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2.2.2 Bonding and structure
(a) ionic bonding as electrostatic attraction between positive and negative ions, and the construction of ‘dot-and-cross’ diagrams
(b) explanation of the solid structures of giant ionic lattices, resulting from oppositely charged ions strongly attracted in all directions e.g. NaCl
(c) explanation of the effect of structure and bonding on the physical properties of ionic compounds, including melting and boiling points, solubility and electrical conductivity in solid, liquid and aqueous states
(d) covalent bond as the strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
(e) construction of ‘dot-and-cross’ diagrams of molecules and ions to describe:
(i) single covalent bonding
(ii) multiple covalent bonding
(iii) dative covalent (coordinate) bonding
(f) use of the term ‘average bond enthalpy’ as a measurement of covalent bond strength
(g) the shapes of, and bond angles in, molecules and ions with up to six electron pairs (including lone pairs) surrounding the central atom as predicted by electron pair repulsion, including the relative repulsive strengths of bonded pairs and lone pairs of electrons
(h) electron pair repulsion to explain the following shapes of molecules and ions: linear, non-linear, trigonal planar, pyramidal, tetrahedral and octahedral
(i) electronegativity as the ability of an atom to attract the bonding electrons in a covalent bond; interpretation of Pauling electronegativity values
(j) explanation of:
(i) a polar bond and permanent dipole within molecules containing covalently-bonded atoms with different electronegativities
(ii) a polar molecule and overall dipole in terms of permanent dipole(s) and molecular shape
(k) intermolecular forces based on permanent dipole–dipole interactions and induced dipole–dipole interactions
(l) hydrogen bonding as intermolecular bonding between molecules containing N, O or F and the H atom of –NH, –OH or HF
(m) explanation of anomalous properties of H2O resulting from hydrogen bonding, e.g.:
(i) the density of ice compared with water
(ii) its relatively high melting and boiling points
(n) explanation of the solid structures of simple molecular lattices, as covalently bonded molecules attracted by intermolecular forces, e.g. I2, ice
(o) explanation of the effect of structure and bonding on the physical properties of covalent compounds with simple molecular lattice structures, including melting and boiling points, solubility and electrical conductivity
(d) explanation of:
(i) metallic bonding as strong electrostatic attraction between cations (positive ions) and delocalised electrons
(ii) a giant metallic lattice structure, e.g. all metals
(e) explanation of the solid giant covalent lattices of carbon (diamond, graphite and graphene) and silicon as networks of atoms bonded by strong covalent bonds
(f) explanation of physical properties of giant metallic and giant covalent lattices, including melting and boiling points, solubility and electrical conductivity in terms of structure and bonding
(g) explanation of the variation in melting points across Periods 2 and 3 in terms of structure and bonding (see also 2.2.2 o)
Approaches to teaching the content
Assessment of what students already know is an important component of making teaching decisions. The key is to have students make connections among the various interrelated concepts in bonding. Students have limited knowledge from GCSE with few connections and even fewer cross-connections. If there are gaps in students’ understanding or if they are missing conceptual links, learning new material is going to be difficult. In fact students do not arrive in the classroom with a clean slate to which knowledge is added.
The scientific educational literature is full of research that has been done about the difficulty students have in understanding chemical bonding. Many of these misconceptions are robust and remain even after teaching the topic. Common difficulties are: why bonding occurs, confusing ionic bonding with covalent bonding – more in the next section.
Have you ever wondered if we teach the topic chemical bonding in the correct order? Chemical bonds are still taught as pure ionic and covalent; as well, the role of electronegativity in bonding is often not made clear in ionic bonding. Researchers believe that there are four limitations in the teaching approach.
- Bonding concepts are arranged on the basis of bond strength – students struggle with the concept of bond strength.
- Teaching order is by tradition ionic, covalent, polar covalent, hydrogen bonds and London forces.
- Electronegativity is introduced when polar bonds are introduced.
- The teaching order is not in line with the bond strength.
The major problems associated with this order of teaching are that students learn that all types of bonding are independent of each other and that there is no association between them. Researchers have proposed the covalent, polar covalent and ionic bonding sequence for effective teaching. That way you are comparing the product aspect of bonding rather than the process aspect of bonding.
Teachers working with pre-16 students tend to over-rely on the ‘octet rule’ as an infallible tool for students to use in determining formulae and bonding. This so-called rule contributes to the students’ problems with bonding. This rule has limited application in bonding, and therefore should not be used in teaching chemical bonding, because the octet rule only works successfully for carbon, nitrogen, oxygen and aluminium.
A range of different terms is used to describe bonds, especially intermolecular bonds. These include ‘van der Waals’, ‘London forces’, ‘intermolecular forces’, ‘attractive forces’ and ‘attractions’. Such language over-complicates the picture. Terms such as ‘induced dipole-dipole bonds’ or ‘permanent dipole – permanent dipole bonds’ are much more descriptive. We need to be consistent in using our bonding terminology.
Teachers should be aware that their own knowledge of content can cause issues with how information is presented. They should take care not to oversimplify the content.
Common misconceptions or difficulties students may have
- Students confuse electron transfer (the process) and electrostatic attraction (the outcome). The latter is ionic bonding.
- Often there is a belief that mobile electrons cause conduction, even in ionic compounds. In ionic compounds, it is the free ions that carry electrical charge.
- Confusion over electrical conductance – the key idea is that ionic compounds will conduct when molten or in solution because then the ions are mobile.
- Mistakenly thinking that ions formed by the loss of electrons have a negative charge – whereas it is of course positive as the electron has a negative charge.
- Why do Group 13 and 15 elements rarely form ionic compounds and Group 14 elements never form ionic compounds?
- Students often think that boiling simple covalent molecules involves breaking the covalent bonds. Covalent bonds are intramolecular whereas boiling breaks intermolecular forces.
- They also think that all molecules obey the octet rule – they will meet examples where the octet rule is not adhered to here.
- Students mistakenly think that any molecule that contains polar bonds must itself be polar – the polarities can cancel out, as in symmetrical molecules.
- Students become confused between intermolecular forces and covalent bonds – covalent bonds are intramolecular.
- Students often confuse the conduction of electricity by metals and graphite (electrons) with that of ionic compounds (ions).
- They see the formulae of ionic compounds written as ‘KCl’ or ‘CaCl2’. There is no distinction between these formulae and ‘NH3’ or ‘H2O’, which are covalent.
- There are only two types of bonds – covalent and ionic. The vast majority of compounds fall between these two extremes.
- Covalent bonds are weaker than ionic bonds.
- Students find it difficult to describe how London forces are formed.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
The understanding of bonding is linked in with earlier topics in the syllabus, such as Formulae and equations and Oxidation numbers. It is important to highlight that bonding plays a very important role in Chemistry and will come back in the following topics: Alkanes, Alkenes, Bond enthalpies, Halogenoalkanes, Transition elements, Alcohols, Amines, Amino acids, and Carboxylic acids.
Hydrogen bonding is important in many chemical processes. It is responsible for water’s unique solvent capabilities. Hydrogen bonds hold complementary strands of DNA together, and they are responsible for determining the three-dimensional structure of folded proteins including enzymes and antibodies.
The paper addresses the latest developments in the design of novel anti-tumour agents based on platinum and palladium.
Play hangman with the keywords. Teacher writes out the amount of letters on the board. Students need to find the correct keyword and then explain what this keyword means.
Pass around a model of the NaCl structure. Get the students to describe it in their own words.
Demonstrations of water boiling and sugar dissolving, ice melting, iodine subliming and propanone evaporating can all be used to investigate students’ thinking about chemical bonding – make the events explicit by carrying them out in the students’ presence and using molecular models to probe thinking about which bonds break and form.
Play ‘True or false’. Students draw an element card each and say whether a covalent bond or ionic bond would form.
Write down a few concept terms on the board so that students can use them to setup a concept map. Compare this concept map with one made after they have studied the topic.
ICT: Research the boiling points of the hydrides of the Group 16 elements.
ICT: Research the anomalous properties of water.
Plenary or revision activity. Students could make a poster on shapes to consolidate their knowledge. Various examples e.g. Compound chem, Andy Brunning.
The activity is a practical investigative approach to period 3 elements. All the period 3 elements are on display and the students need to think about the structure and bonding. Clue cards are available in Teacher Resource 2, and answers are given in Teacher Resource 1.
Investigating the relationship between bonding, structure and physical properties of substances. Students would be able to predict the bonding and structure in unknown compounds.
Students build the different molecular shapes using Molymods; Molymod sells a ‘Shapes of Molecules’ box.
Quick 5 minute plenary to check students’ basic understanding. An interactive quiz on ionic and covalent bonding, made by John Ewart, can be accessed by clicking on the link.
Students change the electronegativity of atoms in a molecule to see how it affects polarity. They can see how the molecule behaves in an electric field. They change the bond angle to see how shape affects polarity. They can see how it works for real molecules in 3D. They can see can change the electronegativity of the atoms and see how it effects it polarity.
An interactive tutorial and quiz. This computer course is designed to teach the user the VSEPR rules. Once the user is familiar with the rules, a set of worked examples is available to show how the rules are applied to unfamiliar molecules. Then there is a set of problems in which the user must determine the geometry of a series of molecules which will be randomly selected by the computer.
Students scan each side of the cube with the Augment app (available on both iOS and Android) to reveal the six main molecule shapes. These can then be manipulated to explore the molecule. This is a TES activity from Paul McCormack using augmented reality.
A card sort activity. The cards can be laminated for use as a starter or revision activity. Four types of bonding structures – metallic, giant ionic, giant covalent, simple covalent and pictures, descriptions, properties and example elements/compounds to arrange under the headings.
A four minute animation that describes the properties of water that support life. These properties include solvency, cohesion and adhesion, high surface temperature, high heat capacity, high heat of vaporisation, and varying density.
This resource was originally developed to support the Chemistry B (Salters) specification, but is equally useful for Chemistry A. The activity develops understanding of induced dipole–dipole interactions in the context of organic molecules.
Ice cubes are floated on cooking oil and students observe what happens to the liquid water produced as the ice melts and the density changes.
In an electrolysis experiment, the ions migrate towards electrodes of opposite charge. In this experiment the migration of an intensely coloured purple plume of manganate(VII) ions is seen to move towards the positive terminal.
RSC’s ’Starter for Ten’ pack on bonding.
This resource is designed to provide strategies for dealing with some of the misconceptions that students have, in the form of ready-to-use classroom resources.
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