IB Chemistry HL — Structure 2.1: The Ionic Model

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What charge does a Group 16 (oxygen group) element form?

This is the Higher Level lesson. The core S2.1.1–S2.1.3 (ions, bonding, lattice, properties) is shared with SL; the HL-only content — lattice enthalpy — carries a purple HL badge.

Core — SL & HL

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👆 build formulae · click properties · compare lattice enthalpies

1. Making ions by electron transfer S2.1.1

When a metal reacts with a non-metal, electrons transfer from the metal to the non-metal. The metal becomes a positive cation, the non-metal a negative anion — both reaching a full, stable outer shell. Predict the charge from the group: Group 1 → 1+, Group 2 → 2+, Group 13 → 3+; Group 15 → 3−, Group 16 → 2−, Group 17 → 1−.

Before: neutral atomsSodium has 1 outer electron; chlorine has 7. Toggle to transfer that single electron.

2. Build an ionic formula S2.1.2

An ionic compound is neutral overall, so the positive and negative charges must cancel. Pick a cation and an anion — the builder balances the charges for you (and adds brackets around polyatomic ions when there's more than one).

Ionic formula builder

Cation (+)

Anion (−)

NaClNa⁺ + Cl⁻ → charges balance 1 : 1

The seven polyatomic ions you must know: ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, hydrogencarbonate HCO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, phosphate PO₄³⁻.

3. The giant ionic lattice & properties S2.1.3

Ionic compounds don't exist as molecules — they form a giant 3D lattice of alternating cations and anions, held by strong electrostatic attractions in all directions. We write them with the empirical formula (the simplest ratio). Click a property to see how the lattice explains it.

4. Lattice enthalpy HL

Lattice enthalpy is a measure of the strength of ionic bonding — the energy change when one mole of a solid ionic lattice forms from its gaseous ions. The bigger the value, the stronger the lattice. It follows straight from Coulomb's law, so it depends on two things: the ionic charges (bigger charges → much stronger attraction) and the ionic radii (smaller ions get closer → stronger attraction). Compare the two effects:

Lattice enthalpy comparison · click a bar

Values are approximate (kJ mol⁻¹). Down a group the anion gets bigger, so the lattice enthalpy falls; doubling the charges (Na⁺Cl⁻ → Mg²⁺O²⁻) increases it several-fold.

Learn The Ionic Model step by step

Free interactive mini-lessons: short screens, worked examples and quick checks. Pick your exam board to open its course map, then tap L on this topic:

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Common mistakes examiners see

What charge does a Group 16 (oxygen group) element form?✗ 6−. ✓ 2− — it gains 2 electrons to fill its outer shell (e.g. O²⁻, S²⁻).

Why don't solid ionic compounds conduct electricity?✗ They have no charged particles. ✓ They DO have ions, but the ions are fixed in the lattice and can't move. When molten or dissolved, the ions are free to move and carry charge.

What is the formula of calcium nitrate?✗ CaNO₃. ✓ Ca(NO₃)₂ — Ca²⁺ needs two NO₃⁻ ions to balance, so the polyatomic ion is bracketed and subscripted.

Are ionic compounds made of molecules?✗ Yes, like NaCl molecules. ✓ No — they are giant lattices. The formula (e.g. NaCl) is the empirical ratio, not a molecule.

Why is the lattice enthalpy of MgO far larger than that of NaCl?✗ Because MgO is heavier. ✓ MgO has ions with 2+ and 2− charges (vs 1+ and 1−) and smaller ionic radii, so the electrostatic attraction is much stronger. HL

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The Ionic Model