IB Chemistry: The Periodic Table & Periodicity — FAQ

Periodicity is the repeating pattern of physical and chemical properties across the periodic table, caused by the regular repetition of electron configurations as atomic number rises. In the current IB Chemistry guide it sits under Structure 3.1: The periodic table: classification of elements, first examined in 2025, covering radius, ionisation energy, electronegativity and melting point trends.

Atomic radius shrinks across a period because each added electron enters the same outer shell while the nuclear charge grows by one proton each time. That extra positive charge pulls the electron cloud in tighter, since shielding from inner shells stays roughly constant. Down a group, radius grows instead — a new occupied shell wins out over rising nuclear charge.

Electronegativity rises across a period and falls down a group, following the same logic as ionisation energy: rising nuclear charge and shrinking radius pull bonding electrons in harder. Fluorine sits at the top of the Pauling scale at 4.0; francium and caesium sit near the bottom, around 0.7 — the two extremes examiners like to quote in data-based questions.

Periodicity isn't an isolated topic. Melting point trends across Period 3 (sodium to argon) connect directly to Structure 2's bonding models: metallic bonding in Na, Mg and Al gives way to a giant covalent network in silicon, then simple molecular structures (P4, S8, Cl2, Ar) held by weak London forces.

Start by identifying whether the question wants a trend described or a cause explained. Always link the trend to nuclear charge, shielding and atomic radius — examiners award marks for the reasoning chain, not just the direction of the trend. Under Structure 3.1, expect at least one 'explain' question worth several marks on every Paper 2.

First ionisation energy generally rises across a period but dips twice — once between Group 2 and Group 13, again between Group 15 and Group 16. Both anomalies come from sub-shell structure, not falling nuclear charge, and IB examiners test this specifically to separate students who've memorised the trend from those who understand it.

Yes — the IB provides a printed periodic table inside the Chemistry data booklet for every Paper 1 and Paper 2, so you don't need to memorise atomic numbers or full electron configurations for most elements. What you do need to memorise is why the trends happen, because the booklet won't explain the reasoning for you.

Paper 1 tests periodicity mostly through multiple-choice (Paper 1A) and data-based short-answer items (Paper 1B), often asking you to read or compare a trend from a given graph or table. Paper 2 goes deeper, with structured or extended-response questions worth four to six marks asking you to explain a trend using nuclear charge, shielding and radius in sequence.

SL and HL cover the same core trends — atomic radius, ionisation energy, electronegativity and melting point — under Structure 3.1. HL adds a distinct layer on top: the d-block transition elements, including anomalous electron configurations, variable oxidation states, coloured complexes and catalytic behaviour, none of which appear on the SL paper.

Periodicity isn't conceptually hard, but it's a frequent source of dropped marks because students describe a trend instead of explaining it. Marking mock papers, the commonest lost mark is writing 'ionisation energy increases' with no cause attached — examiners want the reasoning: nuclear charge, shielding, or a named sub-shell exception.

The most common mistakes are describing a trend without explaining it, confusing shielding with nuclear charge, and forgetting the Group 2→13 and Group 15→16 ionisation energy dips. A close second: mixing up atomic radius with ionic radius, which behave differently for cations and anions of the same element.

You don't need any chemistry background here — ask your child to explain a trend out loud, the way they'd explain it to a friend. If they can only say 'it just does' when you ask why atomic radius decreases, that's the exact gap an examiner will catch too, and it's worth flagging well before the mock.

Look for revision notes that walk through why each trend happens rather than just naming it, worksheets built around real Period 3 data so you can practise explaining the dips, and timed past-paper-style questions to build exam pacing. Reasoning-first practice consistently outperforms passive re-reading for this exact topic.