The Ionic Lattice
Quick Notes
- Ionic solids are made of cations and anions arranged in a repeating 3-D lattice structure.
- The arrangement maximizes attraction between opposite charges and minimizes repulsion between like charges.
- Coulomb’s Law explains the strength of ionic interactions:
- Larger charges give stronger attraction
- Smaller ions give stronger attraction (shorter distance between nuclei)
- The structure is rigid, stable, and strong due to the repeating pattern of alternating charges.
Full Notes
Ionic compounds, like NaCl or MgO, exist as solid lattices – a highly ordered structure of positive and negative ions. This structure results from strong electrostatic forces between oppositely charged particles.
Ionic Lattice Structure
In an ionic solid, cations and anions are arranged in a regular 3-dimensional grid.
The goal of this arrangement is to:
- Maximize attractive forces between opposite charges (cation–anion)
- Minimize repulsive forces between like charges (cation–cation or anion–anion)
Each ion is surrounded by multiple oppositely charged ions, leading to strong net attractions throughout the crystal.
Note – although the specific crystal structure of compounds (e.g., cubic, hexagonal) is not assessed on the AP exam, the concept of alternating ions in a repeating pattern is essential knowledge.
Coulomb’s Law and Lattice Stability
Coulomb’s Law:
F ∝ (q1 × q2) / r2
F = force of attraction between ions
q1 and q2 = charges on the ions
r = distance between ion centers
This means that stronger ionic bonds occur when:
- Ion charges are larger (e.g., 2⁺ and 2⁻ instead of 1⁺ and 1⁻)
- Ions are smaller, so their nuclei are closer together
These factors result in a greater electrostatic force, which makes the lattice more stable and gives the compound a higher melting point.
Properties of Ionic Lattices
The structure of the ionic lattice explains many macroscopic properties of ionic solids:
- High melting and boiling points – due to strong electrostatic forces throughout the lattice
- Brittleness – when layers shift, like charges can align and repel, causing the crystal to break
- Electrical conductivity:
- Solid state → non-conductive (ions fixed in place)
- Molten or dissolved in water → conductive (ions free to move)
Particulate Model Representation
In a particulate model of an ionic lattice:
- Positive and negative ions alternate in a regular 3D arrangement
- Ions are shown as spheres, sized approximately according to ionic radius
- Spacing reflects relative distances and strength of attractions
Sometimes ionic lattice structures are shown as unit cells – with ions being placed in corners of a cube to represent how the ions are arranged. In these diagrams, the ions are often shown as points or small spheres with spaces between them. Remember that in reality, ions are touching and are packed as tightly together as possible.
Which compound has a stronger ionic lattice: NaCl or MgO?
- Compare charges: NaCl → Na⁺ and Cl⁻ (±1); MgO → Mg²⁺ and O²⁻ (±2).
- Apply Coulomb’s Law: higher charges and smaller ionic radii increase attraction.
- Assess size: Mg²⁺ and O²⁻ are smaller than Na⁺ and Cl⁻, so r is smaller.
Answer: MgO has a stronger lattice, higher melting point, and requires more energy to break.
Summary
- Ionic solids form 3D lattices of alternating cations and anions arranged to maximize attraction and minimize repulsion.
- Coulomb’s Law: higher charges and smaller ions create stronger ionic bonds.
- Typical properties: high melting points, brittleness, and electrical conductivity when molten or dissolved.
- Visual models should show alternating ions, relative sizes, and close packing.