IB Chemistry: Atomic Structure & Subatomic Particles FAQ

Most lost marks come from confusing mass number with atomic number, forgetting isotopes share identical electron counts, or misapplying the chromium/copper electron configuration exceptions. Examiners also deduct method marks when working for weighted isotopic mass calculations isn't shown, even if the final answer is correct.

Not conceptually — it's some of the most memorisable content in the whole DP Chemistry course. Where students actually struggle is precision: writing full electron configurations without slips, reading successive ionisation energy graphs correctly at HL, and not muddling isotope averaging with a simple mean of two numbers.

Atomic number is the number of protons in the nucleus and defines which element you're looking at. Mass number is protons plus neutrons combined, and it changes between isotopes of the same element. Get these two swapped in an exam and you'll likely lose the mark even with correct working elsewhere.

Yes — neutral isotopes of the same element always have identical proton and electron counts; only the neutron number, and therefore the mass number, differs between them. That's why isotopes behave almost identically in chemical reactions but differ in physical properties like density and nuclear stability.

Three: protons, neutrons and electrons. You need their relative masses, relative charges and location in the atom, plus how gaining or losing electrons produces ions. Structure 1.2 in the current DP Chemistry guide expects you to link these directly to atomic number, mass number and isotope notation.

SL students write full electron configurations up to atomic number 36 and identify s- and p-block position on the periodic table. HL students go further, covering d-block configurations, the chromium and copper exceptions, and using successive ionisation energy data as evidence for the existence of sub-shells.

A graph of successive ionisation energies for one element shows big jumps between main shells and smaller breaks within a shell — that pattern of small breaks is the accepted evidence for sub-shell existence. HL students are expected to read a log-scale graph and explain each jump in terms of shielding and nuclear attraction.

Multiply each isotope's mass number by its fractional (or percentage ÷100) abundance, then add the results together — this gives the weighted average relative atomic mass shown on the periodic table. Mark schemes award a method mark for showing this working, separate from the mark for a correctly rounded final figure.

Treat these as facts to memorise, not rules to derive. Chromium is [Ar] 3d⁵ 4s¹ and copper is [Ar] 3d¹⁰ 4s¹, both because a half-filled or fully-filled d sub-shell is more stable than the 'expected' 4s² arrangement. Examiners test these two specific elements on multiple-choice and short-answer questions almost every exam session.

Nail the definitions first — proton, neutron, electron, isotope, ion — until you can state them cold, then move straight to past-paper questions on isotope maths and electron configuration. I've taught this topic for years and the students who slip up are almost never confused about theory; they just haven't drilled the calculation format enough.

Both — atomic structure (Structure 1.2) and core electron configuration (Structure 1.3) are examined at SL and HL alike. The extra HL-only material is d-block exceptions, ionisation energy evidence for sub-shells, and configurations extending to higher atomic numbers than SL students need.

A little, but from a different angle. IB Physics covers nuclear structure, isotopes and radioactive decay through energy and stability, while IB Chemistry uses the same particles — protons, neutrons, electrons — to explain bonding and periodicity. Students taking both subjects usually find the shared vocabulary reinforces learning rather than duplicating it.

Past-paper practice matters most here, because the same three or four question types — isotope calculations, electron configuration, and at HL, ionisation energy graphs — recur almost every exam session. Look for a question bank organised by syllabus subtopic rather than by exam paper, so mistakes on one specific skill get caught and corrected quickly.