An ionic compound is one giant lattice, not a set of pairs
Definition
Giant ionic lattice
A regular, repeating three-dimensional arrangement of very large numbers of oppositely charged ions held together by ionic bonds.
- A solid ionic compound is a single enormous structure rather than a collection of separate units.
- Positive and negative ions alternate in a fixed, repeating pattern in three dimensions.
- The pattern continues for as long as the crystal does, so even a visible grain contains a vast number of ions.
- The word giant records that the number of ions is not fixed by the formula.
- The formula gives the ratio of the ions present, not how many of them there are.
Key Idea
- A lattice is a repeating arrangement, so every ion sits in the same relationship to its neighbours.
- The formula is a ratio, so NaCl\text{NaCl}NaCl means one Na+\text{Na}^{+}Na+ for every Cl−\text{Cl}^{-}Cl− rather than a pair of ions.
Every ion is held by attractions acting in all directions
Definition
Ionic bond
The strong electrostatic attraction between oppositely charged ions.
- Each positive ion is surrounded by negative ions, and each negative ion by positive ions.
- In sodium chloride, every ion has six nearest neighbours of the opposite charge.
- The attractions act in every direction through the structure, not along single lines between chosen pairs.
- Breaking the structure apart means overcoming very many strong attractions at once.
- Larger charges pull harder, so the attractions in MgO\text{MgO}MgO are stronger than those in NaCl\text{NaCl}NaCl.
Note
- The attraction acts between oppositely charged ions, which is the detail that turns a vague force into an ionic bond.
- Six nearest neighbours in sodium chloride is a concrete figure worth carrying into a description.
Describing and drawing a lattice
- A full description names both the arrangement and the force.
- The arrangement is a regular, repeating three-dimensional pattern of oppositely charged ions.
- The force is strong electrostatic attraction between those oppositely charged ions.
- Saying only that the forces are strong leaves the description unfinished, because it does not name them.
- A diagram should show many ions in a repeating grid with their charges marked.
- Drawing sodium chloride as two ions side by side describes a molecule, which an ionic compound does not form.
Self review
- What is meant by a giant ionic lattice?
- What holds the ions in the lattice together?
- Why does the formula NaCl\text{NaCl}NaCl not describe a molecule of two ions?
- How many nearest neighbours of opposite charge does each ion have in sodium chloride?
