How IB Chemistry Structure 1.3 electron configurations are actually marked on Paper 1
Master IB Chemistry Structure 1.3 electron configurations for Paper 1. Learn the five sub-orbital rules, notation styles, and transition-metal exceptions that decide a 7.
IB Chemistry Structure 1.3 sits inside the Structure 1 sub-topic of the IB Diploma Programme chemistry syllabus, and on Paper 1 it is one of the highest-yield, lowest-noise areas a candidate can prepare. The sub-topic asks students to describe and explain the electron configuration of atoms and ions of the first 36 elements using full spdf notation, orbital box diagrams, and the shorthand noble gas form. Three or four marks on Paper 1, depending on the examination paper, come directly from Structure 1.3, and another two or three marks typically appear in Section B of Paper 2 when an examiner threads a configuration question into a periodicity or bonding context. A candidate who can write any first-36 configuration in two of the three accepted notations, and who can justify the chromium and copper exceptions without hesitation, walks into the exam with points other candidates are still arguing for in their heads.
What IB Chemistry Structure 1.3 actually tests on the exam
The IB Chemistry Structure 1.3 sub-topic is short on the syllabus, but the marking rewards a specific kind of literacy. The examiner is not asking whether you can recite the Aufbau order; that information is given to you in Section 5 of the IB Chemistry data booklet, in the form of an energy-level diagram with sub-level labels. What the examiner is testing is whether you can read that diagram, apply it to a real atom or ion, and present the configuration in a notation that the rubric accepts.
Three notations appear in mark schemes, and the syllabus expects you to be fluent in all of them. The first is the full spdf notation, for example 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷ for cobalt. The second is the orbital box diagram, where each box represents one orbital and the arrows represent electrons with their spins. The third is the noble gas shorthand, for example [Ar] 4s² 3d⁷ for cobalt. The shorthand is the one most students default to under time pressure, but it is also the one that produces the most avoidable errors when candidates forget the 4s-before-3d ordering rule once ions are formed.
Two skills sit behind the notation. The first is sub-orbital capacity: knowing that s holds two electrons, p holds six, d holds ten, and f holds fourteen. The second is Hund's rule: within a sub-level, electrons occupy orbitals singly before pairing. IB Chemistry mark schemes treat Hund's rule violations as a one-mark deduction even when the total count is correct, because the examiner is reading for the model, not the number. A candidate who writes all six p-electrons in one box has answered a different question to the one that was set.
In practice, the strongest Paper 1 candidates spend twenty to thirty seconds per configuration question and reserve the longer three-minute questions for the calculation or mechanism items. If a candidate can hold a 1.3 question to under forty-five seconds across the paper, the time saved flows directly into the harder 10- and 15-mark items on Paper 2 Section B. For most candidates reading this, Structure 1.3 is the section of the paper where the time-per-mark ratio is best, and preparation should reflect that.
The five sub-orbital rules IB examiners expect you to apply in order
IB mark schemes rarely give marks for the final configuration alone. Marks are awarded for the reasoning chain, and the reasoning chain is built from five rules that candidates must apply in a specific sequence. Memorising the order is what separates a Level 5 answer from a Level 7 answer on a two-mark configuration question.
Rule 1: Determine the number of electrons
For a neutral atom, the number of electrons equals the atomic number, and the periodic table on the data booklet gives you this directly. For a monatomic ion, the charge adjusts the electron count: a +2 ion has two fewer electrons than the neutral atom, and a −1 ion has one more. Students lose the most marks here by treating the charge as a proton count rather than an electron count, which inverts the whole configuration. Writing 1s² 2s² 2p⁶ for a sodium ion instead of for neon is the classic version of this error.
Rule 2: Apply the Aufbau order from the data booklet
The Aufbau order is supplied in Section 5 of the IB Chemistry data booklet, and the syllabus explicitly tells candidates to use it rather than memorise it. In a Paper 1 setting, the data booklet should be open to Section 5 before the first configuration question is read, because the time cost of flipping back is not worth the memorisation saving. The order is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, and so on. The 4s-before-3d ordering is the rule that creates most of the marking complexity, because it changes when the atom is ionised.
Rule 3: Fill the sub-levels to capacity
Once the right number of sub-levels has been identified, fill each to its capacity before starting the next. This is where Hund's rule enters. Within a 3d sub-level, for example, the five d orbitals must each receive one electron before any of them receives a second. The arrows in the box diagram are conventionally drawn pointing up first, then down, and the spin-pairing symbol on a mark scheme is explicit: one arrow per box, then paired arrows.
Rule 4: Re-order on ionisation
This is the rule that catches out SL students most often. When a transition metal atom forms a +2 ion, the two electrons leave from the 4s sub-level, not the 3d. The shorthand is that 4s fills before 3d during construction, but empties before 3d during ionisation. The 3d sub-level is therefore written after 4s in the neutral atom configuration, but before 4s in the ion configuration. IB examiners test this routinely on Paper 1 with questions of the form "Write the electron configuration of the Cu²⁺ ion using spdf notation." Candidates who write [Ar] 3d⁹ 4s⁰ are correct; candidates who write [Ar] 3d⁷ 4s² are answering the wrong ion.
Rule 5: Check the transition metal exceptions
Chromium and copper break the Aufbau pattern in their neutral atom configurations, and the IB mark scheme expects candidates to know this. Chromium is [Ar] 4s¹ 3d⁵ rather than [Ar] 4s² 3d⁴, because a half-filled d sub-level is energetically more stable than a partially filled s. Copper is [Ar] 4s¹ 3d¹⁰ rather than [Ar] 4s² 3d⁹, because a fully filled d sub-level is more stable than a partially filled s. The exceptions are limited to chromium and copper at IB level. The syllabus does not require candidates to memorise exceptions beyond the first transition series, and examiners do not test them.
When the five rules are applied in order, a configuration answer of two to three lines takes about thirty seconds to write. When the rules are not applied in order, candidates spend two to three minutes and often end up with an answer the examiner cannot award full marks to. The order is the preparation strategy, not the final string of symbols.
spdf notation, orbital boxes, and noble gas shorthand: when to use which
The IB Chemistry mark scheme accepts three notations, and the choice of notation is part of the answer. Most candidates default to noble gas shorthand because it is the fastest to write, but the choice has consequences for the marks awarded on specific question stems.
spdf notation is the safest choice when the question stem says "in terms of sub-levels" or "in full." The notation is unambiguous, the order of sub-levels is explicit, and the examiner can award each mark without inferring intent. Candidates writing 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁷ communicate the model, not just the count. The notation is also the one to default to when the question asks for an ion, because the ordering change at ionisation is visible in the symbols themselves.
Orbital box diagrams are the right choice when the question explicitly asks for a diagram, when Hund's rule is being assessed, or when the mark scheme allocates a mark for correct spin pairing. The conventional box diagram uses a small square for each orbital, a horizontal line under each sub-level, and a label such as "3d" above the group of five boxes. Arrows are drawn as half-arrows, conventionally pointing up first, then down for the paired electron. Two errors dominate: drawing paired arrows before all boxes are singly occupied, and forgetting to label the sub-levels. Both are one-mark deductions each, and both are entirely preventable with a five-second check before submitting the answer.
Noble gas shorthand is the right choice for neutral atoms of elements from period 3 onwards, where the shorthand saves significant writing time. The notation is [noble gas core] outer electrons, and the noble gas core is taken from the previous period. Sodium is [Ne] 3s¹, potassium is [Ar] 4s¹, and iron is [Ar] 4s² 3d⁶. The shorthand is least useful for the first twenty elements, where the full notation is shorter anyway, and for ions, where the re-ordering rule on ionisation is easier to express in full spdf form.
| Notation | Best used for | Time cost | Common error |
|---|---|---|---|
| Full spdf (1s² 2s² 2p⁶ ...) | Ions; explicit ordering questions | Highest | Forgetting 4s empties before 3d on ionisation |
| Orbital box diagram | Hund's rule questions; "draw a diagram" stems | Medium | Pairing before all orbitals singly occupied |
| Noble gas shorthand ([Ar] ...) | Neutral atoms of period 3 and beyond | Lowest | Using the wrong noble gas core (e.g. [Kr] for iron) |
A practical preparation tactic is to write each of the first 36 elements in all three notations once during revision, then never write the orbital box version again unless the question stem demands it. The shorthand is the daily driver; the spdf form is the safety net for ion questions; the box diagram is a specific skill, not a general one.
Transition metal configurations: the IB Chemistry Paper 1 trap zone
Transition metal configuration questions are the most common place where Structure 1.3 candidates drop from a Level 6 to a Level 5. The trap is not the notation; the trap is the model. The IB mark scheme wants to see the d-block electrons written explicitly, and it wants to see the 4s/3d ordering respected on both the neutral atom and the ion.
For the first transition series, the configurations to drill are scandium through zinc. The neutral atom pattern is [Ar] 4s² 3dⁿ where n runs from 1 to 10, with the chromium and copper exceptions at n = 5 and n = 10. The +2 ion pattern is [Ar] 3dⁿ⁻², because the two 4s electrons leave first. The +3 ion pattern for the common ions is [Ar] 3dⁿ⁻³. Candidates who can write the +2 configuration of any first-series transition metal in under twenty seconds, in full spdf form, have removed a significant slab of avoidable error from their Paper 1 score.
The IB examiner also tests the reverse direction. A question stem may give the configuration [Ar] 3d⁸ and ask the candidate to identify the element and the charge. The element is nickel, and the charge is 0; the 4s² is implicit in the noble gas core. A second question may give [Ar] 3d⁷ and ask the same; the element is cobalt, charge 0. A third question may give [Ar] 3d⁶ and ask the element and the charge; the element is iron, charge +2. The reverse direction tests whether the candidate understands the construction order, not just the spelling of the answer.
For most candidates reading this, the highest-leverage preparation move on transition metal configurations is a single A4 sheet with three columns: element symbol, neutral atom configuration, and +2 ion configuration. Working through the sheet three times, once in shorthand, once in spdf, and once as box diagrams, locks the model in a way that drilling the exceptions alone never does. The exceptions are the symptoms; the model is the disease.
Common pitfalls and how to avoid them in IB Chemistry Structure 1.3
Most Structure 1.3 errors fall into four families, and each family has a one-line fix that can be drilled in under a week. The pitfall list below is ordered from highest frequency to lowest frequency on IB mark schemes.
- Confusing atomic number with mass number when starting the configuration. The configuration is built from the electron count, and the electron count comes from the atomic number for a neutral atom. A candidate who reads the mass number of chlorine-35 and writes 17 electrons is correct; a candidate who writes 35 electrons has answered a different question. Fix: write the atomic number above the element symbol in the working margin before the configuration.
- Filling 3d before 4s in the construction of the neutral atom. The Aufbau order places 4s below 3d in energy for atoms up to calcium, so 4s fills first. Candidates who write [Ar] 3d² 4s² for calcium are not following the data booklet diagram. Fix: re-read the diagram before writing the first sub-level, and recite "4s, 3d" out loud if necessary.
- Forgetting to remove the 4s electrons first when forming an ion. This is the most heavily penalised single error in the sub-topic. The 4s electrons are the highest in energy once the 3d sub-level is occupied, and they leave first. Fix: write the word "ionise" above the line before re-writing the configuration for an ion, and re-check the 4s sub-level is empty or has the correct reduced count.
- Writing paired electrons before the sub-level is half-filled. Hund's rule is explicit: electrons fill singly until the sub-level is half-full, then they pair. Candidates who write a 2p configuration as a single box with six arrows have lost a mark. Fix: count the boxes in the sub-level, write the arrows one at a time up to half the box count, then pair.
A fifth error is less common but more expensive: the candidate writes the configuration in the wrong notation. A question stem that says "in full" expects spdf notation; a stem that says "using a diagram" expects a box diagram. Candidates who answer in shorthand when the stem said "in full" usually lose a mark even when the configuration is correct. Reading the stem twice takes five seconds and saves a mark.
How Structure 1.3 feeds the rest of the IB Chemistry syllabus
Structure 1.3 is a foundation sub-topic, and the configurations a candidate writes in Paper 1 Section A reappear in three other syllabus areas: Structure 2 (bonding), Structure 3 (periodic table), and Reactivity 3 (mechanisms, where the lone pair on a Lewis base is read directly from the configuration). A preparation strategy that treats Structure 1.3 as a stand-alone memorisation unit is leaving marks on the table in those later sub-topics.
In Structure 2, the configuration explains the number of valence electrons available for bonding, the number of lone pairs on a central atom, and the expanded octet exceptions for period 3 and beyond. A question that asks for the shape of the sulfate ion, for example, requires the candidate to recognise that the sulfur atom has access to 3d orbitals and can therefore accommodate twelve electrons rather than eight. That recognition is a Structure 1.3 skill applied in a Structure 2 context.
In Structure 3, the configuration explains the periodic trend in atomic radius, ionisation energy, and electronegativity. A question that asks for the explanation of the dip in first ionisation energy between beryllium and boron uses the configuration 2s² versus 2s² 2p¹ directly. The candidate who cannot write the two configurations in five seconds each is the candidate who runs out of time on the explanation question in Paper 2 Section B.
In Reactivity 3, the configuration identifies the lone pair on a Lewis base such as ammonia, water, or a halide ion. A mechanism question that asks the candidate to draw a curly arrow from a lone pair to an electrophile requires the candidate to locate the lone pair in the configuration and place it in the box diagram. The arrow follows the electron, and the electron's position is set in Structure 1.3.
For most candidates, the right preparation strategy is to treat Structure 1.3 as a daily warm-up rather than a stand-alone revision block. Five minutes at the start of each study session, writing the configurations of the first 36 elements in shorthand, is more efficient than a four-hour weekend block. The skill is a fluency skill, and fluency skills are built by frequency, not duration.
Paper 1 versus Paper 2: how Structure 1.3 scoring differs across the IB Chemistry exam format
On Paper 1, Structure 1.3 questions are typically one- or two-mark multiple choice or short response items, and the marking is binary: the configuration is either right or wrong, and the examiner is not looking for partial credit on a multiple-choice option. The candidate has twenty to thirty seconds per question, and the strategy is to identify the question, write the configuration in shorthand, and move on. Spending more than forty-five seconds on a single Structure 1.3 question is a time-management error.
On Paper 2 Section A, Structure 1.3 questions are typically one- or two-mark short response items embedded in a larger context. A question may give the configuration of a metal ion and ask the candidate to identify the element, the charge, and the period. Each part is a separate mark, and partial credit is available. The strategy is to read the stem, identify what is being asked, and answer each part in the notation the stem requests.
On Paper 2 Section B, Structure 1.3 is rarely tested as a stand-alone question. The configurations appear as supporting evidence for a periodicity, bonding, or mechanism argument, and the candidate is expected to write them as part of a longer answer. The strategy here is to write the configuration in the line above the argument it supports, so the examiner can see the link without searching the page. A configuration written in the margin without a connecting phrase is a mark the candidate is hoping the examiner will infer; in practice, the examiner rarely does.
The total mark allocation for Structure 1.3 across the two papers is small, typically three to five marks in total, but the mark density per minute of preparation is high, and the error rate is low for candidates who drill the sub-topic. A candidate who can hold the sub-topic to under three errors per paper across a full mock examination has converted Structure 1.3 from a risk into a foundation. That is the conversion this sub-topic is for.
Conclusion and next steps for IB Chemistry Structure 1.3 preparation
IB Chemistry Structure 1.3 is a small sub-topic with a high mark density, and the preparation strategy is to drill the first 36 elements in three notations, master the 4s/3d ordering rule on ionisation, and learn the chromium and copper exceptions as the model rather than as facts. The skills built here flow directly into Structure 2, Structure 3, and Reactivity 3, and the time invested pays back across the rest of the syllabus. A six-week preparation plan that allocates five minutes per day to configuration writing, and a single two-hour block to the transition metal exceptions, is more than sufficient for a Level 7 on this sub-topic. IB Courses' IB Chemistry HL programme breaks each candidate's Paper 1 configuration error pattern against the rubric and turns the Structure 1.3 sub-topic into a reliable marks bank for the final examination.