S2.4.2 Position in the bonding triangle Notes

Electronegativity and Bond Character: Understanding the Bonding Triangle

The Three Corners of the Bonding Triangle

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

1. Ionic bonding: Complete transfer of electrons (e.g., NaCl).
2. Covalent bonding: Sharing of electrons (e.g., H₂).
3. 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:

1. Ionic-covalent: Polar covalent bonds with partial ionic character.
2. Covalent-metallic: Bonds in metalloids or semiconductors.
3. 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.
   $$
   \Delta\chi = \chi_{\text{higher}} - \chi_{\text{lower}}
   $$
2. Calculate $\bar{\chi}$: Take the average of the electronegativities.
   $$
   \bar{\chi} = \frac{\chi_{\text{higher}} + \chi_{\text{lower}}}{2}
   $$

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

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

Covalent Compounds

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

Metallic Compounds

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

Mixed Character Compounds

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