Structure 2.2 The covalent model (HL)
Structure 2.2.11
​Understandings:
  • Resonance structures occur when there is more than one possible position for a double bond in a molecule.
Learning outcomes:
  • Deduce resonance structures of molecules and ions.​
Additional notes:
  • Include the term “delocalisation”.
Linking questions:
  • Structure 1.3 Why are oxygen and ozone dissociated by different wavelengths of light?
This video covers resonance structures. 
This video covers molecules and ions with delocalised electrons. 
Structure 2.2.12
​Understandings:
  • Benzene, C6H6, is an important example of a molecule that has resonance.
Learning outcomes:
  • Discuss the structure of benzene from physical and chemical evidence.​
Linking questions:
  • Reactivity 2.1, 2.2 How does the resonance energy in benzene explain its relative unreactivity?
  • Reactivity 3.4 What are the structural features of benzene that favour it undergoing electrophilic substitution reactions?
This video covers the physical and chemical evidence for the structure of benzene. 
Structure 2.2.13
​Understandings:
  • Some atoms can form molecules in which they have an expanded octet of electrons. 
Learning outcomes:
  • Represent Lewis formulas for species with five and six electron domains around the central atom.
  • Deduce the electron domain geometry and the molecular geometry for these species using the VSEPR model.​
Linking questions:
  • Structure 3.1 How does the ability of some atoms to expand their octet relate to their position in the periodic table?
This video covers how to deduce the molecular and electron domain geometries for molecules and ions with up to six electron domains. 
Structure 2.2.14
​Understandings:
  • Formal charge values can be calculated for each atom in a species and used to determine which of several possible Lewis formulas is preferred.
Learning outcomes:
  • Apply formal charge to determine a preferred Lewis formula from different Lewis formulas for a species.​
Linking questions:
  • Structure 3.1, Reactivity 3.2 What are the different assumptions made in the calculation of formal charge and of oxidation states for atoms in a species?
This video covers how to determine formal charges. 
Structure 2.2.15
​Understandings:
  • Sigma bonds σ form by the head-on combination of atomic orbitals where the electron density is concentrated along the bond axis.
  • Pi bonds π form by the lateral combination of p-orbitals where the electron density is concentrated on opposite sides of the bond axis.
Learning outcomes:
  • Deduce the presence of sigma bonds and pi bonds in molecules and ions.​
Additional notes:
  • Include both organic and inorganic examples.
This video covers sigma and pi bonds. 
Structure 2.2.16
​Understandings:
  • Hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals for bonding.
Learning outcomes:
  • Analyse the hybridization and bond formation in molecules and ions.
  • Identify the relationships between Lewis formulas, electron domains, molecular geometry and type of hybridization.
  • Predict the geometry around an atom from its hybridization, and vice versa.​
Additional notes:
  • Include both organic and inorganic examples. Only sp, sp2 and sp3 hybridization need to be covered.
This video covers sp, sp2 and sp3 hybridisation.