Crystalline Solids: MDCAT Chemistry notes
Crystalline Solids MDCAT notes: crystalline vs amorphous solids, anisotropy, isomorphism, polymorphism, and ionic, covalent, molecular and metallic solids.
Crystalline and amorphous solids
| Property | Crystalline solid | Amorphous solid |
|---|---|---|
| Arrangement | Regular, long-range order | No long-range order |
| Melting | Sharp melting point | Softens over a range |
| Heat of fusion | Definite | Not definite |
| Cleavage | Breaks along definite cleavage planes | Irregular fracture |
| Direction | Anisotropic | Isotropic |
| Examples | NaCl, KCl, Fe, Cu, glucose, diamond, rhombic sulphur | Glass, plastics, rubber, glue |
Glass is often described as a supercooled liquid.
Properties of crystalline solids
- Geometrical shape: definite, regular shape with flat faces.
- Cleavage planes: a crystal breaks along definite planes where the lattice parts most easily.
- Anisotropy: properties such as electrical conductivity, thermal conductivity and refractive index differ with direction. Graphite conducts along its layers but hardly across them. Melting point, density and molar mass are not directional.
- Symmetry: repetition of faces, edges and angles when a crystal is rotated through 360°.
- Habit of a crystal: its overall shape under given growth conditions.
Isomorphism, polymorphism and allotropy
- Isomorphism: different compounds with the same crystal structure and the same atomic ratio, but different atoms. Examples: $\mathrm{NaNO_3}$ and $\mathrm{CaCO_3}$; $\mathrm{K_2SO_4}$ and $\mathrm{K_2CrO_4}$. Isomorphs have different physical and chemical properties and can crystallize together as mixed crystals. Common among ionic solids.
- Polymorphism: one compound existing in more than one crystalline form, e.g. $\mathrm{CaCO_3}$ as calcite and aragonite. Polymorphs have the same formula unit and chemical properties but different physical properties.
- Allotropy: polymorphism of an element, e.g. diamond and graphite; rhombic and monoclinic sulphur.
- Transition temperature: the temperature at which one crystalline form changes into another; for sulphur it is 95.5 °C.
Types of crystalline solids
| Type | Particles and forces | Examples | Properties |
|---|---|---|---|
| Ionic | Ions, electrostatic attraction | NaCl, KCl | Hard, brittle, high m.p., conduct when molten |
| Covalent (network) | Atoms, covalent bonds throughout | Diamond, silica ($\mathrm{SiO_2}$), SiC | Hardest, very high m.p. |
| Molecular | Molecules, van der Waals forces | $\mathrm{I_2}$, dry ice, ice, white phosphorus | Soft, low m.p. |
| Metallic | Positive ions in a sea of electrons | Cu, Zn, Fe, Al, Na | Conduct, malleable, ductile, lustrous |
Diamond and silica are giant covalent structures. B, C and Si form covalent solids; Cu is metallic. The term atomic solid is used for solids built from atoms rather than molecules or ions; in past papers it has been applied to both covalent network solids and metallic solids.
Metallic solids
- Electron sea theory: free, delocalized electrons explain electrical and thermal conductivity.
- Conductivity decreases on heating because vibrating metal ions scatter the electrons.
- Malleable: beaten into sheets. Ductile: drawn into wires. Layers of ions slide without breaking the bonding.
- The valence bond theory of metals was proposed by Linus Pauling.
Carbon and its relatives
- Graphite: layers of carbon in hexagonal rings; soft, conducts along layers.
- Boron nitride (BN) has a layered structure similar to graphite.
- Graphene: a single layer of graphite; very light yet stronger than steel.
Common MDCAT traps
- Glucose is crystalline; glass, plastic and rubber are amorphous.
- Melting point and molar mass are not anisotropic properties.
- Isomorphs have the same atomic ratio but different atoms; polymorphs have the same formula.
- Allotropy is for elements; polymorphism is for compounds.
- Iodine forms molecular crystals, not covalent network crystals.
Quick revision
- Amorphous solids lack a definite heat of fusion.
- Graphite shows anisotropic electrical conductivity.
- Covalent network solids are the hardest.
- Metals conduct heat and electricity through free electrons.
- Sulphur's transition temperature is 95.5 °C.