Example of Concept-Based Teaching: The Concept of Chemical Bonds
One example of concept-based teaching that has been experimented in Chemistry tutorials has been in the learning of chemical bonds. From our experience, chemical bonding is often taught as a topic rather than conceptually, so curriculum developers classify substances, based on a list of properties, into four different groups of lattices (ionic, molecular, covalent and metallic) and elaborate on and discuss each of these structures based on the chemical bonds that exist between the particles. These types of chemical bonds (ionic, covalent and metallic bonds) are often discussed as different entities. According to Hurst (2002), this oversimplified presentation misleads Chemistry students and may actually cause learning impediments. Taber and Watts (2000) point out that students are expected to acquire some familiarity with the theoretical frameworks of Chemistry and to develop some level of proficiency in applying their knowledge regarding chemical bonds in order to produce valid scientific explanations. Presenting the bonds as different entities, as is often done in textbooks, can be misleading and it may fail to represent the key unifying ideas in Chemistry, thus resulting in very disjointed understanding of the discipline.
An approach which teachers of highly able learners can adopt, as proposed by Nahum, Mamlok-Naaman, Hofstein and Krajcik (2007) in the teaching of chemical bonds and the structures and properties of substances with different lattices, is to use a similar conceptual model to describe all bonds. This is done from a submicroscopic level (understanding of the principles that are common to all types of chemical bonds between two atoms in the gas state) and only then progressing towards the microscopic and macroscopic levels (structures and properties of molecules and clusters, which involve much greater complexities). This approach is based on an understanding of the common principles and concepts suggested for all chemical bonds (such as electrostatic attractions; bond strengths, electron densities and overlapping of bonding orbitals) and then using these ideas to explain the structures and properties of molecules and lattices. Such an approach is consistent with Hurst (2002), who concluded that the bonding theory and related concepts need to be taught in a uniform manner.
According to this approach, the focus is on the bonds that might be formed between two atoms. On the one hand, all chemical bonds (including hydrogen bonds and van der Waals bonds) are presented using the model of interactions between two atoms in the gas state. The idea is to bring across to students the concept of continuous bond strengths of chemical bonds. On the other hand, there is emphasis on the importance of the ability of students to distinguish between different bonds by their lengths, energies and other important characteristics such as directionality. Thus, although all these bonds can be presented on a continuous scale of bond strength, students should acquire a qualitative understanding regarding the strength of these bonds and their characteristics.
One example of building a qualitative understanding of bond strength is reflected in getting students to understand the unique nature of the hydrogen bond and recognise the situations in which it might occur. More specifically, if the hydrogen bonds between water molecules in the liquid state and the polar covalent bonds between hydrogen and oxygen in a single molecule of water are discussed, students should be able to explain the following: firstly, the common principles and concepts regarding the hydrogen bond and the polar covalent bond, i.e. both bonds are directional and can be explained by the equilibrium point at which the repulsive and attractive forces are equal, and, secondly, the hydrogen bond between an oxygen atom on one water molecule and the hydrogen atom on an adjacent water molecule is much longer and as a result is much weaker (based on Coulomb’s law) than the polar covalent bond between oxygen and hydrogen in a single water molecule. These different bond strengths result from different energy balances. Consequently, the energies required for breaking each bond are largely different, and this is reflected in the properties of water. By using such a coherent conceptual model for all bonds, our students’ ability to apply their knowledge of chemical bonds in a variety of contexts improved, and this aligned well with their learning performances. In fact, over the 2-year course of study, this systematic approach fostered in both students and teachers a much deeper understanding of the underlying key concepts of bonding that resulted in a firmer grasp of chemical properties and reactions later in the course.