Bonding Models (SL IB Chemistry)

Bonding Models

• Models simplify complex systems and allow us to predict and test theories
• As we have seen, chemical bonding broadly falls into three types: ionic, covalent and metallic
• The bonding types can be used to explain chemical and physical properties of substances
• However, like all models, there are limitations and inaccuracies that arise from oversimplification
• For example, take a substance like aluminium chloride, AlCl_3. The compound consists of a metal and non-metal, so the traditional bonding approach would be to predict it has ionic bonding and the associated properties of an ionic compound such as high melting point and boiling point
  • Aluminium chloride actually melts at 192^oC, so it does not behave like an ionic compound
• The fact we know polar covalent bonds exist arise provides evidence that bonding type is not something that can be easily compartmentalised
• Bonding is best thought of as a continuum of the three different bonding types like the area of an equilateral triangle

A bonding model

Chemical bonding is a continuum of ionic, covalent and metallic bonding

• The location of an element or compound in the bonding triangle is determined by the electronegativity values of the elements present
• The bonding triangle is anchored by two axes, electronegativity difference on the y-axis and average electronegativity on the x-axis
• The symbol for electronegativity is the Greek letter (chi) χ, pronounced 'ky' as in 'sky'
  • The average electronegativity of two elements, A and B would be: 
  • The difference in electronegativity between two elements A and B would be:

The Bonding Triangle

The bonding triangle is used to determine the percentage of ionic, covalent and metallic character in an element or compound

• Since elements have zero difference in electronegativity they would be found along the x-axis depending on the electronegativity value of the individual element
• Ionic compounds have a large difference in electronegativity so would be located in the top centre part or apex of the triangle
• Covalent compounds with a low difference in electronegativity would be found in the bottom right and polar covalent compounds in the zone between ionic and covalent