IB Chemistry: Ionisation Energy & Emission Spectra FAQ

They're both evidence for electron sub-shells: emission spectra show discrete lines from electrons dropping energy levels; successive ionisation energy graphs show jumps between shells. According to the IB, this sits under Structure 1.2 and 1.3 in the current Chemistry guide (first exams 2025) and is tested via data-based Paper 1 and 2 questions.

Each line is emitted when an excited electron falls from a higher energy level to a lower one, releasing a photon of a specific frequency. The lines converge at higher frequency because energy levels get closer together further from the nucleus — that convergence point corresponds to ionisation energy.

Plot log(ionisation energy) against the number of electrons removed. Each big jump marks a new, lower shell being broken into — closer to the nucleus, more strongly held. The number of electrons before the first big jump tells you the number of electrons in the outer shell, which gives the group number.

If electrons could hold any energy, spectra would be continuous rainbows. Instead you see distinct lines at fixed frequencies, proving electrons only occupy discrete energy levels and can only emit photons matching the exact energy gap between two of those levels.

State the trend, then explain it using nuclear charge, shielding and atomic radius — never just one factor. Across a period, ionisation energy generally increases due to increasing nuclear charge with similar shielding; down a group it decreases due to increasing atomic radius and shielding outweighing nuclear charge.

Oxygen has a lower first ionisation energy than nitrogen because oxygen's fourth p-electron is paired, and electron-electron repulsion in that filled orbital makes it easier to remove than nitrogen's unpaired, half-filled arrangement. Similarly, boron dips below beryllium because of the extra stability of beryllium's filled 2s subshell.

Confusing first ionisation energy trends (period/group) with successive ionisation energy graphs (same element, removing electron after electron) — they test different things. I've marked scripts where students explain a group trend when the question actually gave a successive-removal graph for one element.

HL students use the Rydberg-style relationship between spectral line frequency and energy level differences, calculating the ionisation energy of hydrogen from the convergence limit of its emission spectrum using $E=hf$. This connects Structure 1.3 directly to quantitative Paper 2 questions.

Yes — HL goes further, requiring students to calculate ionisation energy from spectral convergence frequencies and to explain finer sub-shell detail (s, p, d splitting) that SL only needs qualitatively. Both levels must explain periodic trends and successive ionisation graphs.

Both. Paper 1 (multiple choice) often tests trend recognition and graph reading quickly, while Paper 2 asks for full written explanations linking spectra or ionisation data to electron configuration — sometimes worth 4-6 marks per sub-question. It rarely appears in Paper 3 data-based questions directly.

Expect 'state', 'explain', 'deduce' and 'sketch'. 'Deduce' questions usually want you to work out group or period position from a successive ionisation graph; 'explain' questions want the shielding/nuclear charge/radius reasoning written out, not just named.

Past paper questions on Structure 1 are the single best resource, since this topic repeats in similar formats yearly. On RevisionPrep, the Chemistry Revision Notes cover the theory concisely, and the Topical Worksheets group past-paper-style questions by exact sub-topic so you can drill graph interpretation specifically.

Costs vary widely, from free past papers on the IB's own resource pages to structured platforms charging a subscription for organised notes and worksheets. For a topic like this one, where marks are lost to muddled explanations rather than lack of knowledge, well-organised, topic-specific practice tends to be worth more than sheer volume of resources.