Solids are broadly classified into crystalline and amorphous based on the internal arrangement of constituent particles.
| Property | Crystalline Solid | Amorphous Solid |
|---|---|---|
| Internal order | Long-range (extends throughout) | Short-range only |
| Melting point | Sharp (fixed) | Range of temperatures |
| Cleavage | Clean cleavage planes | Irregular fracture |
| Isotropy | Anisotropic (direction-dependent) | Isotropic |
| Examples | Quartz, NaCl, Cu, Diamond | Glass, Rubber, Polythene |
Key point for NEET: Amorphous solids are sometimes called pseudo-solids or supercooled liquids (e.g., glass). They flow very slowly over long time periods.
Further classification by bonding:
- Ionic solids (NaCl, MgO): strong electrostatic forces; high MP; hard; poor electrical conductors (conduct in molten state)
- Covalent/network solids (diamond, ): covalent bonds throughout; very hard; very high MP
- Molecular solids (ice, ): van der Waals/H-bonds; soft; low MP
- Metallic solids (Cu, Fe): electron sea model; lustrous; malleable; good electrical conductors