Classify molecular, ionic, metallic and covalent-network substances from property data.
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Method and worked examples
Classify substances using several properties together. Molecular substances contain separate molecules and often have relatively low melting points because melting overcomes intermolecular attractions. Ionic substances contain oppositely charged ions in a continuous lattice; they commonly have high melting points and conduct when molten or dissolved, if mobile ions are present. Their solid ions cannot move through the lattice.
Metals conduct as solids and liquids because they contain mobile delocalised electrons. Covalent networks contain extended covalent bonding and generally have high melting temperatures. Diamond does not conduct electricity, but graphite conducts along its layers because some electrons are delocalised. A single property can fit more than one category, so combine conductivity, melting behaviour and structural information.
Example 1
X has a high melting point, does not conduct as a solid and conducts when molten. Classify X.
Ionic: fixed ions cannot carry current in the solid, but molten ions are mobile.
Example 2
Y conducts as a solid, is malleable and has a high melting point. Classify Y.
Metallic: mobile electrons carry charge and bonding allows layers to move without losing all attraction.