Bonding & Properties (DP IB Chemistry): Revision Note
Bonding & properties
Understanding the triangular bonding diagram
The triangular bonding diagram is a visual model used to classify chemical bonds by comparing electronegativity values. It shows bonding as a continuum between three idealised types:
Ionic bonding at the top (apex)
Covalent bonding at the bottom right
Metallic bonding at the bottom left
The position of a substance on the diagram is based on two values:
x-axis (horizontal): average electronegativity of the elements involved
y-axis (vertical): difference in electronegativity between the two elements
Each compound is plotted using these values in the format (x, y). This determines how much covalent, ionic, or metallic character the bonding has.
Determining the position of a compound
The electronegativity of elements and binary compounds can be used to find their position in the triangular bonding diagram
For example, sodium chloride:
Sodium (Na)
Has an electronegativity value of 0.9
As a pure element, the difference in electronegativity is 0
This places sodium in the bottom left of the triangle, 100% metallic
Chlorine (Cl2)
Has an electronegativity value of 3.2
As a diatomic molecule, the difference in electronegativity is 0
This places chlorine in the bottom right of the triangle, 100% covalent
Sodium chloride (NaCl)
Has an average electronegativity value of
= 2.05
Has a difference in electronegativity of
= 3.2 - 0.9 = 2.3
This places sodium chloride near the triangle's apex, with around 75% ionic character
This explains NaCl’s high melting point and ability to conduct electricity when molten.
Sodium chloride triangular bonding diagram

Worked Example
Use the bonding triangle (Section 17 of the Data Booklet) and electronegativity values (Section 9 of the Data Booklet) to mark the location for the following substances:
a) phosphorus
b) caesium iodide
c) brass (a copper-zinc alloy)
Answers
a) phosphorus
Has an electronegativity value of 2.2
Since it's a pure element, the difference in electronegativity is 0
This places phosphorus at (0, 2.2), at the bottom middle of the triangle as 100% covalent
b) caesium iodide
Caesium has an electronegativity value of 1.0
Iodine has an electronegativity value of 2.7
The average electronegativity of caesium iodide is:
= 1.85
The difference in electronegativity is:
= 2.7 - 1.0 = 1.7
This places caesium iodide at (1.85, 1.7) in the triangle, as an ionic compound
c) brass (a copper-zinc alloy)
Copper has an electronegativity of 1.9
Zinc has an electronegativity of 1.6
The average electronegativity of brass is:
= 1.75
The difference in electronegativity is:
= 1.9 - 1.6 = 0.3
This places brass at (1.75, 0.3) in the triangle, on the border of metallic and covalent

Percentages of bonding type
The triangular bonding diagram can help estimate the percentage of ionic or covalent character in a compound
For example, comparing aluminium chloride (AlCl3) and aluminium oxide (Al2O3):
Aluminium (Al) has an electronegativity value of 1.6
Chlorine (Cl) has an electronegativity value of 3.2
Oxygen (O) has an electronegativity value of 3.4
Aluminium chloride (AlCl3)
Has an average electronegativity of
= 2.4
Has a difference in electronegativity of
= 3.2 - 1.6 = 1.6
This places aluminium chloride at (2.4, 1.6), with around 60% ionic character
Aluminium oxide (Al2O3)
Has an average electronegativity of
= 2.5
Has a difference in electronegativity of
= 3.4 - 1.6 = 1.8
This places aluminium oxide at (2.5, 1.8), with around 50% ionic character
Percentage of ionic or covalent character diagram

This helps explain the differences in their properties:
Al2O3 has a much higher melting point (2072 °C) due to stronger ionic bonding
AlCl3 melts at just 192 °C due to weaker covalent interactions
Examiner Tips and Tricks
You do not need to calculate exact percentage ionic character in exams.
Use the bonding triangle (Section 17, Data Booklet) to compare materials qualitatively based on electronegativity data (Section 9, Data Booklet).
The triangular bonding diagram allows for:
Accurately assessing real bonding behaviour
Predicting properties like melting point, solubility and electrical conductivity
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