Question
Classify crystalline solids into ionic, covalent (network), metallic, and molecular types. Compare their properties and give examples.
(JEE Main, NEET, CBSE 12 — property-to-type matching is a standard MCQ)
Solution — Step by Step
| Type | Particles at lattice points | Bonding | Examples |
|---|---|---|---|
| Ionic | Cations and anions | Electrostatic (ionic bonds) | NaCl, MgO, CaF2 |
| Covalent (network) | Atoms | Covalent bonds throughout | Diamond, SiC, SiO2 (quartz) |
| Metallic | Metal ions in electron sea | Metallic bonds | Fe, Cu, Ag, Au |
| Molecular | Molecules | van der Waals, H-bonds | Ice, dry ice (CO2), naphthalene, I2 |
| Property | Ionic | Covalent Network | Metallic | Molecular |
|---|---|---|---|---|
| Melting point | High | Very high | Variable (mostly high) | Low |
| Hardness | Hard, brittle | Very hard | Variable, malleable | Soft |
| Conductivity (solid) | No | No (except graphite) | Yes | No |
| Conductivity (molten/aq) | Yes | No | Yes | No |
| Solubility in water | Many soluble | Insoluble | Insoluble | Polar: soluble; Nonpolar: insoluble |
Ionic solids have strong electrostatic attraction but shatter when struck (ions of same charge align and repel). They conduct when molten because ions become mobile.
Covalent network solids have continuous covalent bonds in all directions — breaking requires breaking covalent bonds, so melting points are extremely high. Diamond is the hardest natural substance because of this 3D network.
Molecular solids are held together only by weak intermolecular forces (van der Waals). Individual molecules have strong internal bonds, but the crystal is held together weakly — hence low melting points.
graph TD
A[Crystalline Solids] --> B["Ionic: NaCl, MgO"]
A --> C["Covalent Network: Diamond, SiO2"]
A --> D["Metallic: Fe, Cu"]
A --> E["Molecular: Ice, Naphthalene"]
B --> F["High MP, Brittle, Conducts when molten"]
C --> G["Very high MP, Very hard"]
D --> H["Variable MP, Malleable, Conducts"]
E --> I["Low MP, Soft"]Why This Works
The type of bonding determines every physical property. Stronger bonds mean higher melting points and greater hardness. The key insight: in molecular solids, we are not breaking the covalent bonds within molecules — we are only overcoming the weak forces between molecules. That is why ice melts at 0 degrees C even though the O-H bond inside water is very strong.
Electrical conductivity depends on the presence of mobile charge carriers. Metals always have free electrons. Ionic solids have fixed ions (no conduction) until melted (ions move). Molecular and covalent network solids have no charge carriers at all (with the notable exception of graphite, which has delocalised pi electrons).
Alternative Method
For JEE and NEET, use this quick identifier:
- Conducts as solid? → Metallic (or graphite)
- Conducts when molten but not as solid? → Ionic
- Never conducts? → Molecular or covalent network
- Very high MP + never conducts? → Covalent network (diamond, quartz)
- Low MP + never conducts? → Molecular (naphthalene, ice)
Common Mistake
The classic trap: graphite. Graphite is a covalent network solid (carbon atoms bonded in sheets), but it conducts electricity because of delocalised pi electrons between layers. Students either classify it as metallic (wrong — it is covalent) or say it cannot conduct (wrong — it can, along the layers). Graphite is the exception that is always tested.
Also, quartz (SiO2) is a covalent network solid, not molecular. Students sometimes think SiO2 is molecular like CO2. But SiO2 forms a continuous 3D network (each Si bonded to 4 O atoms), which is why quartz has a very high melting point (1713 degrees C) while CO2 sublimes at -78 degrees C.