UNIT 1: SOLID STATE (TYPES OF SOLIDS) CBSE XIITH CHEMISTRY KEY NOTES PART I

 1. Molecular Solids

Constituents: Molecules (non-polar, polar, or hydrogen-bonded)

Forces of attraction:

• Non-polar: Weak van der Waals forces

• Polar: Dipole–dipole forces

• H-bonded: Hydrogen bonding

Properties:

• Soft, low melting point

• Generally insulators (no free ions or electrons)

• Exist as gases or liquids at room temperature; some are solids due to H-bonding

Examples:

• Non-polar → I₂, CO₂ (dry ice)

• Polar → HCl, SO₂

• H-bonded → H₂O (ice), NH₃

Tip: “Molecules are soft & non-conductors”

2. Ionic Solids

• Constituents: Positive & negative ions arranged in 3-D lattice
• Forces: Strong electrostatic forces between ions

Properties:

Hard & brittle

High melting points

Non-conductors in solid state (ions fixed), but good conductors in molten or aqueous state (ions free to move)

Examples: NaCl, KCl, MgO, CaF₂

Tip: “Ions → strong bonds → high mp → conduct when molten”

 3. Metallic Solids

Constituents: Positive metal ions in a sea of delocalised electrons

Forces: Metallic bonds (electrostatic attraction between lattice & electrons)

Properties:

• Hard, malleable, ductile

• Good conductors of heat & electricity (free electrons)

• Variable melting points

Examples: Fe, Cu, Al, Ag

Tip: “Sea of electrons → shine + conduct”

4. Covalent / Network Solids

Constituents: Atoms connected by covalent bonds in a giant network

Forces: Strong directional covalent bonds throughout the structure

Properties:

• Very hard, very high melting points

• Brittle in some cases

• Non-conductors, except graphite (delocalised electrons)

Examples: Diamond, SiO₂ (quartz), Graphite

Tip: “Covalent bonds everywhere → extremely hard”

Memory Hack:

 MIMC → Molecular – Ionic – Metallic – Covalent

• M → Molecules, soft, insulators

• I → Ions, strong forces, conduct when molten

• M → Metals, electron sea, conduct always

• C → Covalent bonds, super hard, graphite exception

 Quick Comparison Table

Property	Molecular	Ionic	Metallic	Covalent / Network
Constituents	Molecules	Ions	Metal cations + e⁻ sea	Atoms linked covalently
Forces	van der Waals / H-bond	Electrostatic	Metallic bonds	Covalent bonds
Hardness	Soft	Hard, brittle	Variable, generally hard	Very hard
Melting Point	Low	High	Variable	Very high
Conductivity	Non-conductors	Conduct when   molten	Good conductors	Usually insulators (Graphite is conductor)
Examples	I₂, H₂O	NaCl, MgO	Fe, Cu, Al	Diamond, SiO₂, Graphite