Ionic bonding in magnesium oxide
4 marks · NormalThis question is about magnesium oxide.
Draw a dot-and-cross diagram to show the ionic bonding in magnesium oxide (MgO). Clearly show all the electrons, their arrangement, and the charges on the ions.
Why are both ions in MgO drawn with full outer shells?
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(a) Fully correct dot-and-cross diagram showing 2 electrons transferred from Mg to O, Mg^{2+} [2,8], O^{2-} [2,8], correct charges and correct use of dots/crosses. · must show 2 electrons transferred from Mg to O, 8 electrons around each ion, Correct charges (+2 on Mg, -2 on O), Clear indication of transferred electrons with two symbols
(b) Because both ions have a full outer shell after electron transfer · must show Reference to full outer shell (noble gas configuration)
Must show: Mg ion with 8 electrons (2,8), O ion with 8 electrons (2,8), 2+ charge on Mg ion, 2- charge on O ion, Use dot and cross to show origin of the electrons
Common mistakes: Missing charges; Not showing electron transfer
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Marking points
B1 (a) · 1 mark · Draws Mg ion [2,8], O ion [2,8], showing correct numbers of electrons.
B2 (a) · 1 mark · Shows that 2 electrons have been transferred from Mg to O (with dots/crosses or lines/arrows).
B3 (a) · 1 mark · Correctly labels charges: Mg^{2+} and O^{2-}.
B4 (b) · 1 mark · States both ions have a full outer shell (noble gas configuration) after bonding.
Worked solution
The diagram shows Mg with 2,8 electrons (all crosses), O with 8 electrons (all dots), two of O's electrons being crosses (to show they were transferred), surrounded by brackets, with Mg^{2+} and O^{2-} outside the brackets.
Both ions are drawn with full outer shells because, after electron transfer, each achieves a noble gas electron configuration.