● Covalent bonds result from the overlap of atomic orbitals. A sigma bond () is formed by the
direct head-on/end-to-end overlap of atomic orbitals, resulting in electron density concentrated
between the nuclei of the bonding atoms. A pi bond () is formed by the sideways overlap of
atomic orbitals, resulting in electron density above and below the plane of the nuclei of the
The linear combination of atomic orbitals to form molecular orbitals should be covered in the context
of the formation of sigma () and pi () bonds.
● Formal charge (FC) can be used to decide which Lewis (electron dot) structure is preferred from
several. The FC is the charge an atom would have if all atoms in the molecule had the same
electronegativity. FC = (number of valence electrons) – ó(number of bonding electrons) – (number
of non-bonding electrons). The Lewis (electron dot) structure with the atoms having FC values
closest to zero is preferred.
● Exceptions to the octet rule include some species having incomplete octets and expanded octets.
Molecular polarities of geometries corresponding to fi ve and six electron domains should also be
● Delocalization involves electrons that are shared by/between all atoms in a molecule or ion as
opposed to being localized between a pair of atoms.
● Resonance involves using two or more Lewis (electron dot) structures to represent a particular
molecule or ion. A resonance structure is one of two or more alternative Lewis (electron dot)
structures for a molecule or ion that cannot be described fully with one Lewis (electron dot)
Applications and skills
● Prediction whether sigma () or pi () bonds are formed from the linear combination of atomic
● Deduction of the Lewis (electron dot) structures of molecules and ions showing all valence
electrons for up to six electron pairs on each atom.
● Application of FC to ascertain which Lewis (electron dot) structure is preferred from different Lewis
(electron dot) structures.
● Deduction using VSEPR theory of the electron domain geometry and molecular geometry with
fi ve and six electron domains and associated bond angles.
● Explanation of the wavelength of light required to dissociate oxygen and ozone.
● Description of the mechanism of the catalysis of ozone depletion when catalysed by CFCs and
Ozone - student presentation
Research and present either of the 3 topics below:
Student work example - Ozone: a case study in resonance, molecular polarity and formal charge
Student work example - Ozone is an essential component of the stratosphere
Student work example - Catalytic destruction of ozone