Magnesium Oxide

Magnesium oxide is the go-to compound for explaining why some ionic solids melt at extreme temperatures. Because both of its ions carry a double charge, MgO is the example examiners use to test whether you can link ionic charge to lattice strength.

Key facts

Property	Detail
Formula	MgO
Relative formula mass (Mr)	40 (Mg 24, O 16)
Bonding / structure	Giant ionic lattice (Mg2+ and O2- ions)
Melting point	2852 C
Classification	Basic oxide
Main exam roles	Ionic bonding diagram, melting-point reasoning, antacid

Where this compound appears in 0620

• Ions and ionic bonds (Topic 2): the dot-and-cross diagram for MgO showing two electrons transferred from magnesium to oxygen.
• Reactions of metals (Topic 10): burning magnesium in oxygen and the observations involved.
• Oxides (Topic 8): a basic (metal) oxide that neutralises acids; the hydroxide is used as an antacid.
• Structure and properties: relating the very high melting point to the giant ionic structure.

Reactions you must know

Formation (burning magnesium):

2Mg(s) + O2(g) -> 2MgO(s)

Magnesium burns with a brilliant white flame; the white ash is MgO. In the dot-and-cross diagram, each Mg atom (2,8,2) loses two electrons and each O atom (2,6) gains two, so both ions reach the configuration 2,8.

With acids (basic oxide):

MgO(s) + 2HCl(aq) -> MgCl2(aq) + H2O(l) MgO(s) + H2SO4(aq) -> MgSO4(aq) + H2O(l)

With water (slightly):

MgO(s) + H2O(l) -> Mg(OH)2(aq)

The weakly alkaline suspension is milk of magnesia, used as an antacid to neutralise excess stomach acid.

Why the melting point is so high

Melting a giant ionic lattice means overcoming the electrostatic attraction between every neighbouring pair of ions. That attraction depends on the product of the ionic charges: in MgO the charges are 2+ and 2-, whereas in sodium chloride they are only 1+ and 1-. The stronger attraction in MgO is why it melts at 2852 C compared with 801 C for NaCl. Its very high melting point makes MgO useful as a refractory (heat-resistant) furnace lining.

Worked exam question

Magnesium has the electronic configuration 2,8,2 and burns in oxygen to form magnesium oxide.

(a) Describe, in terms of electron transfer, how magnesium atoms and oxygen atoms form the ions in magnesium oxide. (3 marks)

(b) Magnesium oxide melts at 2852 C, far higher than sodium chloride at 801 C. Explain this difference. (3 marks)

Mark scheme

• (a) Each magnesium atom loses two electrons to form an Mg2+ ion [1]; each oxygen atom gains two electrons to form an O2- ion [1]; both ions then have a full outer shell / noble gas configuration (2,8) [1]
• (b) MgO contains ions with charges of 2+ and 2-, while NaCl has 1+ and 1- ions [1]; the higher charges give stronger electrostatic attraction between the ions in MgO [1]; so more energy is needed to break the lattice, giving the higher melting point [1]

Examiner note: in (a) electrons are transferred, not shared — the word “share” describes covalent bonding and scores zero. In (b) the charges of both compounds must be stated for the comparison mark.

Common exam mistakes

• Describing the bonding as “sharing” electrons. MgO is ionic: electrons are transferred from magnesium to oxygen, not shared.
• Writing the formula as Mg2O2 or MgO2. The ions Mg2+ and O2- combine in a 1:1 ratio, so the formula is simply MgO.
• Explaining the high melting point as just “strong bonds”. You must compare the ionic charges (2+/2- against 1+/1-) to earn the marks.
• Vague burning observations. The correct observation is a brilliant white flame/light leaving a white solid (MgO) — not “smoke” or “sparks” — and the light is bright enough to warn against looking directly at it.