Structure and Bonding
Blogs about Chemistry
Students always find structure and bonding challenging. The concepts they are grappling with in bonding are abstract, and the links between the structure and the bonding are not always easy for students to visualise. This is a topic that needs a lot of scaffolding to help students to build up their understanding, looking down from the big-picture ideas about the properties of a type of substance, and a solid grasp of the fundamentals of atomic structure and energetics before students have a realistic chance of really understanding bonding without holding serious misconceptions about the behaviour of atoms, ions, and electrons.
I have debated endlessly with myself about the best order for teaching bonding in. Most schemes of work tend to teach ionic, covalent and then metallic. Given the importance of balancing teaching from big-picture thinking as well as building up from the building blocks of bonding, I have come to the conclusion that starting with metallic bonding is preferential, as students are most familiar with the macro-scale properties of metals and can most easily link bonding with properties via structure. Furthermore, metallic bonding only requires students to deal with the charges on positive ions, rather than dealing with positive and negative ions simultaneously. When teaching about the formation of ions, any talk of formation of ions with a full outer shell "because they are more stable" or "happy atoms" must be avoided at all costs: this leads to serious misconceptions later. One way of avoiding these sorts of student misconceptions is to use some transition metals as examples for metallic bonding, for example, copper or zinc. The excellent RSC Chemical Misconceptions activity uses iron as an example. Furthermore, an elementary discussion about the strength of the bond produced vs. the energy required to remove further electrons can take place using a concept cartoon activity. The material on metallic bonding provided here is explicitly designed to help students effectively link between the arrangement of particles and macro-scale properties of metals.
Moving on to ionic bonding, students then appreciate that there is a link between particles and properties, although this becomes more abstract for ionic substances which they are less familiar with. A lesson could wither start with determining some of the properties experimentally and then going on to build up the particle picture, or working from the ground up and students then planning and carrying out an investigation to determine whether or not some substances are ionic. Students understanding of the formation of ions is developed as they learn about the formation of negative ions and use their understanding of why metals form ions of certain charges to understand the charges on negative ions. Once again the stimulus materials provided here are designed to scaffold the students' thinking and help them to understand the links between the micro- and macro-scale phenomena. I have also provided some extension material on lattice structures designed for students taking Triple Award Chemistry. The RSC Chemical Misconceptions sheet is an excellent way for the teacher to check whether or not students have grasped the key learning required before moving on to the next topic.
I tend to think that covalent bonding should be taught completely separately as it does not involve the formation of ions at all. This then avoids confusion for students as they have had time to consolidate their ideas about ionic bonding, writing formula etc. with an intermediary topic such as Acids, bases and salts or the Reactivity series. My teaching methods in terms of providing students with a firm foundation for covalent bonding are similar to the strategies above: allow student to really understand the links between the bonding and the properties and use thinking maps to help students to scaffold their explanations (worksheets here and here). Once again, however, all talk of atoms needing to fill their outer shells must be avoided as this is shown to be false two weeks into A-level teaching. Instead I frame my explanations around the idea that making bonds releases energy, so atoms tend to form the maximum number of covalent bonds possible. There are some excellent opportunities for stretch and challenge for students that 'get it', for example, drawing a dot-and-cross- diagram for a hydroxide ion and bringing together students' ideas about charges and ions.
After covering the three types of bonding, it is essential that students are given the opportunity to practise selecting the correct type of bonding to discuss in a particular situation or problem. This is called interleaving and is an essential aspect of students being able to chunk their learning of each kind of bonding so that the techniques can be moved into the long-term memory without worrying that students will forget the basic principles. I have provided an example task that students could complete individually or discuss in pairs to help with this process of interleaving.