Question
How do we classify a solid as ionic, covalent (network), metallic, or molecular? Compare their properties and give examples.
(CBSE 12 + JEE Main + NEET)
Solution — Step by Step
| Property | Ionic | Covalent Network | Metallic | Molecular |
|---|---|---|---|---|
| Particles | Cations + Anions | Atoms (covalent bonds) | Metal cations + electron sea | Molecules (van der Waals) |
| Melting point | High (600-3000°C) | Very high (1700-3500°C) | Variable (low to high) | Low (-272 to 400°C) |
| Hardness | Hard but brittle | Very hard | Variable, malleable | Soft |
| Conductivity (solid) | No | No (except graphite) | Yes | No |
| Conductivity (melt/solution) | Yes | No | Yes (liquid) | No |
| Examples | NaCl, MgO | Diamond, SiO₂, SiC | Fe, Cu, Al | Ice, I₂, sugar |
Ask these questions in order:
- Is it a metal? → Metallic solid
- Does it dissolve in water and conduct electricity in solution? → Ionic solid
- Is it extremely hard with a very high melting point? → Covalent network solid
- Is it soft with a low melting point? → Molecular solid
Ionic bonds are strong (high melting point, hard). But if you apply force that shifts one layer, positive ions align with positive ions — the repulsion shatters the crystal. Metals, by contrast, have a “sea of electrons” that allows layers to slide without losing bonding — hence metals are malleable.
Graphite is a covalent network solid that CONDUCTS electricity. Each carbon is hybridized, leaving one -electron delocalized across the layers. These delocalized electrons carry current. This makes graphite unique among covalent solids.
flowchart TD
A["Identify type of solid"] --> B{"Is it a metal?"}
B -- Yes --> C["METALLIC SOLID"]
B -- No --> D{"Conducts in solution/melt?"}
D -- Yes --> E["IONIC SOLID"]
D -- No --> F{"Very high melting point + very hard?"}
F -- Yes --> G["COVALENT NETWORK SOLID"]
F -- No --> H["MOLECULAR SOLID"]
G --> I["Examples: diamond, SiO₂"]
H --> J["Examples: ice, naphthalene, I₂"]
E --> K["Examples: NaCl, KCl, CaO"]
C --> L["Examples: Fe, Cu, Na"]Why This Works
The classification is based on the type of bonding that holds the solid together. Strong ionic bonds and covalent network bonds create high melting points. The metallic electron sea model explains both conductivity and malleability. Weak intermolecular forces (van der Waals, hydrogen bonding) in molecular solids mean they melt easily.
The electrical conductivity test is particularly diagnostic: if it conducts in solid state, it must be metallic. If it conducts only when molten or dissolved, the ions must be locked in a lattice (ionic). If it never conducts, the particles are neutral molecules.
Alternative Method
For NEET MCQs, use the melting point as a quick filter: below 300°C is almost certainly molecular. Above 1000°C is ionic or covalent network. Then check conductivity to distinguish. Metals are easy to identify from the periodic table position.
Common Mistake
Students classify diamond as a molecular solid because “it is made of carbon atoms.” Diamond is a COVALENT NETWORK solid — every carbon is covalently bonded to 4 others in a 3D lattice. There are no discrete molecules. Similarly, (quartz) is covalent network, not ionic, even though it contains two different elements.