4.2 Bonding and structure

A crystalline lattice is a regular, repeating arrangement of particles. Classify the lattice by asking what repeats: ions in a giant ionic lattice, atoms in a giant covalent or metallic lattice, or discrete molecules in a simple molecular lattice.

In a giant ionic lattice, oppositely charged ions alternate through the structure and are held by electrostatic attraction in all directions. The arrangement and relative ion sizes determine the packing; sodium chloride and magnesium oxide are source-supported examples of cubic ionic lattices.

Covalent substances may form simple molecular lattices, such as iodine, buckminsterfullerene and ice, where discrete molecules are arranged together, or giant molecular lattices, such as diamond, graphite and silicon(IV) oxide, where covalent bonding extends through a large network. Do not treat all covalent substances as one lattice type.

A giant metallic lattice contains positive metal ions arranged regularly and surrounded by a sea of delocalised electrons. The ions may form layered or cubic arrangements; copper is a source-supported example. The delocalised electrons are part of the metallic structure, not separate molecules between the ions.

Giant ionic substances have high melting and boiling points because strong electrostatic attractions hold oppositely charged ions throughout the lattice. They are brittle because shifting layers can bring like charges together and split the crystal. Ions are fixed in a solid but mobile when molten or dissolved, so conductivity depends on state; ion–dipole interactions can support solubility in water.

Giant metallic substances are strong and generally have high melting and boiling points because positive metal ions attract delocalised electrons. Mobile electrons allow conduction in both solid and liquid states. Layers can slide while the metallic attraction is maintained, giving malleability; more delocalised electrons and smaller cations strengthen the attraction in the source-supported comparison.

Simple molecular substances generally have low melting and boiling points because only weak intermolecular forces need to be overcome between molecules; they usually do not conduct because they lack mobile charged particles. Polarity or hydrogen bonding can change solubility and attraction strength. The covalent bonds inside each molecule are not broken during melting or boiling.

Giant covalent substances have high melting and boiling points because many strong covalent bonds extend through the network. Diamond and silicon(IV) oxide are hard and lack mobile charge carriers, whereas graphite is soft because its layers attract weakly and can slide, and it conducts along the layers because it has delocalised electrons. Use these as structure-based exceptions, not as a rule that all carbon forms behave alike.

Explain a property by naming the structure, the relevant attraction or mobile particle, and the condition being tested. Do not infer ionic structure from melting point alone, say that every ionic solid conducts, or treat ‘insoluble’ and ‘non-conducting’ as interchangeable evidence.

Start by recording the conditions of the evidence: melting or boiling behaviour, electrical conductivity in the stated state, solubility in water, hardness or malleability, and any physical state. Do not compare a solid conductivity result with a molten or aqueous result as if they were the same test.

Use the evidence to narrow the structure: high melting behaviour suggests strong attractions or an extended network; mobile charge in a solid supports a metallic structure, while mobile charge only when molten or in solution supports an ionic structure. Low melting behaviour and weak conductivity support a simple molecular structure, subject to polarity and hydrogen-bonding exceptions.

Check the remaining observations against the candidate. Distinguish giant covalent materials from simple molecular substances using the extent of covalent bonding and the resulting melting behaviour; use graphite’s delocalised electrons and layered softness, and diamond or silicon(IV) oxide’s hard network, as source-supported contrasts.

State the best-supported structure and the linked causal evidence: particle type and bonding/attraction → mobility or energy needed → observed properties. A single property is rarely decisive; if evidence is incomplete or an exception applies, state the limitation rather than forcing a classification.

Do not identify a structure from melting point alone, call every high-melting substance ionic, or treat insolubility as proof of one structure. Keep the deduction on the four syllabus structure classes and their source-supported property signatures; detailed reaction or energetics analysis belongs elsewhere.