TOPIC 14.1 FURTHER ASPECTS OF COVALENT BONDING AND STRUCTURE

ESSENTIAL IDEA Larger structures and more in-depth explanations of bonding systems often require more sophisticated concepts and theories of bonding. NATURE OF SCIENCE (2.7) Principle of Occam’s razor – bonding theories have been modified over time. Newer theories need to remain as simple as possible while maximizing explanatory power, for example the idea of formal charge.

UNDERSTANDING/KEY IDEA 14.1.A 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 being concentrated between the nuclei of the bonding atoms. A pi bond is formed from the sideways overlap of atomic orbitals, resulting in electron density above and below the plane of the nuclei of the bonding atoms.

APPLICATION/SKILLS Be able to predict whether sigma or pi bonds are formed from the linear combination of atomic orbitals.

SIGMA BONDS All single covalent bonds are sigma bonds. Sigma bonds form by the overlap of orbitals along the bond axis. The following form sigma bonds: s and s (H2) s and p (HCl) p and p (Cl2) Hybrid orbitals and s Hybrid orbitals and hybrid orbitals

PI BONDS All double covalent bonds contain one pi and one sigma bond. All triple covalent bonds contain two pi and one sigma bond. The following form pi bonds: p and p sideways Pi bonds are weaker than sigma bonds since their electron density is farther away from the positive nucleus. The double bonds break more readily and are more reactive than those with only sigma bonds.

UNDERSTANDING/KEY IDEA 14.1.B Formal charge (FC) can be used to decide which Lewis 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 structure with the atoms having FC values closest to zero is preferred.

APPLICATION/SKILLS Be able to apply formal charge (FC) to ascertain which Lewis structure is preferred from different Lewis structure.

UNDERSTANDING/KEY IDEA 14.1.C Exceptions to the octet rule include some species having incomplete octets and expanded octets. (This understanding was explained in detail in ppt 4.3. Incomplete octets involve B and Be and expanded octets encompass any structure with the trigonal bipyramidal and octahedral shapes.)

APPLICATION/SKILLS Be able to deduce the Lewis structure of molecules and ions showing all valence electrons for up to six electron pairs on each atom. (Covered in detail in ppt 4.3)

APPLICATION/SKILLS Be able to deduce using the VSEPR theory the electron domain geometry and molecular geometry with five and six electron domains and associated bond angles and polarity. (Covered in detail in ppt 4.3)

UNDERSTANDING/KEY IDEA 14.1.D 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 Sometimes electrons show a tendency to be shared between more than one bonding position and are said to be delocalized. Since they are free from the constraints of a single bonding position, delocalized electrons spread themselves out making the molecule more stable. Delocalization happens when you have a double bond that can have more than one possible position.

PROPERTIES Delocalized electrons give rise to intermediate bond strength and length. This new length is in between a single and double bond. Bond Order – is the number of shared electron pairs divided by the number of bonding positions. The higher the bond order, the greater the electron density. Greater stability due to the fact that delocalization spreads out the electrons and minimizes the repulsions between them. Delocalized electrons allow electrical conductivity.

UNDERSTANDING/KEY IDEA 14.1.E Resonance involves using two or more Lewis structures to represent a particular molecule or ion. A resonance structure is one of two or more alternative Lewis structures for a molecule or ion that cannot be described fully with one Lewis structure alone.

RESONANCE Delocalization cannot be depicted by one Lewis structure so the concept of resonance is introduced. The actual structure of the species is an average of the number of Lewis structures that can be drawn as resonance structures. Benzene is a good example of this.

APPLICATION/SKILLS Be able to explain the wavelength of light required to dissociate oxygen and ozone.

OZONE Ozone, O 3, has a bent shape with a bond angle of 117 ◦ It has 2 resonance structures. The double bond consists of one pi and one sigma bond. The electrons in the pi bond are held less tightly so they become delocalized giving rise to the resonance structure. The bond order is 1.5 which means the length is intermediate and the strength is between a double and single bond.

OZONE The molecule is polar which is explained by formal charge and the uneven distribution of electrons. The lower part of the stratosphere, known as the ozone layer, contains 90% of the atmospheric ozone. Ozone levels are maintained through a cycle of reactions involving the formation and breakdown of oxygen and ozone.

Ref: schooltutoring.com

APPLICATION/SKILLS Be able to describe the mechanism of the catalysis of ozone depletion when catalyzed by CFCs and NOx.