Ionic bonding in magnesium oxide
4 marks · NormalThe table below shows the number of electrons in the outer shell for atoms and the resulting ions in magnesium oxide.
| Particle | Electrons in Outer Shell |
|---|---|
| Mg atom | 2 |
| Mg ion | 8 |
| O atom | 6 |
| O ion | 8 |
Using the data, explain how the outer shell electron configuration changes for magnesium and oxygen when MgO forms.
How does this information support the use of dot-and-cross diagrams in representing magnesium oxide?
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(a) Magnesium loses 2 outer electrons (2→8), oxygen gains 2 (6→8), both achieve full outer shells · must show Magnesium: 2 outer electrons in atom, 8 in ion, Oxygen: 6 outer electrons in atom, 8 in ion, Electron transfer leads to full outer shells
(b) Dot-and-cross diagrams show the transfer, showing both ions with 8 electrons in the outer shell, matching the ions' configuration · must show Dots and crosses represent the transferred electrons, Both ions have complete outer shells in diagram
Common mistakes: Saying number of electrons in atom and ion is the same
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Marking points
M1 (a) · 1 mark · Identifies that magnesium goes from 2 to 8 outer electrons, oxygen from 6 to 8
M2 (a) · 1 mark · Explains this is due to loss/gain of electrons leading to full shells (noble gas configuration)
M3 (b) · 1 mark · States that dot-and-cross diagrams represent electron transfer/outer shell configuration
M4 (b) · 1 mark · Links the diagram's full outer shells to the ions' configurations
Worked solution
a) Magnesium atom has 2 outer electrons; when it forms an ion, it loses 2, resulting in 8 in its next shell. Oxygen atom has 6 outer electrons; it gains 2 to get 8 in its outer shell. Both achieve full (noble gas) outer shells.
b) Dot-and-cross diagrams show which electrons are transferred. After transfer, both ions have full outer shells (8 electrons), matching the data for Mg^{2+} and O^{2-} ions.