Using The Bonding Triangle

Specification Reference S2.4.2

Quick Notes

The position of a compound in the bonding triangle depends on the relative contribution of:
- Ionic bonding
- Covalent bonding
- Metallic bonding
Electronegativity difference helps estimate bonding character:
- Large difference → more ionic
- Small difference → more covalent
- No electronegativity difference (metal + metal) → metallic
Use electronegativity values (from the IB data booklet) to predict properties like:
- Electrical conductivity
- Melting point
- Solubility
Only binary compounds are required here (compounds made of two elements).

The bonding triangle represents the continuum of bonding between:

Ionic corner: strong electrostatic attraction from electron transfer (e.g., NaCl)
Covalent corner: shared electron pairs between atoms (e.g., Cl₂, CO₂)
Metallic corner: delocalized electrons in a lattice of cations (e.g., Fe, Cu)

Most compounds fall between these points, depending on their bonding character.

Electronegativity is a measure of how strongly an atom attracts bonding electrons.

The difference in electronegativity (ΔEN) between two atoms gives a rough guide to bonding type:

Note: This is not a fixed rule, but a useful guideline.

Compound	Elements	ΔEN Difference	Bonding Character	Likely Triangle Position	Key Properties
NaCl	Na (0.9), Cl (3.0)	2.1	Ionic	Close to ionic corner	High melting point, conducts when molten, soluble in water
HCl	H (2.1), Cl (3.0)	0.9	Polar covalent	Between ionic and covalent	Gas at room temp, soluble in water
Cl₂	Cl (3.0), Cl (3.0)	0	Non-polar covalent	Pure covalent corner	Low melting point, non-conductive
MgO	Mg (1.2), O (3.5)	2.3	Ionic	Strongly ionic	Very high melting point, hard, insoluble in organic solvents
AlCl₃	Al (1.5), Cl (3.0)	1.5	Polar covalent with some ionic	In between	Intermediate melting point, poor conductor as solid

Triangle Region	Typical Properties
Ionic side	Hard, brittle, high melting point, conductive (molten/solution)
Covalent side	Low melting/boiling, poor conductivity, often volatile
Metallic side	Malleable, ductile, high thermal and electrical conductivity

Knowing where a compound falls in the triangle helps predict its physical and chemical behaviour.

Use electronegativity difference to estimate where a compound lies in the bonding triangle.
The triangle links bond type to material properties such as melting point, conductivity and solubility.
Only binary compounds are required at this level and you are not expected to calculate percent ionic character.