Topic 14.1: Chemical bonding and structure – Further aspects of covalent bonding and structure
Larger structures and more in-depth explanations of bonding systems often require more sophisticated concepts and theories of bonding.
• Understanding: Covalent bonds result from the overlap of atomic orbitals. A sigma bond (σ) 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 (π)
is formed by the sideways overlap of atomic orbitals, resulting in electron density above and below the plane of the nuclei of the
bonding atoms.
▪ Covalent bonds: electrostatic attraction between a shared pair of electrons and
the positively charged nuclei
• Covalent bonds result from the overlap of atomic orbitals
• Sigma bond (σ): formed by the direct end-to-end overlap of atomic
orbital; electron density between nuclei
• Pi bond (π): formed by the sideway overlap of atomic orbitals;
electron density above and below plane of bonding atoms
• Understanding: 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)-
½ (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.
▪ Formal charge (FC): charge an atom would have if all atoms in the molecule has the same electronegativity
• FC = (number of valence electrons) – ½ (Number of bonding electrons) – (Number of non-bonding electrons)
• Sum of all FC of atoms: must equal to the charge of an ion or a molecule (FC=0)
• Understanding: Exceptions to the octet rule include some species having incomplete octets and expanded octets.
▪ Incomplete octets: stable Lewis structure involving less than 8 valence electrons surrounding central atom (e.g. BF3)
▪ Expanded octets: stable Lewis structure involving more than 8 valence electrons surrounding central atom (e.g. XeF4)
• Central atom has 5 electron domains (trigonal bipyramidal) or 6 electron domains (octahedral)
• Understanding: 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.
▪ Delocalization: involves electrons shared by more than two atoms in a molecule or ion as opposed to being shared between a pair
• Electrons are delocalize in resonance structures
• Bond order: fractional double bond character between two atoms in a resonance molecule or on
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑖𝑟𝑠 𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑖𝑛𝑔 𝑏𝑜𝑛𝑑 𝐴
• Bond order =
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑜𝑛𝑑 𝐴 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑖𝑟𝑠 𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑖𝑛𝑔 𝑁𝑂 3
• Example: NO2- bond order = =
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑁𝑂 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 2
• Understanding: 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) structure alone.
▪ Resonance: molecules or ions with more than two possible Lewis structures
to represent a particular molecule
• Resonance structure (forms): one of two or more alternative
Lewis structure of a molecule or ion that has resonance
• Resonance hybrid: actual electronic structure of the species
▪ Resonance structures occur when there is more than one possible position for a double bond in a molecule
▪ Resonance structures are pure hypothetical and does not actually exist
• Applications and skills: Prediction whether sigma (σ) or pi (π) bonds are formed from the linear combination of atomic orbitals.
σ bond π bond
Orbital combination s+s s+p
p + p (end to end) p + p (lateral sideway overlap)
• Applications and skills: Deduction of the Lewis (electron dot) structures of molecules and ions showing all valence electrons for up
to six electron pairs on each atom.
▪ Deduction of Lewis structure of molecules
• Identify ball and stick diagram of molecule
• Localize the charge onto the central atom
• From charge and number of valence electrons, identify electron domain geometry
• From the number of bonds, determine the molecular geometry (and bond angles)
• Complete the octet by filling lone pairs of electrons and raw any resonance structure
Larger structures and more in-depth explanations of bonding systems often require more sophisticated concepts and theories of bonding.
• Understanding: Covalent bonds result from the overlap of atomic orbitals. A sigma bond (σ) 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 (π)
is formed by the sideways overlap of atomic orbitals, resulting in electron density above and below the plane of the nuclei of the
bonding atoms.
▪ Covalent bonds: electrostatic attraction between a shared pair of electrons and
the positively charged nuclei
• Covalent bonds result from the overlap of atomic orbitals
• Sigma bond (σ): formed by the direct end-to-end overlap of atomic
orbital; electron density between nuclei
• Pi bond (π): formed by the sideway overlap of atomic orbitals;
electron density above and below plane of bonding atoms
• Understanding: 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)-
½ (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.
▪ Formal charge (FC): charge an atom would have if all atoms in the molecule has the same electronegativity
• FC = (number of valence electrons) – ½ (Number of bonding electrons) – (Number of non-bonding electrons)
• Sum of all FC of atoms: must equal to the charge of an ion or a molecule (FC=0)
• Understanding: Exceptions to the octet rule include some species having incomplete octets and expanded octets.
▪ Incomplete octets: stable Lewis structure involving less than 8 valence electrons surrounding central atom (e.g. BF3)
▪ Expanded octets: stable Lewis structure involving more than 8 valence electrons surrounding central atom (e.g. XeF4)
• Central atom has 5 electron domains (trigonal bipyramidal) or 6 electron domains (octahedral)
• Understanding: 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.
▪ Delocalization: involves electrons shared by more than two atoms in a molecule or ion as opposed to being shared between a pair
• Electrons are delocalize in resonance structures
• Bond order: fractional double bond character between two atoms in a resonance molecule or on
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑖𝑟𝑠 𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑖𝑛𝑔 𝑏𝑜𝑛𝑑 𝐴
• Bond order =
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑜𝑛𝑑 𝐴 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑖𝑟𝑠 𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑖𝑛𝑔 𝑁𝑂 3
• Example: NO2- bond order = =
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑁𝑂 𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 2
• Understanding: 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) structure alone.
▪ Resonance: molecules or ions with more than two possible Lewis structures
to represent a particular molecule
• Resonance structure (forms): one of two or more alternative
Lewis structure of a molecule or ion that has resonance
• Resonance hybrid: actual electronic structure of the species
▪ Resonance structures occur when there is more than one possible position for a double bond in a molecule
▪ Resonance structures are pure hypothetical and does not actually exist
• Applications and skills: Prediction whether sigma (σ) or pi (π) bonds are formed from the linear combination of atomic orbitals.
σ bond π bond
Orbital combination s+s s+p
p + p (end to end) p + p (lateral sideway overlap)
• Applications and skills: Deduction of the Lewis (electron dot) structures of molecules and ions showing all valence electrons for up
to six electron pairs on each atom.
▪ Deduction of Lewis structure of molecules
• Identify ball and stick diagram of molecule
• Localize the charge onto the central atom
• From charge and number of valence electrons, identify electron domain geometry
• From the number of bonds, determine the molecular geometry (and bond angles)
• Complete the octet by filling lone pairs of electrons and raw any resonance structure
