Chemistry unit guide
Models of the particulate nature of matter
Moles, formulae, limiting reagents, gases and atomic structure - the arithmetic the rest of the course rests on.
Structure 1 is the unit everything else quietly depends on. Almost every calculation question in the course, in any other unit, ends up passing through a mole ratio at some point, so weakness here shows up as lost marks in thermochemistry, equilibrium and electrochemistry rather than as lost marks in Structure 1. Most students find the content straightforward and still lose marks, because the examiner is testing execution under time pressure rather than understanding.
Topics in this unit
What gets examined
- Converting between mass, moles, concentration and gas volume, in both directions and often twice in one question.
- Empirical formula from percentage composition, then molecular formula from a given relative molecular mass.
- Identifying the limiting reagent and using it - not the reagent named first - to find the theoretical yield.
- Ideal gas law calculations where one quantity is held constant, and recognising when to use pV = nRT rather than the combined gas law.
- Electron configuration using the Aufbau principle, including the chromium and copper exceptions.
- Interpreting first ionisation energy trends across a period and down a group, and the dips that reveal sub-shell structure.
How to revise it
Write the balanced equation before anything else
The single most common cause of a wrong answer in this unit is a correct calculation on an unbalanced equation. Balance first, every time, even when the question looks like it only wants a unit conversion.
Carry units through the working
Writing mol dm⁻³ × dm³ = mol beside your numbers catches the majority of slips before they reach the answer line, and the working itself earns method marks when the final answer is wrong.
Practise limiting reagent questions specifically
They are the most reliably examined part of this unit and the most reliably mishandled. Calculate moles of both reactants, divide each by its coefficient, and use the smaller. Do it that way every time and it stops being a judgement call.
Learn the ionisation energy graph shape, not the values
You are never asked to recall numbers. You are asked to explain a dip between groups 2 and 13, or between 15 and 16, and both explanations are about sub-shell occupancy and pairing.
Where marks get lost
- Rounding intermediate values instead of carrying full precision to the final answer.
- Giving answers to the wrong number of significant figures - the rule is the least precise piece of data you were given.
- Using the reagent that appears first in the equation rather than the one that actually limits.
- Forgetting to convert cm³ to dm³ before using concentration.
- Writing 4s before 3d in a written configuration when the question asks for the ion, where 4s empties first.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.