Question
What are the four types of crystalline solids? How do their bonding, melting points, conductivity, and hardness differ, and how do we identify the type from a given substance?
(JEE Main, NEET, CBSE 12 — classification with property comparison is a guaranteed 1-mark question)
Solution — Step by Step
Constituent particles: Cations and anions held by strong electrostatic (Coulombic) forces.
Examples: NaCl, , ,
Properties:
- High melting points (NaCl: 801 degC, MgO: 2852 degC)
- Hard but brittle (layers slip and like-charges repel, causing cleavage)
- Do not conduct in solid state (ions are fixed), but conduct when molten or dissolved in water
- Soluble in polar solvents (water), insoluble in non-polar solvents
Constituent particles: Atoms bonded by a continuous 3D network of covalent bonds.
Examples: Diamond (), silicon carbide (), quartz (), boron nitride ()
Properties:
- Very high melting points (diamond: 3550 degC — breaking covalent bonds requires massive energy)
- Extremely hard (diamond is the hardest known natural substance)
- Do not conduct electricity (all electrons are locked in covalent bonds). Exception: graphite — delocalized electrons in layers allow conduction along the planes
- Insoluble in virtually all solvents
Constituent particles: Metal cations in a sea of delocalized electrons.
Examples: Fe, Cu, Ag, Au, Al
Properties:
- Variable melting points (Hg: -39 degC to W: 3410 degC)
- Malleable and ductile (layers can slide without breaking bonds — the electron sea adjusts)
- Excellent electrical and thermal conductors (free electrons carry charge and heat)
- Lustrous (free electrons absorb and re-emit light)
Constituent particles: Molecules held by van der Waals forces, dipole-dipole interactions, or hydrogen bonds.
Three sub-types:
- Non-polar molecular: , , naphthalene (London dispersion forces only) — very low melting points
- Polar molecular: , (dipole-dipole forces) — slightly higher melting points
- Hydrogen-bonded molecular: Ice (), solid — highest among molecular solids but still lower than ionic
Properties: Low melting points, soft, non-conducting, often volatile.
flowchart TD
A["Crystalline Solid"] --> B{"What are the constituent particles?"}
B -->|"Ions (cation + anion)"| C["Ionic solid"]
B -->|"Atoms in 3D covalent network"| D["Covalent solid"]
B -->|"Metal cations + electron sea"| E["Metallic solid"]
B -->|"Molecules"| F["Molecular solid"]
C --> G["Hard, brittle, high MP, conducts when molten"]
D --> H["Very hard, very high MP, non-conducting*"]
E --> I["Malleable, variable MP, conducts"]
F --> J["Soft, low MP, non-conducting"]Why This Works
The properties of each solid type follow directly from the strength and nature of the bonding. Melting point correlates with bond strength: covalent network > ionic > metallic (variable) > molecular. Conductivity depends on whether charge carriers (free electrons or mobile ions) exist. Hardness depends on whether the lattice can deform without breaking bonds.
The elegant part: once you identify the type of bonding, ALL properties follow logically. You do not need to memorise each property independently.
Common Mistake
The classic trap: “Does diamond conduct electricity?” No — all four valence electrons of each carbon are used in covalent bonds, leaving no free electrons. But “Does graphite conduct?” Yes — each carbon uses only 3 electrons for bonding, leaving one delocalized electron per atom. Students who memorise “covalent solids do not conduct” without learning the graphite exception lose easy marks. NEET 2023 tested exactly this.
Quick identification: if it conducts when solid, it is metallic. If it conducts only when molten/dissolved, it is ionic. If it sublimes easily (naphthalene, dry ice), it is molecular. If it is extremely hard and high-MP, it is covalent network.