The correct option is Hard and brittle solids.
Explanation
Ionic compounds are formed by the transfer of electrons between atoms, resulting in positive and negative ions. These ions are held together by strong electrostatic forces of attraction (ionic bonds) arranged in a rigid, three-dimensional crystal lattice structure.Detailed Analysis:
- Hardness and Solid State: The strong electrostatic attraction between oppositely charged ions holds the lattice firmly together. Consequently, ionic compounds are solids at room temperature and are generally hard because significant energy is required to break the inter-ionic forces.
- Brittleness: Despite being hard, ionic compounds are brittle. When external force is applied to the crystal lattice, layers of ions may shift. If this movement brings ions of similar charges (e.g., cation over cation) directly opposite each other, the resulting strong electrostatic repulsion causes the crystal to fracture or shatter immediately.
- Incorrect Options:
- Soft malleable solids: This characteristic is typical of metals (metallic bonding), where the "sea of electrons" allows layers of atoms to slide over one another without breaking the structure.
- Gaseous or Liquid at room temperature: These states are generally characteristic of covalent compounds, which possess weak intermolecular forces (such as van der Waals forces).
Key Takeaway:
Ionic compounds are hard due to strong electrostatic forces holding the lattice together, but they are brittle because mechanical stress aligns like charges, causing repulsion that shatters the crystal structure.