Bonding and physical properties — AQA A-Level Chemistry
Test yourself on Bonding and physical properties with AQA A-Level practice questions.
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Bonding and physical properties explained
Four structures cover every solid on this course.
Read the full explanation
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.
The structures of the following crystals as examples of these four types of crystal structure: diamond graphite ice iodine magnesium sodium chloride.
Six named crystals illustrate the four types. Diamond is giant covalent, each carbon bonded to four others tetrahedrally at 109.5°, so it is extremely hard, melts very high and does not conduct. Graphite is also giant covalent but each carbon bonds to only three, leaving one delocalised electron per atom, so it conducts along its layers, which are held together by weak London forces and slide easily. Iodine is molecular, a lattice of I₂ molecules held by London forces. Ice is molecular, held by hydrogen bonds in an open tetrahedral cage. Magnesium is giant metallic with 2⁺ ions and two delocalised electrons per atom. Sodium chloride is giant ionic with 6:6 co-ordination.
Your focus
- Classify a named solid into one of the four crystal types from data on its melting point and conductivity.
- Name the force overcome when each of the four structure types melts.
- Explain why melting a molecular solid does not break any covalent bonds.
Show all 6 objectives
- Explain why graphite conducts electricity and is soft while diamond does neither, using the bonding in each.
- Describe the arrangement of particles in the sodium chloride lattice and in a lattice of iodine molecules.
- Predict the relative melting points of magnesium, iodine and diamond and justify the order.
Bonding and physical properties exam tips
Marking Points
- one mark for naming the type of crystal structure
- one mark for identifying the particles at the lattice points, whether ions, atoms, molecules or ions and electrons
- one mark for naming the force or bond overcome on melting
- one mark for linking the strength of that force to the melting point or conductivity
- one mark for the correct structure type assigned to the named crystal
- one mark for the arrangement described, such as each carbon bonded to four others in diamond or three in graphite
- one mark for naming the force broken when that crystal melts
- one mark for the property explained from that structure, such as conduction in graphite from delocalised electrons
Examiner 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.
- 💡Learn the six crystals as a table of structure, bonding, melting point and conductivity.
- 💡For graphite, mention both the strong covalent bonds within layers and the weak forces between them.
- 💡Quote co-ordination numbers where you know them; they show you can describe the lattice, not just name it.
Common Mistakes
- 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
- saying graphite conducts because it contains ions rather than delocalised electrons
- giving the bond angle in diamond as 90° or 120° instead of 109.5°
- describing iodine as a giant covalent structure because it is a solid that sublimes
- explaining the low density of ice without naming hydrogen bonds holding an open lattice