Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Malleability and ductility
Layers of positive metal ions can slide past one another while remaining attracted to the delocalised electron sea. Metals can therefore be hammered into sheets and drawn into wires.
Ionic brittleness
If ionic layers shift, ions of like charge may become adjacent. Strong repulsion splits the crystal. Ionic structures are strongly bonded but brittle.
Melting-point comparison
Ionic and giant covalent structures usually have high melting points because strong forces or bonds extend throughout the structure. Simple molecular substances have lower melting points because only weak intermolecular forces are overcome.
Conductivity comparison
Metals and graphite conduct as solids because they contain mobile delocalised electrons. Ionic compounds conduct only when molten or dissolved. Most simple molecular and other giant covalent substances do not conduct.
Selecting materials
A material should be chosen by linking a required property to its structure. Diamond suits cutting, graphite suits electrodes and lubricants, and metals suit wires and shaped components.
Practical or data skill
Identify unknown structure types from melting point, state, solubility and conductivity data. Justify each classification with at least two observations.
Examination tip
Name the mobile charge carrier: ions in ionic melts and solutions; electrons in metals and graphite.
Review questions and suggested answers
Question 1
Why are metals malleable?
Suggested answer
Ion layers slide while attraction to delocalised electrons remains.
Question 2
Why are ionic solids brittle?
Suggested answer
Layer movement brings like charges together, causing repulsion.
Question 3
Which solids conduct?
Suggested answer
Metals and graphite.
