Metallic Bonding
3 marks · NormalThe melting point of magnesium (Group 2) is much higher than that of sodium (Group 1).
Using information about metallic bonding, calculate the number of delocalised electrons contributed per atom in each metal, and explain how this relates to their melting points.
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Also accept: Any correct reference to group number and number of delocalised electrons per atom
Common mistakes: Confusing number of electrons with overall charge; Not linking number of delocalised electrons to bond strength
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Marking points
M1 · 1 mark · Identifies that sodium contributes 1 and magnesium contributes 2 delocalised electrons per atom.
M2 · 1 mark · Links number of delocalised electrons to strength of electrostatic forces/metallic bonding.
A1 · 1 mark · Explains that the higher melting point of magnesium is due to its stronger metallic bonding resulting from more delocalised electrons and higher ionic charge.
Worked solution
Sodium (Group 1) has one electron in its outer shell, so each sodium atom contributes 1 delocalised electron. Magnesium (Group 2) has two outer electrons, so each atom contributes 2 delocalised electrons.
More delocalised electrons mean stronger electrostatic attraction between positive ions and electrons.
Magnesium ions are 2+ and sodium ions are 1+, so the attraction is much stronger in magnesium, resulting in a higher melting point.