Answers · Ions and ionic bonding

Ions and ionic bonding — return questions key

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

1. BR1 · Return

Model answer. (a) An electron is negative. A bromine atom has equal numbers of protons and electrons; gaining one electron gives it one more electron than protons, so the ion is Br⁻, an anion, not Br⁺. (b) The two electrons moving from calcium to bromine are the electron transfer: it makes the Ca²⁺ and Br⁻ ions. The ionic bond is what holds these ions together afterwards: the strong electrostatic attraction between the oppositely charged ions. (c) High melting and boiling points. It does not conduct as a solid, but it conducts when molten and when aqueous. (d) Calcium bromide is a giant lattice of Ca²⁺ and Br⁻ ions. Each ion is held by strong electrostatic attractions to the oppositely charged ions around it. Many of these must be overcome to melt the solid, which needs a lot of energy, so the melting point is high (it melts at 730 °C). In the liquid the ions are still close together and still attract each other strongly. To boil it, the ions must be pulled far apart, so these attractions must be overcome too, which needs even more energy: the boiling point is even higher.

A full answer includes

(a) I wrote that an electron is negative.
(a) I wrote that the bromide ion has one more electron than protons, so it is Br⁻.
(b) I wrote that the electron transfer only makes the ions.
(b) I wrote that the ionic bond is the strong electrostatic attraction between the oppositely charged ions.
(c) I predicted a high melting point.
(c) I predicted a high boiling point.
(c) I predicted that it does not conduct as a solid.
(c) I predicted that it conducts when molten.
(c) I predicted that it conducts when aqueous.
(d) I wrote that calcium bromide is a giant lattice of 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 the ions in the liquid still attract each other strongly.
(d) I wrote that boiling must overcome these remaining attractions, so the boiling point is high too.
I did not write that melting or boiling breaks bonds inside molecules: calcium bromide has none.

Also accept

(a) An electron is negative, so gaining one gives Br⁻. (b) The transfer makes the ions; the bond is the strong electrostatic attraction between them. (c) High; no as a solid, yes molten and aqueous. (d) Giant lattice; many strong attractions need a lot of energy to overcome; strong attractions remain in the liquid and must be overcome to boil it.

Watch for

“Gaining an electron makes an ion positive.” — An electron carries a negative charge, so gaining one makes the ion negative.
“The ionic bond is the electron moving from the metal to the non-metal.” — The electron transfer makes the ions. The bond is the strong electrostatic attraction between the oppositely charged ions afterwards.
“Solid calcium bromide conducts, because it is made of ions.” — The ions are there, but in the solid they are held in fixed positions and cannot move.
“Once calcium bromide has melted, the attractions between its ions are gone.” — In the liquid the ions are still close together and still attract each other strongly. Boiling must overcome these attractions, which is why the boiling point is high too.

2. BR2 · Return

Model answer. (a) A potassium atom has only 1 outer electron. When the ions form, that electron moves to the iodine atom: it is the cross in the iodide ion’s outer shell. It cannot also stay on the potassium ion, so the drawing shows the same electron twice. Once it has gone, the potassium ion’s outer shell is the full shell underneath: 8 crosses, in square brackets with charge +. The iodide ion is drawn correctly: 7 dots and 1 cross, with charge −. (b) The X ion’s own electrons are the dots: 6, so an X atom has 6 outer electrons and X is in Group VI. The 2 crosses are the electrons it gained, one from each M atom, which gives it a charge of 2−. Each M ion is 1+, so each M atom lost 1 electron. A full outer shell of 2 can only be the first shell, so each M ion has just that one shell: the M atom had 2 + 1 = 3 electrons, arranged 2,1. The element with 3 electrons, and so 3 protons, is lithium, in Group I. The compound is lithium oxide or another lithium compound of a Group VI element.

A full answer includes

(a) I wrote that a potassium atom has only 1 outer electron.
(a) I wrote that this electron moved to iodine: it is the cross in the iodide ion.
(a) I wrote that the drawing shows this one electron twice.
(a) I wrote that the potassium ion’s outer shell should be the full shell underneath: 8 crosses.
(b) I wrote Group VI for X: its own outer electrons are the 6 dots.
(b) I wrote that each M atom lost 1 electron, because its ion is 1+.
(b) I wrote that a full outer shell of 2 is the first shell, so the M ion has no other shells.
(b) I wrote that an M atom had 2 + 1 = 3 electrons.
(b) I identified M as lithium.
I did not count the X ion’s 2 crosses as X’s own electrons.
I did not put M in Group II: its 2 crosses are what is left after it lost 1.

Also accept

(a) Potassium’s one outer electron is drawn twice: it moved to iodine, so the potassium ion should show its full shell of 8 crosses. (b) X is in Group VI. M is lithium: after losing 1 electron its ion keeps only a first shell of 2, so the atom had 3 electrons.

Watch for

“The potassium ion keeps its outer electron and also gives one to iodine.” — A potassium atom has only 1 outer electron. Once it has moved to iodine, it is no longer on the potassium ion.
“The iodide ion is drawn wrongly: the electron it gained should be a dot.” — The cross is right: it shows that this electron came from potassium. The fault is on the potassium ion.
“M is in Group II, because each M ion shows 2 crosses.” — The 2 crosses are what the M ion has left after losing 1 electron. Its charge is 1+, so the atom had 2 + 1 = 3 electrons: lithium, in Group I.
“X is in Group VIII, because its ion has 8 outer electrons.” — Two of the 8 are crosses that came from M. The X atom had 6, the dots: Group VI.