S2.4.1 Bonding as a continuum Notes

Bonding and Material Properties

1. You’ve likely learned about the three primary types of chemical bonds: ionic, covalent, and metallic.
2. While these categories are helpful, most bonds don’t fit neatly into one type: they exist on a spectrum.
3. This idea of bonding as a continuum is elegantly captured in the bonding triangle, first developed by van Arkel and Ketelaar.

The Bonding Triangle

1. Each corner of the triangle corresponds to a "pure" bonding type:
   1. Ionic Bonding: Found in compounds like NaCl, where electrons are transferred from one atom to another.
   2. Covalent Bonding: Found in molecules like $H_2$ or $SiO_2$, where electrons are shared between atoms.
   3. Metallic Bonding: Found in metals like Cu or Fe, where electrons are delocalized and free to move.
2. The sides of the triangle represent intermediate bonding types:
   1. Ionic–Covalent Continuum: Bonds with partial ionic and covalent character, such as in AgCl.
   2. Covalent–Metallic Continuum: Bonds with partial covalent and metallic character, such as in silicon.
   3. Metallic–Ionic Continuum: Bonds with partial metallic and ionic character, such as in alloys.
3. The position of a material in the bonding triangle is determined by two key parameters:
   1. Electronegativity Difference (Δχ): This measures the ionic–covalent character of the bond.
   2. Mean Electronegativity ($\bar{\chi}$): This measures the metallic–covalent character of the bond.

Applications of Bonding Models: Explaining Material Properties

Understanding bonding as a continuum allows us to explain the properties of materials, such as melting points, conductivity, and hardness, in terms of their position on the bonding triangle.

Melting and Boiling Points

The strength of bonding interactions determines a material’s melting and boiling points:

1. Ionic Compounds: High melting and boiling points due to strong electrostatic forces between ions (e.g., NaCl).
2. Covalent Molecular Substances: Low melting and boiling points because only weak intermolecular forces need to be overcome (e.g., $CO_2$.
3. Covalent Network Solids: Extremely high melting points due to strong covalent bonds throughout the structure (e.g., diamond, $SiO_2$.
4. Metallic Substances: Variable melting points depending on the strength of metallic bonding (e.g., mercury melts at -39°C, while tungsten melts at 3422°C).