Metallic Bonds and Molecular Crystals: Properties & Types

Understanding Metallic Bonds

A metallic bond is a type of chemical bond where the electrostatic attraction between positive metal ions (kernels) and delocalized electrons holds the metal ions in their respective positions. In metal atoms, the valence electrons are loosely bound and are free to move from one atom to another within the metallic crystal lattice. These free electrons, also known as conduction electrons or delocalized electrons, are uniformly distributed throughout the metal lattice.

When metal atoms lose these conduction electrons, they become positive ions, or kernels (atoms without their valence electrons). The electrostatic (or coulombic) interaction that develops between these positive kernels and the sea of negative electrons is known as a metallic bond.

Common examples include copper, aluminum, silver, and iron. Even in a molten state, the metallic bond persists and is not completely broken until the metal boils.

Properties of Metals

• They are hard and cannot be compressed easily.
• They are nonvolatile.
• They have high melting points (m.p.) and boiling points (b.p.).
• They have a metallic luster.
• They are good conductors of heat and electricity.
• They are malleable (can be changed into sheets) and ductile (can be drawn into wires).

An Introduction to Molecular Crystals

Molecular crystals are solids in which atoms or molecules (either polar or nonpolar) are held together by van der Waals forces. Based on the specific type of van der Waals forces, they are classified into two main types.

Crystals with Dipole-Dipole Forces

These van der Waals forces are present in polar molecular crystals, such as solid ammonia (NH₃), solid hydrogen chloride (HCl), and ice.

Crystals with London Forces

These forces, also known as dispersion forces, are present in all molecular solids, both polar and nonpolar. Examples of nonpolar molecular solids include iodine, sulfur, phosphorus, and solid carbon dioxide (dry ice). Polar molecular solids, which also exhibit London forces, include ice, sugar, solid ammonia, and solid hydrogen chloride.

Properties of Molecular Crystals

• They are soft.
• They have low boiling points (b.p.) and melting points (m.p.).
• They are volatile.
• They are poor conductors of electricity.
• Polar molecular crystals are soluble in polar solvents, while nonpolar crystals are soluble in nonpolar solvents.
• They have low density and low values for lattice energy, heat of fusion, and heat of sublimation.