Characteristics of Ionic Compounds

Ionic compounds are formed through the electrostatic attraction between oppositely charged ions. These unique chemical structures result in several distinct physical and chemical properties:

• High Melting and Boiling Points: Strong ionic bonds in the crystal lattice require significant energy to break.
• Crystalline Structure: They typically exist as solid crystals with a regular, repeating 3D arrangement of ions.
• Electrical Conductivity:
  • Solid state: Poor conductors because ions are fixed in a rigid lattice.
  • Molten or Aqueous state: Good conductors because ions are free to move and carry current.
• Solubility: Most dissolve in polar solvents (like water) but not in non-polar solvents (like oil).
• Brittleness: Hard but brittle; shifting ions can cause repulsion and fracture the crystal.

Why are they brittle?

When force is applied, the layers of the crystal shift. This causes ions of the same charge to align next to each other. The resulting electrostatic repulsion forces the layers apart, causing the crystal to shatter rather than deform.

Brittleness Explained via Crystal Lattice
As seen in the NaCl crystal lattice structure, ions are held in a strict alternating pattern of positive and negative charges. When mechanical force is applied, layers shift, forcing ions of the same charge into alignment. The resulting electrostatic repulsion overcomes the ionic bonds, causing the crystal to fracture instantly.

Summary Table

Property	Description
Bonding	Electrostatic attraction
Physical State	Hard, crystalline solid
Conductivity	Conductive when molten or dissolved

Covers the UPSC and State PCS syllabus.

Practice Questions

1. Why do ionic compounds exhibit high melting points?

• (a) Because they are covalent in nature.
• (b) Because of the weak van der Waals forces between molecules.
• (c) Because of strong electrostatic forces of attraction between oppositely charged ions.
• (d) Because they have a low molecular weight.

View Answer & Explanation

Correct Answer: (c)

The ions in an ionic compound are held together by strong electrostatic forces in a crystal lattice, which require a significant amount of heat energy to overcome.

2. Under which condition can an ionic compound conduct electricity?

• (a) Only in solid state.
• (b) Only in gaseous state.
• (c) When dissolved in water or in a molten state.
• (d) Never, they are insulators.

View Answer & Explanation

Correct Answer: (c)

In the solid state, ions are fixed in a lattice. In molten or aqueous states, the lattice breaks, allowing ions to move freely and carry electric charge.

3. Why do ionic crystals shatter when struck by a hammer?

• (a) The ions become metallic.
• (b) Layers shift, bringing like-charged ions together, causing repulsion.
• (c) The heat generated by the blow melts the crystal.
• (d) Ionic bonds are inherently weak.

View Answer & Explanation

Correct Answer: (b)

Shifting the layers brings ions of the same charge into alignment, resulting in electrostatic repulsion that causes the structure to fracture.

4. Why are most ionic compounds soluble in water but insoluble in non-polar organic solvents?

• (a) Because water is a non-polar solvent.
• (b) Because the dipole nature of water molecules overcomes the electrostatic forces of the crystal lattice.
• (c) Because ionic compounds are actually covalent.
• (d) Because organic solvents have higher boiling points.

View Answer & Explanation

Correct Answer: (b)

Water is a polar solvent with a high dielectric constant. The partial charges on water molecules interact with the ions, providing enough energy to break the lattice (hydration energy), whereas non-polar solvents lack the charge required to pull the ions apart.