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1.6.1Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Evidence of Ions — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 17 of 36 Unit quiz

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Part 1 of 5What Is an Ionic Compound

1.6.1 Evidence of Ions

Chemists cannot see ions directly, so the model is supported by several lines of evidence.

Ions
  • Atoms that have gained or lost electrons so that they carry a charge, such as Na⁺ and Cl⁻.
Ionic bond
  • The electrostatic attraction between oppositely charged ions acting in all directions.
Lattice
  • Ions are arranged in a regular repeating pattern called a giant ionic lattice in which each ion is surrounded by others of opposite charge.
Crystals
  • Regular shapes of ionic crystals, such as cubes of sodium chloride, reflect the regular arrangement inside.

Part 2 of 5Electron Density Maps

Electron Density Maps
What they show
  • Contour lines join points of equal electron density, so closely packed rings show where electrons are most likely to be found.
Around each ion
  • For sodium chloride the contours form separate circles around each nucleus, showing electrons stay with their own ion.
Between ions
  • The density falls to almost zero between a Na⁺ and a Cl⁻ ion, so no electrons are shared as they would be in a covalent bond.
Sizes
  • The maps show the chloride ion as larger than the sodium ion, in agreement with measured ionic radii.

Part 3 of 5Electrical and Migration Evidence

1
Solid ionic compounds do not conduct electricity because the ions are fixed in position in the lattice.
2
When melted or dissolved in water the ions are free to move and carry charge, so the liquid or solution conducts.
3
During electrolysis of copper(II) chloride solution, copper forms at the negative electrode and chlorine at the positive electrode.
4
This shows Cu²⁺ ions move to the cathode and Cl⁻ ions move to the anode, confirming opposite charges.
5
In a migration experiment, a coloured salt on damp filter paper between electrodes splits into two coloured patches moving in opposite directions.
6
A blue patch of Cu²⁺ ions travels towards the negative electrode while a yellow chromate ion patch travels towards the positive electrode.

Part 4 of 5Physical Properties as Evidence

Physical Properties as Evidence
High melting point
  • A lot of energy is needed to overcome the strong attractions between ions throughout the lattice.
Brittle
  • A blow shifts a layer so like charges meet, and the repulsion splits the crystal.
Solubility
  • Many dissolve in water because polar water molecules attract and surround the ions, releasing hydration energy.
Non-conductor of solid
  • Fixed ions cannot carry current, which supports the idea of separate charged particles rather than mobile electrons.

Part 5 of 5Exam Checklist & Tips

1.6.1 – Key Points for the Exam

  • Ion An atom or group of atoms that has gained or lost electrons and so carries an electrical charge.
  • Electron density map A diagram using contours to show where electrons are most likely to be found.
  • Electrolysis The breakdown of an ionic compound by passing an electric current through it when molten or dissolved.
  • Migration The movement of ions towards the electrode of opposite charge.
  • Lattice A regular repeating three-dimensional arrangement of particles.

Common Mistakes

  • Saying solid sodium chloride conducts because it contains charged ions; the ions cannot move.
  • Thinking ionic compounds contain molecules of NaCl rather than a lattice.
  • Describing electron density minima between ions as a covalent bond.
  • Reversing the direction of migrationpositive ions go to the negative electrode.

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