S2.2 (AHL) The Covalent Model

• (AHL) Resonance structures occur when there is more than one possible position for a double bond in a molecule.

• (AHL) Benzene, C6H6, is an important example of a molecule that has resonance.

• (AHL) Some atoms can form molecules in which they have an expanded octet of electrons. 

• (AHL) Formal charge values can be calculated for each atom in a species and used to determine which of several possible Lewis formulas is preferred.

• (AHL) Sigma bonds form by the head-on combination of atomic orbitals where the electron density is concentrated along the bond axis. Pi bonds form by the lateral combination of p orbitals where the elctron density is concentrated on opposite sides of the bond axis

AHL

Structure 1.3—Why are oxygen and ozone dissociated by different wavelengths of light? 

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?

Structure 3.1—How does the ability of some atoms to expand their octet relate to their position in the periodic table?

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?