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Bonding & Properties (DP IB Chemistry) : Revision Note

Stewart Hird

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DP Chemistry: SLDP Chemistry: HL

Bonding & Properties

Determining the position of a compound in the bonding triangle

  • We can use the electronegativity of elements and binary compounds to find their position in the bonding triangle

  • For example, take sodium chloride:

Substance

Average electronegativity

Electronegativity difference

Where it is in the triangle

Na

0.9

0.9 - 0.9 = 0.0

Bottom left of the triangle. Na is 100% metallic

Cl2

3.2

3.2 - 3.2 = 0.0

Bottom right corner. Cl2 is 100% covalent

NaCl

straight capital sigma straight chi space equals space fraction numerator straight chi subscript straight A space plus straight chi subscript straight B over denominator 2 end fraction space equals fraction numerator 3.2 space plus space 0.9 over denominator 2 end fraction space equals space 2.1

increment straight chi space equals space straight chi subscript straight A minus straight chi subscript straight B space equals space 3.2 space minus space 0.9 space equals space 2.3

Centre top of the triangle. It is about 70% ionic on the triangle

Sodium chloride bonding triangle

Sodium chloride bonding triangle

The location of sodium chloride on the bonding triangle using electronegativity values

Worked Example

Use the bonding triangle and electronegativity values from Section 9 of the data booklet to mark the location for the following substances:

a) phosphorus

b) caesium iodide

c) brass (an alloy of copper and zinc)

Answer

Substance

Average electronegativity

Electronegativity difference

a) phosphorus

2.2

0

b) caesium chloride

straight capital sigma straight chi space equals space fraction numerator straight chi subscript straight A space plus straight chi subscript straight B over denominator 2 end fraction space equals fraction numerator 0.8 space plus space 2.7 over denominator 2 end fraction space equals space 1.8

increment straight chi space equals space straight chi subscript straight A minus straight chi subscript straight B space equals space 2.7 space minus space 0.8 space equals space 1.9

c) brass (an alloy of copper and zinc)

straight capital sigma straight chi space equals space fraction numerator straight chi subscript straight A space plus straight chi subscript straight B over denominator 2 end fraction space equals fraction numerator 1.9 space plus space 1.6 over denominator 2 end fraction space equals space 1.8

increment straight chi space equals space straight chi subscript straight A minus straight chi subscript straight B space equals space 1.9 space minus space 1.6 space equals space 0.3

Bonding triangle for worked examples

Percentages of Bonding Type

  • We can also use the bonding triangle to assess the percentage of ionic or covalent character in a compound

  • Take aluminium chloride and aluminium oxide as an example: 

Substance

Average electronegativity

Electronegativity difference

Bonding character 

aluminium oxide

Σχ = χA +χB2 =1.6 + 3.42 = 2.5

∆χ = χA-χB = 3.4 - 1.6 = 1.8

 Ionic. About 60% ionic

 aluminium chloride

Σχ = χA +χB2 =1.6 + 3.22 = 2.4

∆χ = χA-χB = 3.2 - 1.6 = 1.6

Polar covalent. About 50% covalent 

Percentage of ionic or covalent character diagram

Bonding triangle for different percentage bonding character

The percentage of bonding type character in aluminium oxide and aluminium chloride can be assessed using the bonding triangle

  • From this analysis, we can see why there is a significant difference in properties between aluminium oxide and aluminium chloride

    • The melting point of the oxide is 2072 oC, whereas the chloride is 192oC

  • The bonding triangle continuum allows for a more accurate assessment of bonding type and the prediction of associated properties

Examiner Tips and Tricks

Calculations of percentage ionic character are not required.

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

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.

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