IBChemistryDPHLRevision NotesModels of Bonding & StructureFrom Models to MaterialsBonding & Properties

Bonding & Properties (DP IB Chemistry): Revision Note

Stewart Hird

Written by: Stewart Hird

Reviewed by: Richard Boole

Updated on

DP Chemistry: HLDP Chemistry: SL

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 straight capital sigma straight chi equals fraction numerator 3.2 space plus space 0.9 over denominator 2 end fraction = 2.05

      • Has a difference in electronegativity of increment straight chi = 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

Sodium chloride bonding triangle
The location of sodium chloride, and its component elements, on the bonding triangle using electronegativity values

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:

straight capital sigma straight chi equals fraction numerator stretchy left parenthesis 1.0 plus 2.7 stretchy right parenthesis over denominator 2 end fraction = 1.85

  • The difference in electronegativity is:

increment straight chi = 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:

straight capital sigma straight chi equals fraction numerator stretchy left parenthesis 1.9 plus 1.6 stretchy right parenthesis over denominator 2 end fraction = 1.75

  • The difference in electronegativity is:

increment straight chi = 1.9 - 1.6 = 0.3

  • This places brass at (1.75, 0.3) in the triangle, on the border of metallic and covalent

Bonding triangle for worked examples

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 straight capital sigma straight chi equals fraction numerator stretchy left parenthesis 1.6 plus 3.2 stretchy right parenthesis over denominator 2 end fraction = 2.4

      • Has a difference in electronegativity of increment straight chi = 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 straight capital sigma straight chi equals fraction numerator stretchy left parenthesis 1.6 plus 3.4 stretchy right parenthesis over denominator 2 end fraction = 2.5

      • Has a difference in electronegativity of increment straight chi = 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

Triangular graph showing bond types: metallic, covalent, polar covalent, and ionic. Includes aluminium chloride and oxide, with electronegativity values.

  • 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

👀 You've read 1 of your 5 free revision notes this week
An illustration of students holding their exam resultsUnlock more revision notes. It's free!

By signing up you agree to our Terms and Privacy Policy.

Already have an account? Log in

Test yourselfFlashcards

Did this page help you?

Previous:Bonding ModelsNext:Properties of Alloys
Author
Reviewer
Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

Richard Boole

Reviewer: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Download notes on Bonding & Properties