Ionic bonding and ionic latticesEdexcel International A Level Chemistry: Revision notes
Section 1
Evidence for the existence of ions
Ionic compounds contain ions: charged particles formed when atoms lose or gain electrons. Three kinds of evidence support this.
- Physical properties. Ionic compounds have high melting and boiling points, are hard but brittle, do not conduct when solid, and conduct when molten or dissolved. This fits a lattice of charged particles that are fixed in the solid but free to move in the liquid.
- Migration of ions. When a crystal of a coloured compound such as potassium manganate(VII) is placed on damp filter paper between electrodes, the purple MnO₄⁻ ion moves towards the positive electrode. Only charged particles move in an electric field.
- Electron density maps. X-ray diffraction shows electron density concentrated in separate, almost circular regions around each nucleus, falling to almost zero between them. The electrons belong to individual ions and are not shared.
Ionic solids do not conduct because their ions cannot move, not because they have no charged particles. Never say the electrons are free to move in molten sodium chloride.
Section 2
Forming ions and dot-and-cross diagrams
Metals lose their outer electrons to form cations; non-metals gain electrons to form anions. Atoms usually end up with a full outer shell.
- Na → Na⁺ + e⁻ (2,8,1 → 2,8)
- Cl + e⁻ → Cl⁻ (2,8,7 → 2,8,8)
- Ca → Ca²⁺ + 2e⁻ and O + 2e⁻ → O²⁻
In a dot-and-cross diagram the outer electrons of one atom are drawn as dots and those of the other as crosses. After transfer, each ion is drawn in square brackets with its charge, showing a full outer shell. The formula comes from balancing charges: Ca²⁺ with 2F⁻ gives CaF₂.
In a dot-and-cross diagram for an ionic compound, show only the outer shell, put each ion in brackets and write its charge outside the bracket.
Section 3
Giant ionic lattices and the bonding
An ionic crystal is a giant lattice: a regular three-dimensional arrangement in which each ion is surrounded by ions of opposite charge. It is not made of separate molecules.
Ionic bonding is the strong net electrostatic attraction between oppositely charged ions throughout the lattice. Each ion attracts all the oppositely charged ions around it and repels like-charged ions, but the attractions outweigh the repulsions, so the net effect is strong bonding. Large amounts of energy are needed to separate the ions, so melting points are high.
Do not describe ionic bonding as the attraction between one pair of ions, or as the transfer of electrons. Transfer forms the ions; the bonding is the attraction between them throughout the lattice.
Section 4
Charge and radius: the strength of ionic bonding
The attraction between two ions increases as their charges increase and as the distance between their centres decreases.
- Higher charge gives a stronger attraction. MgO (2+ and 2−) melts at 2852 °C, whereas NaCl (1+ and 1−) melts at 801 °C.
- Smaller ions can approach more closely, so the attraction is stronger. LiF melts at a higher temperature than NaCl, which melts higher than KBr.
When comparing compounds, state the charges first, then the radii, then link both to the strength of the attraction and the energy needed to overcome it.
Section 5
Trends in ionic radii
Down a group the ions have more electron shells. The outer electrons are further from the nucleus and more shielded, so the radius increases: Li⁺ < Na⁺ < K⁺ and F⁻ < Cl⁻ < Br⁻.
Isoelectronic ions have the same number of electrons and the same configuration. N³⁻, O²⁻, F⁻, Na⁺, Mg²⁺ and Al³⁺ each have 10 electrons (1s²2s²2p⁶). The number of protons rises from 7 to 13, so the same electrons are pulled in more strongly and the radius decreases from N³⁻ to Al³⁺.
For isoelectronic ions the shells and shielding are identical, so the only variable is the nuclear charge. Say this explicitly in your answer.
Must know
- Evidence for ions: physical properties, migration towards an electrode, electron density maps
- Ions form by loss or gain of electrons; show full outer shells in dot-and-cross diagrams
- Ionic bonding is the strong net electrostatic attraction between oppositely charged ions in a giant lattice
- Stronger bonding with higher charge and smaller radius
- Radius increases down a group; radius decreases across N³⁻ to Al³⁺ because nuclear charge increases
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ionic bonding and ionic lattices
- A student lays a strip of filter paper, soaked in aqueous sodium nitrate, between two electrodes connected to a 20 V d.c. supply. A single crystal of potassium manganate(VII), KMnO₄, is placed in the middle of the strip. After 20 minutes a purple band has spread along the paper towards the electrode connected to the positive terminal, while the rest of the paper remains colourless.Explain why this experiment provides evidence that potassium manganate(VII) contains ions.2 marks
- Electron density maps of sodium chloride, made using X-ray diffraction, show contour lines of electron density. The contours form separate, almost circular patterns around the sodium and chlorine nuclei, and the electron density falls to almost zero in the region between the two kinds of ion.Explain what the electron density map shows about the bonding in sodium chloride.2 marks
- Calcium fluoride, CaF₂, occurs naturally as the mineral fluorite and is used to make windows for infrared spectrometers. Calcium has atomic number 20 and fluorine has atomic number 9. Solid calcium fluoride does not conduct electricity, but it does conduct when molten, and its melting point is 1418 °C.Describe, in terms of electrons, how calcium atoms and fluorine atoms form calcium fluoride.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).