4.2 Bonding and structure
- Syllabus
- 9701–2028–2029
- Topic
- 4.2
- Level
- AS
Learning objectives
4.2.1Describe, in simple terms, the lattice• Describe, in simple terms, the lattice structure of a crystalline solid which is:- (a) giant ionic, including sodium chloride and magnesium oxide- (b) simple molecular, including iodine, buckminsterfullerene C60 and ice- (c) giant molecular, including silicon( IV) oxide, graphite and diamond- (d) giant metallic, including copper4.2.2Describe, interpret and predict the effect• Describe, interpret and predict the effect of different types of structure and bonding on the physical properties of substances, including melting/boiling point, conductivity and solubility4.2.3The type of structure and bonding present• Deduce the type of structure and bonding present in a substance from given information
Four lattice types differ in their repeating particles and attractions
A lattice is a regular, repeating three-dimensional arrangement of particles. Classify a crystalline solid by identifying the particles repeated through the structure and the attraction holding them together.
| Lattice type | Repeating particles | Attraction throughout the solid | Required examples |
|---|---|---|---|
| giant ionic | positive and negative ions | strong electrostatic attraction between oppositely charged ions | NaCl, MgO |
| simple molecular | discrete molecules | intermolecular forces between molecules | iodine, I₂; buckminsterfullerene, C₆₀; ice, H₂O |
| giant molecular (giant covalent) | atoms | covalent bonds in a continuous network | SiO₂, graphite, diamond |
| giant metallic | positive metal ions and delocalised electrons | electrostatic attraction between the ions and delocalised electrons | copper |
Graphite is a layered giant covalent network, whereas C₆₀ consists of separate molecules even though both contain only carbon. Ice is simple molecular: covalent O–H bonds lie within each molecule and hydrogen bonds join neighbouring molecules in the lattice.
A formula alone does not establish lattice type. Use evidence about the particles and bonding: SiO₂ is a network, not a collection of separate SiO₂ molecules, while C₆₀ has many atoms but remains a discrete molecule.
Forces and mobile charged particles explain a solid's physical properties
Melting or boiling requires attractions between particles to be overcome. Electrical conduction additionally requires mobile charged particles. Solubility depends on whether new solute–solvent attractions can compensate for attractions within the solid and solvent.
| Structure | Melting and boiling | Electrical conductivity | Solubility pattern |
|---|---|---|---|
| giant ionic | high: many strong ion–ion attractions must be overcome | not when solid; conducts when molten, and in aqueous solution if soluble, because ions can move | many dissolve in polar water when ion–dipole attractions and hydration compensate for lattice separation; not all ionic solids are soluble |
| simple molecular | usually low relative to giant structures because only intermolecular forces are overcome; hydrogen bonding can raise the values | does not conduct because there are no mobile charged particles | depends on polarity and intermolecular forces; substances tend to dissolve in solvents able to form comparable attractions |
| giant covalent | very high because many covalent bonds must be broken | usually does not conduct; graphite conducts along its layers using delocalised electrons | generally insoluble because separating the network would require breaking covalent bonds |
| giant metallic | often high, but varies with the metal | conducts as a solid and liquid because delocalised electrons can move | generally insoluble; a metal may react with a solvent instead of simply dissolving |
Diamond and SiO₂ have no mobile charge carriers, so they are electrical insulators. Graphite is the important exception: each carbon forms three covalent bonds, leaving delocalised electrons that move along the layers.
Do not infer a property from bond strength alone. In a simple molecular substance, melting separates molecules without breaking their internal covalent bonds; in an ionic solid, melting frees ions to move without turning them into atoms.
Deduce structure by combining melting, conductivity and solubility evidence
Record the conditions attached to every observation, especially whether conductivity was measured in the solid, molten or aqueous state. Match at least two independent properties to a structural model, then explain each match using particles, attractions and charge mobility.
| Combined evidence | Most likely structure | Causal interpretation |
|---|---|---|
| low melting/boiling; non-conductor in every pure state | simple molecular | weak intermolecular forces; no mobile charged particles |
| high melting; non-conducting solid but conducting melt, and conducting aqueous solution if soluble | giant ionic | strong ion–ion attractions; ions are fixed in the solid but mobile in melt or solution |
| high melting; conducting solid and liquid; often malleable | giant metallic | strong metallic attraction; mobile delocalised electrons |
| very high melting; insoluble; non-conductor | giant covalent such as diamond or SiO₂ | continuous covalent network; no mobile charged particles |
| very high melting; solid conductor; soft or layered | graphite | giant covalent layers with delocalised electrons and weaker forces between layers |
Suppose a solid has a melting point near 800 °C, does not conduct as a solid, but its melt and aqueous solution conduct. The high melting point suggests strong attractions, and conduction only after ions become mobile identifies a giant ionic lattice.
One property is rarely decisive: both ionic and giant covalent solids can have high melting points, while both metals and graphite conduct as solids. A defensible deduction combines the full pattern and states why competing structures are rejected.