10 min read - Integrating NOS into the teaching of orbital hybridisation theory (​HL - Structure 2.2.16)​

Orbital hybridisation theory is a relatively complex explanation for how atoms form covalent bonds with other atoms. It is also a good opportunity for teachers to familiarise students with theory development in chemistry.

1. Background
A theory is a proposed explanation for sets of experimental data or observations. 

2. What should students already know by Structure 2.2.16?
Towards the beginning of the course (Structure 1.3), students will have seen electron configurations and orbital box diagrams. The electrons in a carbon atom, for example, are arranged as follows:

Earlier in Structure 2.2​, students will also have explored covalent bonding, the octet rule, and molecular geometries. So by the time we are teaching orbital hybridisation theory they should be familiar with the tetrahedral structure of methane that indicates 4 identical sigma bonds resulting in complete 'octets' for all atoms:

3. What issue is presented?
Knowing that a carbon atom has only 2 half-filled orbitals, we might suggest that it can only form 2 'normal' sigma bonds and then, perhaps, accept a pair of electrons to form a coordinate bond and fill the empty p-orbital.

The issue here is that this prediction would lead to a carbon forming 3 bonds and this is not what the experimental evidence tells us.

4. How can we resolve the issue?
Given that the atomic orbital model was already well supported by experimental evidence (predominantly ionisation energies) and mathematical explanations (in the form of quantum mechanics), Linus Pauling proposed orbital hybridisation theory that would extend our understanding of the behaviour of atomic orbitals.

As you will explain in class, this involves the 'mixing' of different orbitals to form the required number of identical, half-filled orbitals.​

The ability of this theory to explain a wide range of experimental observations  led to its acceptance as a valid theory in 1930s. For his work on the nature of covalent bonding, Pauling won the Nobel Prize in Chemistry in 1954.

5. How might this direction of travel help students?
When students ask '...but why?' it is easy to get bogged down into the nitty gritty details of quantum mechanics and greater purpose in the universe. So perhaps a better way to frame teacher explanations is 'we can see that something must happen from experimental data and this is our best attempt to explain it'. This maintains student focus on the relevant components of a theory with the goal of reducing cognitive load. An example of my explanation of hybridisation can be found here.

6. Summary of the underlying nature of science
Below is a flow diagram that outlines the manner in which we reject, adapt or produce theories in science.

Additional point of interest: Linus Pauling also won the Nobel Peace Prize in 1962 for his advocacy work against nuclear arms.

Engaging students with the nature of models in chemistry is something I have started doing more frequently in recent years. Given that topics Structure 1 and Structure 2 in the new course are explicitly named Models of... , it feels like a better conceptual framing to address these ideas.

When introducing atomic structure (now Structure 1.2 - The nuclear atom), for example, the Mystery Box activity has been useful in provoke some thinking about how we produce scientific models as well as their inherent limitations.

In essence, you fill one or more box with objects, and ask students to sketch a model of what might be inside by playing with the boxes but not opening them. It can be helpful to have one box containing something far removed from a classroom to make a point about their likely paradigm when guessing the contents.

After sharing their thoughts with the rest of the class and considering additional experimentation to improve their models (float?/mass?/X-ray?/etc.) they will be desperate to know what's actual inside the boxes. At this point, it can be interesting to tell them that you also don't know. They will hate you for a short period of time but it demonstrates a key challenge in scientific modelling - we are trying to represent things that we will never be able to see directly. Instead, we must settle for degrees of confidence in the knowledge we produce in place of certainty. (To avoid lasting resentment, it is wise to tell them the contents of the box at a latter point in the lesson.)

This activity ties in nicely to the development of atomic models which have, over time, changed as new evidence comes to light. All atomic models have presented some useful characteristics for understanding matter be it the plum pudding model, the Rutherford model or the Bohr model.

For reference, here are some related ideas that might be explored in a TOK class (TOK concepts highlighted in bold). Scientific models...

• provide the basis for a proposed explanation of matter (a theory).
• must be supported by empirical evidence.
• are not provable in the sense we can be 100% certain.
• are developed with some element of interpretation (so scientific knowledge may not be as objective as we like to think).
• are often adapted or replaced ('paradigm shifts') as new evidence comes to light.

The Structure 1.2 slides containing this activity with some prompt questions can be found here.