Crystalline Solids: Bonding Types and Physical Properties

Classification of Crystalline Solids

In crystalline solids, the atoms, ions, or molecules are held together by various types of cohesive forces, which may be chemical bonds or intermolecular forces. Based on the nature of bonding between their constituent particles, crystalline solids are classified into distinct types.

Ionic Crystals

Ionic crystals are formed by the interaction of positive and negative ions. These ions are held together by strong electrostatic forces of attraction, also known as ionic bonds. Examples include NaCl, KNO₃, Na₂CO₃, and K₂Cr₂O₇.

Properties of Ionic Crystals

• They occur in the solid state at room temperature.
• They have a distinct geometrical shape due to the close packing of cations and anions.
• They are very hard.
• They are very stable.
• They are non-volatile.
• They have high melting points, boiling points, and heats of fusion.
• They have high density due to the close packing of ions.
• They do not conduct electric current in the solid state, but they do conduct electric current in solution or liquid (molten) form.
• They are soluble in polar solvents.
• Their reactions are very fast.
• They exhibit isomorphism and polymorphism.

Covalent Crystals

Also known as atomic solids, covalent crystals are composed of neutral atoms of the same or different elements. These atoms are held together by strong covalent bonds. Examples include diamond, graphite, and carborundum (silicon carbide, SiC).

Properties of Covalent Crystals

• They are very hard.
• They have high melting points, boiling points, and heats of fusion.
• They are poor conductors of electric current, with the exception of graphite.
• They are soluble in nonpolar solvents.
• Their chemical reactions are very slow.

Metallic Crystals

Metallic crystals are characterized by metallic bonds. A metallic bond is defined as the attraction between positive metal ions (kernels) and a surrounding 'sea' of freely mobile electrons. Alternatively, it is the bond in which the electron-kernel attraction holds the metal ions in their respective positions. In metal atoms, valence electrons are loosely bound and are free to move from one atom to another within the metallic crystal lattice. These free electrons are called conduction electrons or delocalized electrons, and they are uniformly distributed throughout the metal lattice.