Ionic bonding in magnesium oxide
2 marks · HardDescribe how a dot-and-cross diagram can be used to show the formation of magnesium oxide (MgO) from magnesium and oxygen atoms. Your answer should include arrangements and charges.
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A dot-and-cross diagram shows two electrons (crosses) from magnesium transferred to oxygen (dots), forming a [Mg]^{2+} ion with no electrons in its outer shell and an [O]^{2-} ion with eight electrons in its outer shell. The charges must be shown.
Must show: Shows 2 electrons transferred from Mg to O, Mg becomes Mg^{2+} with full outer shell lost, O becomes O^{2-} with full outer shell (8 electrons), Charges [2+] on Mg and [2-] on O shown
Also accept: Different electron symbols (dots/crosses) as long as atoms are distinguished
Common mistakes: Missing charge signs; Incorrect electron count in shells
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Marking points
B1 · 1 mark · Describes correct transfer of 2 electrons from magnesium to oxygen and resulting ions.
B2 · 1 mark · Mentions charges on each ion are correctly shown in the diagram ([Mg]^{2+} and [O]^{2-}).
Worked solution
In a dot-and-cross diagram for MgO: draw a magnesium ion ([Mg]^{2+}) showing two electrons (crosses) lost from its shell, and an oxide ion ([O]^{2-}) with eight electrons in its outer shell (dots plus the two crosses gained from Mg). Show [2+] charge on Mg and [2-] on O.