Ionic bonding in magnesium oxide
4 marks · NormalThis question is about the bonding in magnesium oxide.
Complete the dot-and-cross diagram for the oxide ion (O^{2-}) formed from an oxygen atom during the bonding in MgO. Show the arrangement of electrons in its outer shell after the transfer.
Explain why the oxide ion has a 2- charge after bonding occurs.
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(a) O^{2-} ion with 8 electrons in its outer shell, 6 as dots, 2 as crosses (to show the two gained electrons), and charge labelled as 2-. · must show 8 electrons in outer shell, 2 electrons represented as crosses, Charge labelled as 2-
(b) Because oxygen atom gains 2 electrons, so it has 2 more electrons than protons, giving it a 2- charge. · must show Oxygen gains 2 electrons, Now has 2 extra electrons compared to protons, Charge is 2-
Must show: Correct dot/cross electron shell for O^{2-}, Extra electrons shown as crosses
Common mistakes: Only 6 electrons shown in outer shell; No explanation of charge
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Marking points
B1 (a) · 1 mark · Draws O^{2-} ion with 8 outer electrons (6 dots from O plus 2 crosses from Mg).
B2 (a) · 1 mark · Labels ion with 2- charge.
B3 (b) · 1 mark · Explains that oxygen gains 2 electrons.
B4 (b) · 1 mark · Links 2 gained electrons to the overall 2- charge (more electrons than protons).
Worked solution
a) The O^{2-} ion has 8 electrons in its outer shell, with 6 dots (original electrons) and 2 crosses (from magnesium), all in brackets, with 2- as a superscript charge.
b) The oxide ion has a 2- charge because it has gained two electrons from magnesium. This means it has 2 more electrons than protons.