Bonding and structure

In this section of the syllabus students revisit ideas connected with bonding, and reappraise their understanding of the types of bonding that are seen in chemical compounds. A level challenges the neat compartmentalisation of the GCSE picture of bonding in compounds as either ionic or covalent. It achieves this by introducing the idea that most substances are neither purely ionic nor purely covalent. Students learn to appreciate that compounds that are essentially covalent (contain no metal elements) can display bond and molecular polarity giving them ionic character. In a similar way they learn that compounds that are essentially ionic can display covalents character (Fajan's rules). In this way, the idea of covalent and ionic compounds as extremes on a continuum of bonding type is nurtured. AlCl3 and BeCl2 for example are covalent compounds whilst LiI has significant solubility in organic solvents.

Very able students might be asked to research organometallic compounds, which contain covalent bonds between metals and carbon (typically but not exclusively), as an extension activity. Students are required to revisit Lewis structures (dot and cross diagrams) of covalent species and appreciate that the octet rule is not sacrosanct, leading to a consideration of molecules that either expand the octet on an atom, leave an atom without a complete octet, or use dative covalent bonds to protect the octet. They need to understand when and why these variations occur.

VSEPR theory is introduced as a way for students to deduce the shape of a molecule, and this is crucially important on deciding whether or not a molecule containing bond dipoles has an overall molecular dipole. This then allows a proper development of the concept of intermolecular forces, to include dipole-dipole forces, the special case of hydrogen bonding and London Dispersion forces (which some syllabuses insist on incorrectly referring to as van der Waals forces. Be aware of your syllabus approach in this respect). Students need to appreciate that intermolecular forces are not mutually exclusive, and that molecules will often exhibit more than one type of interaction.

The ideas developed are then utilised in developing a more complete understanding of solid structures to include giant ionic, simple covalent, giant covalent and metallic solids (note here that the above terminology is that specified by ofqual, and may or may not be that used in a particular syllabus. For example, some syllabuses will prefer simple molecular to simple covalent. There is no right or wrong here, but syllabuses are sometimes very pedantic regarding terminology, so it is wise to check preferred terms). Students often misunderstand questions relating to this section which may for example ask why methane is a gas at room temperature but water, with a very similar Mr, is a liquid. The crucial point is the difference in the strength of intermolecular forces. Hydrogen bonding in water is about an order of magnitude stronger than the London forces in methane.
