2.1 The ionic model
- Syllabus
- First assessment 2025
- Topic
- 2.1
- Level
- SL
Learning objectives
2.1.1Ion formation• Metal atoms lose electrons → cations (+)• Non-metal atoms gain electrons → anions (−)• Predict ion charges from electron configurations2.1.2Ionic bonding• Electrostatic attraction between oppositely charged ions• Binary ionic compounds: cation first, anion with "-ide" suffix• Deduce formulas and names of ionic compounds, including common polyatomic ions2.1.3Ionic compounds• 3D lattice structures, empirical formulas• Properties: volatility, electrical conductivity, solubility• Explain properties using lattice structure and ion mobility
Forming Ions
| Atom tendency | Electron change | Ion formed |
|---|---|---|
| Metal atom | Loses electrons | Positive cation |
| Non-metal atom | Gains electrons | Negative anion |
Use the electron configuration to see how many electrons are needed to reach the relevant stable arrangement. The number lost or gained determines the magnitude of the ion charge.
Loss of electrons leaves more protons than electrons and gives a positive charge. Gain of electrons gives more electrons than protons and gives a negative charge.
Read the outer-shell electrons before predicting charge: Al loses three electrons to form Al³⁺, while O gains two to form O²⁻. Check the sign by recounting protons and electrons after transfer; do not assume that every metal, especially a transition metal, has only one possible charge.
Ionic Bonding and Formulae
Ionic bonding is the electrostatic attraction between oppositely charged cations and anions. Electron transfer can create the ions, but it is not by itself the definition of the bond.
| Task | Check |
|---|---|
| Formula | Choose subscripts so total positive and negative charge is zero |
| Name | State the cation first, then the anion; binary anions use the -ide ending |
| Polyatomic ion | Keep the ion together and use brackets when more than one is needed |
For a metal ion and a polyatomic ion, balance the charges rather than copying the numerical charge into a subscript without checking the whole formula.
Balance total charge, not ion numbers. Al³⁺ and O²⁻ require 2(+3) + 3(−2) = 0, giving Al₂O₃; Ca²⁺ and NO₃⁻ give Ca(NO₃)₂. These formulae state the simplest ion ratio in a lattice, not the composition of one molecule.
Ionic Lattices: Structure, Strength and Conductivity
An ionic lattice is a three-dimensional, repeating arrangement of cations and anions. Its empirical formula gives the simplest whole-number ion ratio, not a molecule.
Lattice dissociation enthalpy is the positive enthalpy change for separating one mole of a solid lattice into gaseous ions. It becomes larger when ionic charges are higher or ionic radii are smaller, because the electrostatic attraction is stronger.
| Property | Structure-based explanation |
|---|---|
| High melting point / low volatility | Strong electrostatic attractions act throughout the lattice |
| Brittle | A layer shift can bring like charges together, causing repulsion and fracture |
| Solid conductivity | Ions are fixed and cannot carry charge through the solid |
| Molten/aqueous conductivity | Ions are mobile and can carry charge |
| Solubility | Depends on the balance between lattice attraction and ion–solvent attraction |
Use charge density to compare lattice strength: MgO has stronger attractions than NaCl because both ions carry ±2 rather than ±1, so its melting point is higher. For conductivity, the presence of charged particles is not enough—solid NaCl does not conduct until its ions can move. Water often hydrates ions, but solubility still depends on the energy balance rather than on polarity alone.