The theory behind theories

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.