Edexcel International A Level Chemistry
Topic 3: Bonding and Structure
How atoms hold together, and how the type of bonding and the shape of the particles explain melting temperatures, conduction, hardness, polarity and the shapes of molecules.
The one idea
Every property in this topic comes from two questions: which particles are there, and what attracts them to each other?
There are three kinds of strong bonding. In ionic bonding, oppositely charged ions attract each other. In covalent bonding, two nuclei attract a shared pair of electrons. In metallic bonding, metal ions attract delocalised electrons. Whether a solid conducts depends on whether charged particles can move. Whether it melts easily depends on how strong the attraction is. A molecule's shape depends on how electron pairs repel, and its polarity depends on its shape and on electronegativity.
You cannot see a bond, so the topic is built on evidence: physical properties, electron density maps, migration of coloured ions, and a jet of liquid bending towards a charged rod.
| Question | Pages |
|---|---|
| How do ions form, and what is an ionic lattice? | 1 and 2 |
| How do we know ions exist? | 3 and 4 |
| What makes ionic bonding stronger, and when is it not purely ionic? | 5 and 6 |
| What is a covalent bond, and how do we draw it? | 7 and 8 |
| What are the giant covalent structures of carbon? | 9 |
| How do bonds lie between covalent and ionic? | 10 |
| What shape is a molecule? | 11 to 13 |
| Is a molecule polar? | 14 |
| What is metallic bonding? | 15 |
How to use these notes
- Read the pages in order the first time. The page on shapes needs the counting of pairs from the covalent pages, and the page on polar molecules needs both the shapes and the electronegativity pages.
- Each page opens with its one idea in a sentence. If you can explain that sentence to someone else, you know the page.
- Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
- Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
- Use the topic on a page the night before an exam.
The two marked practicals of the topic, the migration of ions and the jets of liquid and a charged rod, are on page 4 and page 14. The specification lists them without a code number under 3.1 and 3.15, so each is tagged on the page under that code. Know the method, the observation and the conclusion for each.
The most heavily marked skills here are definitions (each one has exact wording), explanations that give the reason as well as the observation, and dot-and-cross diagrams with every outer electron shown. For shapes, always give the shape and the bond angle.
Electronegativity values are on the Pauling scale. Check them against the data booklet you use, because editions can differ in the last digit. Ionic radii are in picometres (1 pm = 10⁻¹² m).
Pages
- Ions and electron transfer
Metals lose electrons to become positive ions, non-metals gain electrons to become negative ions, and a dot-and-cross diagram shows where every electron has gone.
- Giant ionic lattices and ionic bonding
Ions do not pair up. They pack into a giant lattice, held by the net electrostatic attraction between oppositely charged ions in every direction.
- Evidence for ions
Nobody has seen an ion, so the case rests on evidence. Two kinds are on this page, the physical properties of ionic compounds and electron density maps. The migration of ions is on the next page.
- Migration of ions
Coloured ions move towards the electrode of opposite charge. Two practicals show it, potassium manganate(VII) on filter paper and copper(II) chromate(VI) in a U-tube. Learn the observation and the reason for each.
- Ionic radius and bond strength
Charge and radius decide how strongly ions attract each other. Radii follow two trends, shells down a group and nuclear charge across a set of isoelectronic ions. Count the electrons first, and the right argument follows.
- Polarisation
A small, highly charged cation can pull on the electron cloud of a large, highly charged anion. The anion is distorted, some electron density lies between the ions, and the bond gains covalent character. Keep polarising power and polarisability apart.
- Covalent bonds and dot-and-cross diagrams
A covalent bond is the electrostatic attraction between two nuclei and the shared pair between them. Learn the definition, the electron density map that supports it, and how to draw single, double and triple bonds.
- Dative covalent bonds
In a dative bond both electrons of the shared pair come from the same atom. The ammonium ion and Al2Cl6 are the two examples you must be able to draw. Look for an atom with a lone pair next to an atom that is short of electrons.
- Giant covalent structures and carbon
Giant atomic structures have very high melting temperatures because every atom is held by strong covalent bonds. Diamond, graphite and graphene are built from the same element with different bonding patterns, which gives different properties and uses.
- Electronegativity and bond polarity
Electronegativity is the pull an atom has on the bonding pair in a covalent bond. Equal pull gives a non-polar bond, unequal pull gives a polar bond, and a very large difference gives ionic bonding. Ionic and covalent are the two ends of one scale.
- Electron-pair repulsion theory
Electron pairs round a central atom repel and move as far apart as they can. That one idea fixes every shape. This page gives the theory, the two definitions you must know, and a counting method that works every time.
- Shapes with no lone pairs
When every pair on the central atom is a bonding pair, the shape depends only on the number of regions, from two (linear) to six (octahedral). Learn the five shapes in this group, then use the counting method to predict new ones.
- Lone pairs and the shapes of NH3 and H2O
Methane, ammonia and water all have four electron pairs. They have different shapes and angles because lone pairs repel more than bonding pairs. This is the most common explanation question in the topic.
- Polar bonds and polar molecules
A bond can be polar while the molecule is not. A molecule is polar only if it has polar bonds and their dipoles do not cancel, so the shape decides. A jet of liquid near a charged rod tests this in the laboratory.
- Metallic bonding
Metals are lattices of positive ions in a sea of delocalised electrons. The attraction between them is the bond. The model explains why metals conduct when solid, why melting temperatures are high, and why a metal bends where an ionic crystal shatters.
Also in this topic: Topic 3 on a page
Spec checklist
Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.
| Spec | You should be able to | Where |
|---|---|---|
| 3.1 | know and be able to interpret evidence for the existence of ions, limited to physical properties of ionic compounds, electron density maps and the migration of ions | What counts as evidence Physical properties Electron density maps Why a colour can show a charge Practical 1: potassium manganate(VII) on filter paper Practical 2: copper(II) chromate(VI) in a U-tube |
| 3.2 | be able to describe the formation of ions in terms of loss or gain of electrons | How ions form |
| 3.3 | be able to draw dot-and-cross diagrams to show electrons in cations and anions | Dot-and-cross diagrams for ions |
| 3.4 | be able to describe ionic crystals as giant lattices of ions | Ionic crystals are giant lattices |
| 3.5 | know that ionic bonding is the result of strong net electrostatic attraction between ions | Ionic bonding Three types of strong bonding |
| 3.6 | understand the effects of ionic radius and ionic charge on the strength of ionic bonding | What makes ionic bonding stronger |
| 3.7 | understand reasons for the trends in ionic radii down a group in the Periodic Table, and for a set of isoelectronic ions, including N³⁻ to Al³⁺ | Ionic radius down a group Isoelectronic ions |
| 3.8 | understand the meaning of the term 'polarisation' as applied to ions | What polarisation means |
| 3.9 | understand that the polarising power of a cation depends on its radius and charge, and the polarisability of an anion also depends on its radius and charge | Polarising power and polarisability |
| 3.10 | understand that covalent bonding is the strong electrostatic attraction between two nuclei and the shared pair of electrons between them, based on the evidence: (i) the physical properties of giant atomic structures (ii) electron density maps for simple molecules | What a covalent bond is Electron density map of hydrogen Giant atomic structures as evidence Three types of strong bonding |
| 3.10(i) | (i) the physical properties of giant atomic structures | Giant atomic structures as evidence |
| 3.10(ii) | (ii) electron density maps for simple molecules | What a covalent bond is Electron density map of hydrogen |
| 3.11 | be able to draw dot-and-cross diagrams to show electrons in covalent substances, including: (i) molecules with single, double and triple bonds (ii) species with dative covalent (coordinate) bonds, including Al₂Cl₆ and the ammonium ion | Dot-and-cross diagrams: single bonds Dot-and-cross diagrams: double and triple bonds What a dative bond is The ammonium ion Al₂Cl₆ |
| 3.11(i) | (i) molecules with single, double and triple bonds | Dot-and-cross diagrams: single bonds Dot-and-cross diagrams: double and triple bonds |
| 3.11(ii) | (ii) species with dative covalent (coordinate) bonds, including Al₂Cl₆ and the ammonium ion | What a dative bond is The ammonium ion Al₂Cl₆ |
| 3.12 | be able to describe the different structures formed by giant lattices of carbon atoms, including graphite, diamond and graphene, and discuss the applications of each | Diamond Graphite Graphene Comparing the three |
| 3.13 | understand the meaning of the term 'electronegativity' as applied to atoms in a covalent bond | Electronegativity |
| 3.14 | know that ionic and covalent bonding are the extremes of a continuum of bonding type and be able to explain this in terms of electronegativity differences, leading to bond polarity in bonds and molecules, and to ionic bonding if the electronegativity is large enough | A continuum from covalent to ionic |
| 3.15 | be able to distinguish between polar bonds and polar molecules and predict whether or not a given molecule is likely to be polar | Polar bonds are not the same as polar molecules Practical: jets of liquid and a charged rod |
| 3.16 | understand the principles of the electron-pair repulsion theory, used to interpret and predict the shapes of simple molecules and ions | The theory Counting the electron pairs Same four pairs, three different shapes Why the angle goes down |
| 3.17 | understand the terms 'bond length' and 'bond angle' | Bond length and bond angle |
| 3.18 | know and be able to explain the shapes of, and bond angles in, BeCl₂, BCl₃, CH₄, NH₃, NH₄⁺, H₂O, CO₂, gaseous PCl₅, SF₆ and C₂H₄ | Two and three regions Four regions: tetrahedral Five and six regions Same four pairs, three different shapes Why the angle goes down |
| 3.19 | be able to apply the electron-pair repulsion theory to predict the shapes of, and bond angles in, molecules and ions analogous to those in 3.18 | Counting the electron pairs Predicting by analogy Predicting by analogy |
| 3.20 | understand that metals consist of giant lattices of metal ions in a sea of delocalised electrons | The metallic model |
| 3.21 | know that metallic bonding is the strong electrostatic attraction between metal ions and the delocalised electrons | The metallic model Three types of strong bonding |
| 3.22 | be able to use the models in 3.20 and 3.21 to interpret simple properties of metals, including electrical conductivity and high melting temperature | Using the model |