Covalent bonding made easy

Neither of these models show the 3D shape of the molecule. Jumping between the 2D representation to the 3D shape of a molecular model can be conceptually challenging for students. A third model (Figure 1c) goes some way to overcome this difficulty. Dashes and wedges represent bonds going into and coming out of the page respectively.

Students will understand the bonding in a simple covalent molecule more clearly by considering all the electrostatic interactions involved. Figure 2 shows the interactions between the two nuclei and electrons in the diatomic molecule, H₂. Within the molecule there are both attractive (proton–electron) and repulsive (electron–electron and proton–proton) forces. The strength of these forces varies with the inverse of the square of the distance between the particles involved. Therefore, as the atoms approach each other, the strength of the attractive and repulsive forces change. At a certain distance, the equilibrium internuclear distance, the net attractive interaction is maximised. With the atoms this distance apart, the molecule is most stable.

It’s important to emphasise the electrostatic nature of the attractions that hold the two atoms together in a covalent bond. From relatively early on in their secondary education, students are familiar with the idea of opposite charges attracting and like charges repelling. Once students have met dot and cross diagrams, I start the next lesson with this activity, taken from Covalent bonding teaching resources. I ask them to annotate the diagram of a hydrogen molecule (Figure 3) to show all the attractive and repulsive forces that would exist. This simple activity encourages students to think beyond the shared electrons idea for covalent bonding.

The idea that substances are held together by electrostatic forces of attraction is described as one of 13 powerful ideas – the ideas being the most conceptually important parts of the science we want students to acquire at school. This shift in focusing on electrostatics can be applied to all types of bonding at 14–16. Students will have deeper understanding of the connections between different types of bonding, and progress to more complex ideas about bonding encountered at post-16.

At this stage, we would draw a molecule of ethene as a dot and cross diagram, as shown in Figure 4. Although this model shows the connectivity and explains the existence of a double bond between the two carbon atoms, it doesn’t explain the molecule’s shape or its reactivity.

To fully understand the distribution of the electrons in ethene, students need to understand the process of hybridisation, which is significantly beyond the 14–16 curriculum. However, the concept of orbital overlap can be introduced. Each carbon atom has four electrons to share. Three are held within identical orbitals (sp²) which orientate as far apart as possible to minimise electrostatic repulsion, and the fourth is held within a p orbital. A simple diagrammatic representation (such as Figure 5) allows the students to see the shape of the ethene molecule and understand its reactivity owing to the high energy electrons held within the weaker pi bond formed from the ‘sideways’ overlap of two parallel p orbitals.