DP Chemistry: 14.1 Further aspects of covalent bonding

14.1 Further aspects of covalent bonding

Written specifically for students to provide help and support for the IB Diploma chemistry programme this page provides full coverage of the syllabus content of Topic 14.1 Further aspects of covalent bonding. It encourages you to think critically and provides many questions with full worked answers so that you can monitor and improve your knowledge and understanding.

Learning outcomes

After studying this topic you should be able to:

Understand:

  • Atomic orbitals can overlap to form covalent bonds. Sigma bonds (σ) are formed by the direct head-on/end-to-end overlap of atomic orbitals. This results in the electron density concentrated between the nuclei of the bonding atoms. Pi bonds (π) are formed by the sideways overlap of atomic orbitals. This results in the electron density being above and below the plane of the nuclei of the bonding atoms.
  • The preferred Lewis structure from several possibilities can be decided using formal charge (FC). Formal charge 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 preferred Lewis structure is the one with the atoms having formal charge values closest to zero.
  • Species that have incomplete octets or expanded octets are exceptions to the 'octet rule'.
  • When electrons are shared by, or between, more than one pair of atoms in a molecule or ion it is known as delocalization (as opposed to the electrons being localized between just two of atoms).
  • Resonance structures are one of two or more alternative Lewis structures for a molecule or ion that cannot be described fully with one Lewis structure alone.

Apply your knowledge to:

  • Predict whether σ or π bonds are formed from the linear combination of atomic orbitals.
  • Deduce the Lewis structure of molecules and ions containing up to six electron pairs on each atom.
  • Apply formal charge to determine which is the preferred Lewis structure from two or more different possible structures.
  • Use VSEPR theory to deduce the molecular geometry involving five and six electron domains and associated bond angles (covered in 4.3 Covalent structures (2)).
  • Explain the wavelength of light required to dissociate oxygen and ozone molecules.
  • Describe the mechanism of ozone depletion catalysed by CFCs and NOx.

Relationships & vocabulary

Nature of science

The way in which bonding theories have been modified over time is an example of Occam's razor as newer theories need to remain as simple as possible whilst maximizing their explanatory power, e.g. including the concept of formal charge.

International-mindedness

How has the concentration of ozone in the ozone layer changed over time and what has the global community done to reduce ozone depletion?

Are the global communities efforts to combat ozone depletion a success or a failure in terms of solving an international environmental concern?

For more examples and links to International mindedness, Theory of knowledge, utilization etc. see separate page which covers all of Topics 4 & 14: Chemical bonding & structure.

Vocabulary

sigma bond (σ)pi bond (π)molecular orbitalresonance hybrid
delocalizationconjugationformal charge(bond order)

Learning slides

You can use this slide gallery for learning or for reviewing concepts and information. It covers all the key points in the syllabus for this sub-topic.

  

Something to think about

Benzene is found in crude oil and is an important industrial chemical, both as a solvent and as a chemical feedstock to make polymers such as polystyrene. It used to be used routinely in schools and universities but since the 1970s it is now illegal to use or store benzene in schools in many countries in the world as it can cause blood disorders and is a known carcinogen.

Benzene (бензол) can still be found in school laboratories in some countries. I took this picture in a school in Kazakhstan.

Even though you will not deal with benzene directly, it provides one of the classic examples of delocalization. It can be used to illustrate the many different ways in which delocalization is depicted in chemistry. Essentially delocalization in bonding means that a pair of electrons are no longer fixed as a bonding pair between two atoms but are free to move between atoms.

One way of representing delocalisation is through the use of resonance hybrids. These use double-headed arrows and show the extreme forms of the bonding with the true structure lying somewhere between these extremes. For example:

Often a single diagram is used with a dotted or full line to represent the delocalization. For example:

or

Sometimes the delocalization is shown using the p orbitals and then overlapping them to form delocalized π bonds. For example:

Unless they specify a particular method in a question all of these ways seem acceptable to the IB as descriptions of delocalization. What is important to understand is that in the case of benzene all the C-C bonds are of equal length and of equal energy due to delocalization. Delocalization increases the stability of benzene by about 160 kJ mol-1. This can be used to explain the relative inertness of benzene compared to ethene in undergoing addition reactions as this extra delocalization energy has first to be overcome.

Test your understanding of this topic

(Note that your teacher may have restricted your access to some or all of these questions and worked answers if they are going to use them as a class test or set them as an assignment.)

For ten 'quiz' multiple choice questions with the answers explained see MC test: Further aspects of covalent bonding.

For short-answer questions see Further aspects of covalent bonding questions.

More resources

1. A good animation of how sigma and pi bonds are formed and how double and triple bonds contain a mixture of sigma and pi bonds by Richard Thornley.

  Sigma and pi bond formation 

2. A useful video with examples on calculating formal charge also by Richard Thornley.

  Formal charge

3. A slide show using pictures put together by Lawrence Kok on Delocalization and formal charge.

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