Topic 3 · page 3 of 15

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.

12 min read3.1

Read first: Giant ionic lattices and ionic bonding

What counts as evidence 3.1

The model of an ionic compound says it is a lattice of positive and negative ions. A model is only worth believing if it explains what we observe. The specification names three kinds of evidence for ions:

  1. the physical properties of ionic compounds (this page),
  2. electron density maps (this page),
  3. the migration of ions in an electric field (next page).

In an exam you are asked to do two things with each: say what is observed, and say what that shows about the particles.

Physical properties 3.1

Property What it shows about the particles
High melting temperature (NaCl melts at 801 °C) There are many strong electrostatic attractions between the ions, and a lot of energy is needed to overcome them.
Does not conduct when solid The ions are held in fixed positions in the lattice, so they cannot move to carry charge.
Conducts when molten or dissolved in water The ions are free to move, so they carry charge through the liquid.
Brittle: shatters when hit A layer of ions shifts, ions of the same charge come face to face, and they repel.

The pattern for conduction is the strongest evidence. A substance made of neutral particles cannot conduct, and a solid of fixed charged particles cannot either. A substance that conducts only when its particles are free to move is made of charged particles.

Two rows of ions. Before a blow, positive ions sit above negative ions. After the top row slips by one ion, positive ions sit above positive ions and negative above negative, with arrows showing repulsion.
Why an ionic crystal is brittle. When the layers slip by one ion, like charges meet and repel, and the crystal splits.

Electron density maps 3.1

X-ray diffraction can measure how the electron density varies through a crystal. The result is drawn as a map. Each contour line joins points of equal electron density, in the same way that contour lines on a hill walker's map join points of equal height. Where the lines form closed rings close together, the density is high.

Electron density map of a slice through sodium chloride. Each ion is surrounded by nearly circular contour rings, bigger around chloride than sodium, and no contours join one ion to the next.
A model of the electron density in a slice through sodium chloride. This is a model, not measured data.

Three features of the sodium chloride map are the evidence for ions:

Check yourself

  1. Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does.
  2. Explain why an ionic crystal shatters when it is hit.
  3. Explain why sodium chloride has a high melting temperature.
  4. A student writes that molten magnesium oxide conducts "because of free electrons". Correct the answer.
  5. Describe two features of an electron density map that show a compound is ionic.
  6. Magnesium oxide does not conduct when solid but conducts when molten. State what this shows, and say how the electron density map of a covalent solid would differ.

Answers cover these until you have tried the questions

  1. In the solid, the ions are held in fixed positions in the lattice, so they cannot move to carry charge. When molten, the lattice has broken down and the ions are free to move, so they can carry charge through the liquid.
  2. A blow moves one layer of ions along by one position. Ions with the same charge are then next to each other and repel, so the crystal splits along that plane.
  3. The lattice is held together by many strong electrostatic attractions between oppositely charged ions. A large amount of energy is needed to overcome them and separate the ions, so the melting temperature is high.
  4. Conduction in molten magnesium oxide is by the movement of ions (Mg²⁺ and O²⁻), which are free to move in the liquid. It is not by electrons.
  5. Two from: the electron density falls to almost zero between the ions; the contours around each ion are nearly circular, so the ions are separate spheres of charge; the contours do not join neighbouring ions.
  6. It shows that the solid contains charged particles which are fixed in the solid and mobile in the liquid, which is what ions in a lattice do. On the map of a covalent solid the contours enclose both nuclei and the electron density between the nuclei is high.