Giant ionic lattices: formulae and model limits
| English | Français |
|---|---|
| ionic lattice/aɪˈɒnɪk ˈlætɪs/ | réseau ionique |
| empirical formula/emˈpɪrɪkl ˈfɔːmjʊlə/ | formule empirique |
What would explain this observation?
- A sodium chloride crystal contains a repeating arrangement of ions. The formula NaCl records the simplest ratio, not a single isolated pair joined to form a molecule.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- An ionic compound is a giant lattice of oppositely charged ions held by strong electrostatic forces acting in all directions. In sodium chloride the Na⁺:Cl⁻ ratio is 1:1. A two-dimensional slice can show alternating charges, while a three-dimensional model shows the repeating arrangement beyond one plane. An interior ion in the sodium chloride structure has six nearest neighbours of opposite charge; an edge drawn on a finite model is a cut through the continuing crystal.
- ionic lattice 离子晶格: A repeating giant arrangement of ions held by electrostatic attraction; empirical formula 最简式: The simplest whole-number ratio of atoms or ions in a compound.
What does NaCl specify for the giant structure?
To infer an empirical formula, count ions in a stated representative region and reduce the ratio to the smallest whole numbers. A model with six Mg²⁺ and twelve Cl⁻ gives MgCl₂ after reducing 6:12 to 1:2. Counts in an arbitrary boundary fragment need not themselves give the bulk ratio, so use the representative information or account for shared positions as specified. Knowledge of other named ionic crystal structures is not required.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- To infer an empirical formula, count ions in a stated representative region and reduce the ratio to the smallest whole numbers. A model with six Mg²⁺ and twelve Cl⁻ gives MgCl₂ after reducing 6:12 to 1:2. Counts in an arbitrary boundary fragment need not themselves give the bulk ratio, so use the representative information or account for shared positions as specified. Knowledge of other named ionic crystal structures is not required.
- Compare dot-and-cross, ball-and-stick and space-filling representations. Dot/cross shows electron origin and charge but not the full lattice geometry. Ball/stick makes an arrangement clear but rods are not physical ionic bonds and spaces/radii may be misleading. A 2D slice omits neighbours above and below; a 3D model can hide ions behind others. State the particular useful feature and limitation of each model.
Which two habits make the investigation or model in this case more defensible?
Compare dot-and-cross, ball-and-stick and space-filling representations. Dot/cross shows electron origin and charge but not the full lattice geometry. Ball/stick makes an arrangement clear but rods are not physical ionic bonds and spaces/radii may be misleading. A 2D slice omits neighbours above and below; a 3D model can hide ions behind others. State the particular useful feature and limitation of each model.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a supplied representative diagram contains four Al³⁺ and six O²⁻ ions. The ratio 4:6 simplifies to 2:3, so its empirical formula is Al₂O₃. Charge check: 2(+3)+3(−2)=0. This ratio deduction does not require knowing the actual geometry of the aluminium oxide crystal.
A representative region contains 10 magnesium ions and 20 chloride ions. After reducing to one magnesium, how many chloride ions remain in the ratio? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A representative region contains 10 magnesium ions and 20 chloride ions. After reducing to one magnesium, how many chloride ions remain in the ratio?
The result is 2 . Known: a supplied representative diagram contains four Al³⁺ and six O²⁻ ions. The ratio 4:6 simplifies to 2:3, so its empirical formula is Al₂O₃. Charge check: 2(+3)+3(−2)=0. This ratio deduction does not require knowing the actual geometry of the aluminium oxide crystal.
Check the conclusion and its limits
- Do not count sticks as electron pairs in an ionic ball-and-stick model. Ions attract several neighbours rather than only the one from which an electron came. A simple lattice slice is a representation of a three-dimensional giant structure, not evidence that the real crystal is a flat sheet.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A sodium ion attracts only the chloride ion that accepted its electron. This claim is false: Do not count sticks as electron pairs in an ionic ball-and-stick model. Ions attract several neighbours rather than only the one from which an electron came. A simple lattice slice is a representation of a three-dimensional giant structure, not evidence that the real crystal is a flat sheet.
Giant ionic lattices: formulae and model limits: To infer an empirical formula, count ions in a stated representative region and reduce the ratio to the smallest whole numbers. A model with six Mg²⁺ and twelve Cl⁻ gives MgCl₂ after reducing 6:12 to 1:2. Counts in an arbitrary boundary fragment need not themselves give the bulk ratio, so use the representative information or account for shared positions as specified. Knowledge of other named ionic crystal structures is not required.
A sodium ion attracts only the chloride ion that accepted its electron.
Do not count sticks as electron pairs in an ionic ball-and-stick model. Ions attract several neighbours rather than only the one from which an electron came. A simple lattice slice is a representation of a three-dimensional giant structure, not evidence that the real crystal is a flat sheet.
A repeating giant arrangement of ions held by electrostatic attraction: write the technical term.
ionic lattice means A repeating giant arrangement of ions held by electrostatic attraction.