Bonding & Properties (SL IB Chemistry)

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

Sodium chloride bonding triangle

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

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	$\mathrm{\Sigma \chi } = \frac{{\mathrm{\chi }}_{\mathrm{A}} +{\mathrm{\chi }}_{\mathrm{B}}}{2} =\frac{1.6 + 3.4}{2} = 2.5$	$∆\mathrm{\chi } = {\mathrm{\chi }}_{\mathrm{A}}-{\mathrm{\chi }}_{\mathrm{B}} = 3.4 - 1.6 = 1.8$	Ionic. About 60% ionic
aluminium chloride	$\mathrm{\Sigma \chi } = \frac{{\mathrm{\chi }}_{\mathrm{A}} +{\mathrm{\chi }}_{\mathrm{B}}}{2} =\frac{1.6 + 3.2}{2} = 2.4$	$∆\mathrm{\chi } = {\mathrm{\chi }}_{\mathrm{A}}-{\mathrm{\chi }}_{\mathrm{B}} = 3.2 - 1.6 = 1.6$	Polar covalent. About 50% covalent

Percentage of ionic or covalent character diagram

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