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2.2.3Interactions Over Small and Large Distances

Properties Of Ionic Compounds — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions

5 min read 9 parts Lesson 15 of 26

📚 AQA GCSE Combined Science Synergy 8465 - Physical Sciences Hub / Topical Revision
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Part 1 of 7Structure

Giant ionic lattice
  • Giant ionic lattice Ions are arranged in a regular, repeating three-dimensional pattern.
Strong electrostatic forces
  • Strong electrostatic forces Each ion is attracted to many oppositely charged neighbours in all directions.
  • These forces act throughout the whole crystal, not just between pairs.
Example
  • Example Sodium chloride has Na+ and Cl- ions alternating throughout the crystal, which is why salt crystals are cube-shaped.

Part 2 of 7Properties and explanations

PropertyExplanation
High melting and boiling pointsLarge amounts of energy are required to overcome the many strong electrostatic forces
Hard but brittleA blow shifts layers so that like charges line up and repel, splitting the crystal
Do not conduct when solidIons are held in fixed positions and cannot move
Conduct when molten or dissolvedIons become free to move and carry charge
Often soluble in waterWater molecules attract and separate the ions

Part 3 of 7Why molten ionic compounds conduct

1Heated above melting point
  • Heated above melting point The lattice breaks down and ions can move.
2Electrodes connected
  • Electrodes connected Positive ions move to the negative electrode, and negative ions to the positive one.
3Current flows
  • Current flows The movement of charged ions carries the current.

Part 4 of 7Comparing melting points

CompoundIonsMelting pointReason
Sodium chlorideNa+, Cl-About 801 °CStrong attraction between 1+ and 1- ions
Magnesium oxideMg2+, O2-About 2850 °CStronger attraction between 2+ and 2- ions needs far more energy to overcome
Potassium chlorideK+, Cl-About 770 °CLarger K+ ion, so slightly weaker attraction

Part 5 of 7Answering exam questions

1
Name the particles, which are ions, not atoms or molecules.
2
Say whether they are free to move or fixed in place.
3
Mention the strong electrostatic forces between oppositely charged ions.
4
Link the property to that explanation clearly, such as 'much energy needed to overcome the forces'.
5
For conductivity, always state the state of the compound: solid, molten or dissolved.

Part 6 of 7Applications and evidence

Ionic compounds in use
Electrolysis
  • Electrolysis Molten or dissolved ionic compounds can be broken down by passing electricity through them, because the free ions move to the electrodes.
Testing a substance
  • Testing a substance A solid that does not conduct but does when melted, and has a high melting point, is probably ionic.
Dissolving in water
  • Dissolving in water Salt dissolves in water, so ions are free to carry charge in the solution.
Solubility and conductivity together
  • Solubility and conductivity together An ionic compound that dissolves in water conducts as a solution,
    • so an insoluble one such as calcium carbonate can only conduct if it is molten,
    • which is difficult because of its very high melting point.
Comparison with metals
  • Comparison with metals Metals conduct when solid because delocalised electrons carry the current,
    • but solid ionic compounds do not,
    • which is a useful way to tell the two apart.

Part 7 of 7Dot and cross diagram for sodium chloride

1Draw the atoms
  • Draw the atoms Show Na with one electron in its outer shell and Cl with seven, using dots for one element and crosses for the other.
2Show the transfer
  • Show the transfer Draw an arrow for the single outer electron moving from sodium to chlorine.
3Draw the ions
  • Draw the ions Place each ion in square brackets with its charge, Na+ with eight electrons in its second shell and Cl- with eight in its third.

Key terms

TermMeaning
Giant ionic latticeA regular arrangement of oppositely charged ions held by electrostatic forces.
Electrostatic forceThe attraction between oppositely charged particles.
MoltenIn the liquid state after melting.
BrittleBreaking or shattering when hit rather than bending.
ElectrolyteA molten or dissolved ionic compound that conducts electricity.

Avoid these mistakes

1
Saying electrons carry current in molten ionic compounds; it is ions that move.
2
Writing that covalent bonds are broken when an ionic solid melts.
3
Explaining the high melting point by 'strong bonds between molecules'.
4
Stating that solid ionic compounds conduct because they contain charged particles, without saying the ions are fixed.
5
Saying ionic compounds are brittle because the ions are weakly attracted, when the attraction is strong.

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