The four types of crystal structure: ionic metallic macromolecular (giant…

The idea

Four structures cover every solid on this course. In an ionic crystal, oppositely charged ions occupy the lattice points and strong electrostatic attraction must be overcome to melt it. In a metallic crystal, positive ions sit in a sea of delocalised electrons. In a macromolecular or giant covalent crystal, the atoms are joined by a continuous network of covalent bonds, so melting means breaking covalent bonds and the melting point is very high. In a molecular crystal, whole molecules sit at the lattice points held by weak intermolecular forces, so melting only separates molecules and the covalent bonds inside them survive untouched. Identify the structure first, then name the force broken, then predict the property.

What earns the marks

• Naming the type of crystal structure.
• Identifying the particles at the lattice points, whether ions, atoms, molecules or ions and electrons.
• Naming the force or bond overcome on melting.
• Linking the strength of that force to the melting point or conductivity.

Common mistakes and how to put them right

• Saying covalent bonds break when a molecular solid such as iodine or ice melts.

• Describing a giant covalent structure as containing molecules.

• Naming a molecular solid as simple covalent without naming the intermolecular force.

• Claiming an ionic solid conducts electricity because it contains charged particles.

Exam tips

• Answer in the order structure, particles, force, property; that is how these questions are marked.
• Never let intermolecular force and covalent bond appear interchangeably in the same answer.
• A contradiction later in an answer cancels an earlier correct statement, so do not hedge.