Introduction to   Topic 4 and Topic 14

One of the first things I do with my new Diploma classes is ask them what Chemistry actually is. It does not seem an unreasonable question to ask since they have chosen to study it for two years. It can cause quite a debate as there is much overlap with other branches of science. I like to show them a picture I found in a Ghanaian text book for teachers.

Of course, I am biased but it does illustrate how important Chemistry is. So what is Chemistry? One of the best ideas that my students come up with is that a chemical reaction involves the breaking and making of bonds to make new substances. I sometimes challenge them by then asking them if they think that the physical change of boiling water is therefore Chemistry as it involves breaking forces of attraction (hydrogen bonds) even though no chemical change takes place. Whatever the outcome of the discussion, it underlines how important the concept of bonding is to understanding Chemistry.

Students often seem to have a lot of misconceptions about bonding that they have picked up en route to their Diploma classes. One is "There are two types of bonding - ionic and covalent". Another is that ionic bonds and covalent bonds are both directional in nature. Because of the physical models they have used or seen some think of a bond as a rigid, fixed physical 'wire' connecting two atoms. They also struggle with the difference between intermolecular bonding (or forces of attraction) and intramolecular bonding (or forces of attraction). In fact one of the most common misconceptions seen in answers to Paper 2 questions is confusing inter- and intra- molecular forces of attraction. This is apparent when candidates try to explain why two compounds which have essentially the same molar mass, such as methoxymethane and ethanol, can have very different boiling points (see Areas of difficulty on Paper 2). Many answers wrongly include a reference to the O-H bond breaking in ethanol when it boils.

This topic takes you through the different types of bonding and intermolecular forces. I think it is useful for you to have some idea of the relative strengths (bond dissociation energies) of the different types of attraction. This will help you understand why silicon oxide, for example, has such a high melting and boiling point compared to carbon dioxide. It will also help you to understand why hydrogen bonding, which is in-between dipole-dipole attractions and covalent bonding in energy, is so important in the structure of nucleic acids. Hydrogen bonds are strong enough to hold the double helix together in DNA but weak enough to break during replication.

Standard level students need to know and understand about electron configurations so the level of Topic 14 is not so very different to that of Topic 4 except that it takes it further and includes formal charge and hybridization etc. If you are a Standard Level student you can more or less follow the 'octet rule' throughout. For Higher Level students the challenge of understanding and applying the concept of hybridization is perhaps the area that you may find most difficulty with. All students also have difficulty with the shapes of simple molecules and ions. Since 5 and 6 pairs of electrons (now known as electron domains) around the central atom follow on logically but are in fact covered separately to 2, 3 and 4 pairs on the syllabus I have put both of these together to give just one page to cover VSEPR theory covered in sub-topics 4.3 and 14.1.