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.
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Links and Resources
This comprehensive resource provides much detailed background material on these topics. The resource would be useful for teachers when planning lessons, but also many of the exercises, with a few dated exceptions, are still useful and pertinent to current syllabuses and can be scanned or retyped and edited to provide class or homework material. This is an invaluable resource for planning, and for setting example problems to support teaching and learning.
This resource is stated to be aimed at gifted and talented students, but the early part of the resource is perfectly suited for use with students during the development of ideas related to the application of VSEPR theory in deducing the shape of molecules.
It includes a grid of basic shapes based on the number of repulsion axes on the central atom in a molecule which can be annotated with additional information such as bond angles and alternative names of shapes as the theory is developed. The resource continues with a number of exercises.
A mention is made of an alternative approach based on molecular orbital theory and references are given. This might be a useful extension activity for more able students but is not emphasised in any of the current syllabuses.
This resource, from the Royal Society of Chemistry's Starter for Ten collection, provides useful worksheets covering a number of the areas in this topic. It makes a useful bridge between GCSE and A level by starting with simple and familiar ideas from GCSE, and then continuing with an activity higlighting common misconceptions before tackling A level concepts. The material can be used to reinforce learning either in class or as homework exercises.
This resource encourages students to move past the idea that molecules form in order to obtain full shells, towards a more complete understanding of bonding as an energetic phenomenon. Molecules have lower energy than the seperate atoms for example. Section one of the resource is useful in this respect. However, the second section attempts to extend this further and is rather unsucessful in trying to present a simplified explanation of molecular orbital (MO) theory of a covalent bond and is actually discussing bonding and antibonding molecular orbitals. Whilst MO theory is a useful extension of the nature of covalent bonding for the very able student, a more conventional approach based on evergy level diagrams is perhaps clearer and simpler.
This is another activity from the Chemical Misconceptions collection, this time centering on ionic bonding. It would constitute a useful activity to elicit student ideas on the nature of bonding in an ionic lattice before a deeper theoretical treatment of the subject, or as a diagnostic at the end of a section to disclose hidden misconceptions.
This is a student resource, from the Royal Society of Chemistry's Chemical Misconceptions volume, aiming at eliciting students' ideas on chemical bonding. It is probably best used as a diagnostic towards the end the module on bonding and structure in order to uncover lingering problems with student perceptions which may otherwise go undetected and be carried forward into other topic areas, thus undermining understanding of those areas.
The resource Chemical Bonding (included below) is a teacher resource which discusses the outcomes of research using this resource as a diagnostic aid with students, and the two are most effective when used together.
This is a sister resource to the previous one Spot the Bonding and looks in greater depth at the types of interactions found within and between molecules. It is a natural follow on from the previous resource.
This resource is an analysis of findings regarding student ideas on chemical bonding obtained by use of the partner resource Spot the Bonding with students.