = 6.
(b) I checked the electrons gained: 2 × 3 = 6.
(b) I described each nitride ion’s outer shell: 5 dots and 3 crosses.
I did not write the ratio the wrong way round, as 2 : 3.

Also accept

(a) Rb⁺ and Cl⁻, 1 : 1; the chloride ion’s outer shell has 7 dots and 1 cross. (b) Mg²⁺ and N³⁻, 3 : 2; 5 dots and 3 crosses.

Watch for

“Rubidium gains 7 electrons to fill its outer shell.” — Rubidium is a Group I metal: losing 1 electron is far easier than gaining 7.
“Magnesium nitride is MgN, one of each.” — Mg²⁺ and N³⁻ do not cancel. 3 Mg²⁺ (6+) balance 2 N³⁻ (6−).
“Each nitride ion has 8 dots.” — The 3 electrons from magnesium keep their crosses: 5 dots and 3 crosses.