S2.4.2 Position in the bonding triangle

Electronegativity and Bond Character: Understanding the Bonding Triangle

Imagine you're tasked with creating a material for a groundbreaking project—perhaps a lightweight yet durable component for a spacecraft or a flexible yet strong material for medical implants. How do you decide on the right material?

The properties of a substance, like its melting point, solubility, or electrical conductivity, are closely tied to the type of bonding between its atoms.

The Bonding Continuum and the Bonding Triangle

In reality, bonds between atoms are rarely purely ionic, covalent, or metallic. Instead, bonding exists on a continuum between these three types.

The van Arkel-Ketelaar bonding triangle is a visual tool that helps classify bonds based on their ionic, covalent, and metallic character.

The Three Corners of the Bonding Triangle

The triangle's corners represent the "pure" forms of bonding:

• Ionic bonding: Complete transfer of electrons (e.g., NaCl).
• Covalent bonding: Sharing of electrons (e.g., H₂).
• Metallic bonding: Delocalized electrons shared across a lattice of metal cations (e.g., Cu).

Intermediate Bond Types

The triangle's sides represent bonds with mixed characteristics:

• Ionic-covalent: Polar covalent bonds with partial ionic character.
• Covalent-metallic: Bonds in metalloids or semiconductors.
• Ionic-metallic: Bonds in alloys or compounds with both metallic and non-metallic elements.

Parameters for Classification

The position of a compound in the bonding triangle is determined by two key parameters:

1. Electronegativity difference (Δχ): Indicates the ionic-covalent character. A larger Δχ suggests greater ionic character.
2. Mean electronegativity (χ̄): Indicates the metallic-covalent character. A higher χ̄ suggests more covalent character.

Determining a Compound's Position in the Bonding Triangle

To locate a bond in the bonding triangle:

1. Calculate Δχ: Subtract the smaller electronegativity from the larger one.
   $$\mathrm{\Delta }\chi ={\chi }_{\text{higher}}-{\chi }_{\text{lower}}$$
2. Calculate χ̄: Take the average of the electronegativities.
   $$\overline{\chi }=\frac{{\chi }_{\text{higher}}+{\chi }_{\text{lower}}}{2}$$

Bond Character and Material Properties

The bonding triangle provides insights into the physical and chemical properties of compounds, such as melting point, solubility, and electrical conductivity.

Ionic Compounds

• Example: Sodium chloride (NaCl)
• Bond Character: Predominantly ionic.
• Properties:
  • High melting and boiling points due to strong electrostatic forces.
  • Soluble in polar solvents like water.
  • Conduct electricity when molten or dissolved, as ions are free to move.

Covalent Compounds

• Example: Water (H₂O)
• Bond Character: Polar covalent.
• Properties:
  • Lower melting and boiling points compared to ionic compounds.
  • Solubility depends on polarity (e.g., polar covalent compounds dissolve in water).
  • Do not conduct electricity in any state, as there are no free ions or delocalized electrons.

Metallic Compounds

• Example: Copper (Cu)
• Bond Character: Purely metallic.
• Properties:
  • High electrical and thermal conductivity due to delocalized electrons.
  • Malleable and ductile.
  • Insoluble in common solvents.

Mixed Character Compounds

• Example: Aluminium chloride (AlCl₃)
• Bond Character: Both ionic and covalent.
• Properties:
  • Behaves as ionic in solid form but exhibits covalent properties in the gas phase (e.g., forming Al₂Cl₆ dimers).
  • Soluble in non-polar solvents like trichloromethane.

Application: Materials with Mixed Bonding Character

Many modern materials combine ionic, covalent, and metallic bonding to achieve unique properties. For example:

• Reinforced concrete: Combines ionic (cement), covalent (polymers), and metallic (steel bars) components. This mixture provides strength, flexibility, and durability.
• Semiconductors: Silicon exhibits both metallic and covalent properties, making it ideal for electronic applications.

Prediction of Properties Using the Bonding Triangle

The bonding triangle is not just a classification tool—it’s a predictive framework. By knowing a compound’s position in the triangle, you can infer:

• Solubility: Ionic compounds dissolve in polar solvents; non-polar covalent compounds dissolve in non-polar solvents.
• Electrical conductivity: Metallic compounds and ionic compounds (when molten or aqueous) conduct electricity, while covalent compounds generally do not.
• Melting and boiling points: Compounds with strong ionic or covalent network bonds have high melting points, while molecular covalent substances may have low melting points.

Limitations of the Bonding Triangle

While the bonding triangle is a powerful tool, it has limitations:

• Mixed Bonding: Some compounds, like AlCl₃, exhibit properties that cannot be fully explained by their position in the triangle.
• Structural Factors: The triangle does not account for molecular geometry or intermolecular forces, which can significantly affect properties.

Reflection and Broader Implications