Ionic compounds — AQA GCSE Chemistry
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Ionic compounds explained
Sodium chloride has a giant ionic lattice that can be represented in several ways.
Read the full explanation
A dot and cross diagram shows the transfer of electrons from sodium to chlorine, forming Na⁺ and Cl⁻ ions, but it usually shows only one formula unit. A ball and stick model uses spheres for ions and sticks for bonds, making the three-dimensional arrangement clear but implying that ions are joined by directional bonds. A two-dimensional diagram shows a flat cross-section of the lattice, which is easy to draw but does not show the full three-dimensional arrangement. A three-dimensional model shows the true arrangement of ions in space, with each Na⁺ surrounded by six Cl⁻ ions and vice versa. Each representation has limitations, so chemists choose the one that best communicates the feature they want to show.
Your focus
- Identify the different ways sodium chloride can be represented.
- Describe the limitations of each representation.
- Choose an appropriate representation for a given purpose.
Ionic compounds exam tips
Marking Points
- Dot and cross diagrams show electron transfer and the resulting ions, but often only for one formula unit.
- Ball and stick models show the three-dimensional arrangement but can imply that ions are joined by directional covalent bonds.
- Two-dimensional diagrams are simple to draw but do not show the full three-dimensional lattice.
- Three-dimensional models show the true spatial arrangement of ions in the lattice.
- Each representation has limitations, so the choice depends on what feature is being emphasised.
Examiner Tips
- 💡When asked about limitations, state what the representation shows well and what it fails to show.
- 💡Use the correct terminology: dot and cross, ball and stick, two-dimensional, three-dimensional.
- 💡Link the representation to the feature it best illustrates, such as electron transfer or spatial arrangement.
Common Mistakes
- Thinking that a dot and cross diagram shows the whole lattice; correct this by stating it usually shows only one formula unit.
- Believing that sticks in a ball and stick model represent covalent bonds; correct this by explaining they are just a way to show the arrangement of ions.
- Assuming a two-dimensional diagram shows the full three-dimensional structure; correct this by stating it is a flat cross-section.