IB Chemistry: Ionic Bonding & Lattice Structures FAQ

Most marks are lost because students describe ionic bonding instead of explaining a property from it. Saying "ionic compounds have strong bonds" earns nothing — you need electrostatic attraction between oppositely charged ions in a giant lattice linked directly to melting point, conductivity or brittleness.

Ionic bonding is the electrostatic attraction between oppositely charged ions arranged in a giant lattice. It forms when a metal transfers electrons to a non-metal, producing a cation and an anion. This falls under Structure 2.1 of the current DP Chemistry guide, first examined 2025.

A lattice is the regular, repeating three-dimensional arrangement of ions extending in all directions. Melting an ionic compound means overcoming every one of those electrostatic attractions at once, which is why ionic solids like MgO have very high melting points compared to molecular substances.

A 7-level answer always links structure to property using correct terminology: electrostatic attraction, delocalised electrons, giant covalent lattice. Weak answers stop at description. Practise past-paper "explain" and "deduce" questions until linking structure to property becomes automatic, not something you have to think through.

State the property, then explain it using the lattice model: ionic compounds conduct electricity only when molten or dissolved because ions become free to move and carry charge. In the solid state ions are fixed in position, so solids don't conduct — a distinction examiners specifically check for.

At Higher Level you calculate lattice enthalpy indirectly using a Born-Haber cycle, applying Hess's Law to sum enthalpy changes such as atomisation, ionisation and electron affinity. Lattice enthalpy itself can't be measured directly, which is exactly why the cycle exists — this sits under Reactivity 1.2 in the current guide.

The top mistake is sign errors — forgetting that electron affinity for chlorine is exothermic (negative) while atomisation and ionisation are always endothermic (positive). The second most common error is drawing the cycle with arrows pointing the wrong way, which flips every value in the final calculation.

The core ionic bonding model in Structure 2.1 is common to both SL and HL. Born-Haber cycles and quantitative lattice enthalpy calculations are additional higher level (AHL) content under Reactivity 1.2, so SL students need the concept and properties but not the full energy-cycle maths.

It appears across all three papers. Paper 1 uses multiple-choice questions on properties and structure; Paper 2 includes structured questions asking you to explain properties or draw Born-Haber cycles at HL; Paper 3 can bring it into data-based or option-linked questions, though it's less common there.

Ionic bonding is electrostatic attraction between ions in a lattice; covalent bonding is shared electron pairs between non-metal atoms; metallic bonding is delocalised electrons attracting a lattice of metal cations. Confusing these three is one of the most frequent errors in Paper 1 distractor questions.

There's no single official percentage per topic, but Structure 2 (bonding and structure) is one of the more heavily assessed areas across Papers 1 and 2 because it underpins later topics like periodicity and organic chemistry. A shaky grasp here tends to cost marks throughout the course, not just in one unit.

The most effective combination is concise revision notes for the core model, topical worksheets to drill "explain the property" style questions, and full mock papers to practise Born-Haber cycles under timed conditions. Repetition on the exact command terms the mark scheme rewards matters more than re-reading the textbook.