Metallic bonding in IB Chemistry Structure 2.3: 4 properties the rubric rewards on Paper 2
IB Chemistry Structure 2.3 metallic model explained: electron sea, lattice strength, and the four properties that decide your Paper 1 and Paper 2 mark band.
The IB Chemistry Structure 2.3 metallic model sits inside sub-topic 2 of the Structure domain, sandwiched between ionic and covalent frameworks in the IB Diploma Programme subject guide. It asks candidates to explain metallic properties using a delocalised "electron sea" of valence electrons moving through a regular lattice of positive ions, then to apply that picture to electrical conductivity, thermal conductivity, malleability, and melting point. Most IB candidates can recite "metals have delocalised electrons" in two seconds. Far fewer can deploy that sentence in the way the mark scheme wants: linking the moving electrons to a measurable property, and linking the lattice to the energy cost of disrupting it. This article is built for the IB Chemistry student who already has the textbook diagram memorised and now needs the answer style, the Paper 1 distractors, and the rubric language that move an answer from a mid-band 4 or 5 to a top-band 6 or 7 on both SL and HL papers.
What the IB Chemistry subject guide actually requires under Structure 2.3
The Structure 2.3 metallic model is short on the syllabus outline — usually a single bullet reading something like "describe and explain the properties of metals in terms of metallic bonding" — but the assessment implication is much wider. Candidates are expected to (1) draw or describe the lattice of positive ions surrounded by delocalised valence electrons, (2) use that picture to justify four named physical properties, and (3) on HL only, extend the model to alloys and the impact of lattice distortion. The IB Diploma Programme assesses Structure 2.3 across both Paper 1 and Paper 2, and the question type is almost always a short structured item on Paper 1 paired with a 4-to-6 mark extended response on Paper 2. For SL candidates, the metallic model usually appears once per examination sitting, frequently as part of a comparison question that bundles metallic, ionic, and giant covalent bonding. For HL candidates, the metallic model is also the foundation of Structure 2.4 alloys and underpins the metallic character discussion in Reactivity 3.4.
Three syllabus phrases matter more than the rest, and IB examiners use them as load-bearing language in the mark scheme. They are "delocalised valence electrons", "regular arrangement of positive ions (lattice)", and "layers/ions can slide over each other". If a candidate's answer omits at least two of those three, in my experience the response is anchored to band 3 or 4 even when the rest of the explanation is correct. The IB is not testing whether the candidate has seen a metal before; it is testing whether the candidate can name what is moving, what is staying, and what is being disrupted when a property is observed.
Where Structure 2.3 lives in the wider Structure domain
Structure 2.3 is not a standalone sub-topic. It directly mirrors the logic of Structure 2.2 (ionic) and Structure 2.1 (covalent), and the IB examiner habitually builds cross-sub-topic comparison questions that ask candidates to explain why sodium conducts as a solid while sodium chloride does not. Candidates who treat Structure 2.3 as an isolated fact pile tend to lose those comparison marks. Treat it as a peer of the ionic and covalent models, with the same template — describe the particle arrangement, then map each property back to a particle-level cause. That template is the single most useful piece of exam technique in the whole Structure domain.
The particle picture: what the rubric expects you to draw and label
Most candidates who underperform on Structure 2.3 lose marks before they have written a single property word. The first scoring opportunity is usually a labelled diagram of the metallic lattice, and the rubric awards credit for three things in that diagram: a regular geometric arrangement of positive metal ions, a clear indication that the electrons are delocalised (often shown as dots, a shaded background, or labelled "sea of electrons"), and an arrow or label that ties the electron movement to the property being discussed. A common mid-band failure is drawing the ions as a tidy 2 × 2 grid and then labelling the negative charges as if they were still attached to specific atoms — that is a covalent picture, not a metallic one, and the mark scheme reads it as a misconception rather than a slip.
The second scoring opportunity is the wording around "positive ions". The IB expects the phrase "positive ions" or "cations", not the looser "metal atoms". In the metallic model the atoms have donated their valence electrons into the delocalised sea, so the particles occupying the lattice points carry a net positive charge. This is a subtle but consistent mark-scheme discriminator on Paper 2. Candidates who say "metal atoms in a lattice surrounded by electrons" leave a mark on the table, because the examiner is looking for the ionisation that the model implies. A high-band answer will say something like: "Positive ions arranged in a regular lattice with valence electrons delocalised throughout the structure, forming an 'electron sea'." That sentence alone can secure two of the structural marks on a 4-mark extended response.
The third scoring opportunity is layering. For HL candidates, the IB also accepts — and frequently rewards — diagrams that show layers of positive ions, because the malleability explanation depends on layers sliding. A 2D cross-section with three labelled layers, even if the geometry is simplified, communicates the malleability mechanism more efficiently than three sentences of prose. For SL, a single labelled rectangle with positive ions, electron sea, and a movement arrow is usually sufficient. Most of the marks for the metallic model are in the diagram plus the first sentence; the property explanations are where the remaining marks live.
Four physical properties: the explanation template that scores 6 or 7
Once the lattice is drawn and labelled, the IB mark scheme expects four named properties, each explained through the metallic model. The four are electrical conductivity, thermal conductivity, malleability, and high melting point. For each one, a high-band answer does three things: names the property, names the particle that moves or is disrupted, and links the two with a direction of cause. A weak answer states the property and stops. A mid-band answer states the property and the particle. A high-band answer states the property, the particle, and the cause-effect relationship. The template — "property → particle → mechanism" — is the same one used for ionic and covalent bonding in Structure 2.1 and 2.2, which is why the comparison questions on Paper 2 are so marker-friendly: they are testing whether the candidate can apply the template to three different bonding models in succession.
Electrical conductivity
The mechanism is that the delocalised electrons are free to move through the lattice when a potential difference is applied, carrying charge. The trap is forgetting to mention that the lattice ions themselves stay in place; it is the electrons that move, not the ions. For HL candidates, the IB also expects the distinction that conductivity is preserved in the solid state, unlike ionic compounds which only conduct when molten or aqueous. That contrast is a frequent comparison-question prompt.
Thermal conductivity
When heated, the delocalised electrons gain kinetic energy and transfer it rapidly through the structure by colliding with other electrons and with the lattice ions. The metallic model explains thermal conductivity in the same way as electrical conductivity, with the electrons as the energy carriers. A common candidate error is to attribute thermal conductivity to "vibration of the lattice" alone, which is the giant covalent picture from Structure 2.4 (diamond). The IB mark scheme explicitly contrasts metallic thermal conductivity with that of giant covalent solids.
Malleability and ductility
When a force is applied, layers of positive ions can slide over each other. Because the electron sea is non-directional, the metallic bonds reform in the new positions, so the metal deforms without shattering. This is the property where the layered diagram pays for itself: a clear labelled cross-section makes the mechanism visible. The most common mark-scheme penalty here is writing "the ions can move" without specifying that layers slide while the electron sea accommodates the new geometry. Candidates should also be aware that the IB accepts "malleability" (hammered into sheets) and "ductility" (drawn into wires) as separate sub-properties of the same mechanism.
High melting point
A lot of energy is needed to overcome the strong electrostatic attraction between the positive ions and the delocalised electron sea. The phrase to memorise is "strong electrostatic attraction between positive ions and delocalised electrons". A weak answer says "strong metallic bonds", which the mark scheme treats as a restatement rather than an explanation. For comparison questions, candidates are expected to note that metallic melting points vary widely (mercury at –39 °C versus tungsten above 3400 °C) because the strength of the attraction depends on charge density of the ions and on the number of delocalised electrons per atom.
Common pitfalls and how to avoid them in Structure 2.3
The single most expensive mistake on Structure 2.3 is writing "metallic bonds" where the rubric wants "electrostatic attraction between positive ions and delocalised electrons". The IB is consistent across marking panels: the mark scheme wants the particle-level cause, not the category label. Candidates who default to "metallic bonds are strong" will be marked down for the same reason a candidate who says "ionic bonds are strong" loses marks in Structure 2.2: the answer is true but the model has not been deployed.
The second pitfall is mixing metallic and covalent language. Saying "the metal atoms share their outer electrons" is a covalent model, and the IB mark scheme treats it as a misconception. If a candidate does not say "delocalised" or "sea of electrons" or "mobile electrons", the examiner assumes the candidate has not understood the metallic model. A useful self-check: in the answer, can you underline the word that names the electrons' behaviour? If you cannot, the answer is under-modelled.
The third pitfall is forgetting that SL and HL differ in alloy content. SL Structure 2.3 usually stops at pure metals. HL Structure 2.3 + 2.4 explicitly introduces alloys as a metallic lattice containing atoms of different sizes, which distorts the regular layers and makes it harder for layers to slide. This is why alloys are harder than pure metals. HL candidates who treat alloys as "metals mixed together" without the lattice-distortion argument lose the comparison marks on Paper 2 Section B.
- Use "delocalised electrons" or "electron sea" — never just "free electrons" without specifying delocalisation across the lattice.
- Say "positive ions" or "cations" — not "metal atoms" — for the particles on the lattice points.
- For malleability, describe layers sliding, not ions moving generally. The mechanism is layer-on-layer, not point-by-point migration.
- For melting point, name the attraction explicitly: electrostatic attraction between positive ions and the delocalised electron sea.
- On HL, link alloys to lattice distortion; on SL, omit alloys unless the question is a comparison.
Paper 1 question types and how to triage them
On Paper 1, Structure 2.3 typically appears as one of the 40 multiple-choice items, often disguised behind a property statement or a diagram. The four most common families of distractors are: properties wrongly attributed to ionic compounds (e.g. "conducts in the solid state"), properties wrongly attributed to giant covalent solids (e.g. "very high melting point due to strong covalent bonds"), correct property with wrong mechanism, and correct property with correct mechanism applied to the wrong material. Most Paper 1 items on the metallic model can be solved in under 90 seconds if the candidate can map the property to the particle movement. The triage rule is: identify the property, then ask "what is moving in the metallic model?" If the candidate can name the particle and the direction of movement in one breath, the distractor falls.
A typical Paper 1 stem might be: "Which property is explained by the ability of layers of positive ions to slide over each other in a metallic lattice?" The distractors will usually include a thermal-conductivity distractor, an electrical-conductivity distractor, and an ionic-bonding distractor. The candidate's job is to pick malleability or ductility, both of which are rewarded. A candidate who chooses "high melting point" has confused the malleability mechanism with the lattice-strength mechanism. The fix is to drill the property-mechanism pair as a single unit, not as a list of four independent facts.
Another Paper 1 pattern is the "which statement is correct?" item, where three of the four options contain the right words in the wrong order. For example, "metallic bonding involves a lattice of positive ions in a sea of delocalised neutrons" is a classic distractor because the words "lattice", "positive ions", and "delocalised" are all correct vocabulary, but the particle named is wrong. The candidate must read the full statement, not skim the first half.
Paper 2 extended responses: the rubric language that moves a 5 to a 7
On Paper 2, the metallic model is most often tested as part of an extended-response comparison, worth between 4 and 6 marks. The IB mark scheme for these items is unusually transparent: each mark corresponds to a specific claim about the model, and the top band requires all of the claims to be linked together rather than listed. A 4-mark item usually has a 1-mark-per-claim structure, and a 6-mark item adds two marks for a sustained comparison that ties each bonding model to a different property. Candidates aiming for 6 or 7 need to write the answer as one connected argument, not four bullet points.
The rubric for a 6-mark metallic question typically reads as: 1 mark for stating the model, 1 mark for naming delocalised electrons, 1 mark for naming positive ions in a lattice, 1 mark for using the model to explain a property, 1 mark for linking two properties to the model, and 1 mark for the quality of scientific language. The last mark is the most underused by candidates, because they assume that correct content automatically earns full marks. In practice, the IB examiner is checking for the use of subject-specific vocabulary — "delocalised", "electrostatic attraction", "lattice" — and the absence of generalist substitutes like "forces" or "particles". A candidate who uses three IB-specific terms in the response typically gains the quality-of-language mark; a candidate who uses only one typically does not.
| Property | Particle that moves / is disrupted | Mark-scheme phrase to use |
|---|---|---|
| Electrical conductivity (solid) | Delocalised electrons | electrons move through the lattice carrying charge |
| Thermal conductivity | Delocalised electrons | electrons transfer kinetic energy through the structure |
| Malleability / ductility | Layers of positive ions | layers slide over each other; electron sea reforms bonds |
| High melting point | Positive ions and electron sea | strong electrostatic attraction between positive ions and delocalised electrons |
| Alloys harder than pure metals (HL) | Lattice distortion | different-sized atoms disrupt layer sliding |
Comparison questions: metallic versus ionic versus giant covalent
The IB's favourite Structure 2 question type is the cross-sub-topic comparison, and the metallic model is almost always one of the three bonding models being compared. The standard prompt is to explain why a metal conducts in the solid state while an ionic compound does not, or why a metal is malleable while a giant covalent solid is brittle. These questions are scored on the candidate's ability to use the same template across three different models, so the marking is not just about getting one model right — it is about getting the contrast visible.
For a high-band answer, the candidate should structure the response as three mini-paragraphs, one per bonding model, each following the property → particle → mechanism template. Then, in the conclusion sentence, the candidate should explicitly state the contrast: "In the metal, the delocalised electrons are mobile in the solid state, so the solid conducts. In the ionic solid, the ions are fixed in the lattice and only become mobile when molten or dissolved, so the solid does not conduct." That final sentence is where most comparison marks are awarded or withheld, and it is the part most candidates skip because they assume the comparison is implied.
For HL, the comparison is often extended to alloys, with the candidate expected to argue that the presence of different-sized atoms in the metallic lattice makes layer sliding harder, hence alloys are harder than pure metals. This is the standard IB-style argument and is worth memorising in the form "lattice distortion prevents layer sliding". A common mid-band error is to say "alloys are mixtures so they are stronger", which is a definitional restatement and does not deploy the metallic model.
Preparation strategy: how to drill Structure 2.3 in four weeks
A focused four-week preparation plan for Structure 2.3 should front-load the diagram, then build the property explanations on top of it. Week 1 is the diagram: produce a clean, labelled metallic lattice three times, once per day, until the labels are automatic. The diagram should include positive ions, delocalised electrons, and at least one arrow linking electron movement to a property. Week 2 is the property explanations: write the four property paragraphs from memory, using only the model language — no generalist substitutes. Week 3 is the comparison: take three past Paper 2 questions on metallic versus ionic versus giant covalent, and write the responses under timed conditions, then mark them against the mark scheme. Week 4 is the alloy extension for HL, and a single timed Paper 1 block of 40 questions with a 5-minute review at the end to triage the metallic items.
Two tactical points are worth flagging. First, the IB mark scheme is the single most useful revision document for Structure 2.3, and it is freely available in the IB Diploma Programme subject reports and in the markscheme booklets published after each examination session. Candidates who read the mark scheme for two past metallic questions will see the same three or four phrases appear every time, and those phrases are the answer template. Second, the Data Booklet is not useful for Structure 2.3 in the way it is for Structure 1.4 or Reactivity 3.1. The metallic model is a model question, not a data question, so candidates should not waste time looking up metallic radii or first ionisation energies when revising this sub-topic. The revision time is better spent on diagram labels and property mechanism pairs.
Conclusion and next steps
The IB Chemistry Structure 2.3 metallic model rewards a specific, particle-level answer style: positive ions, delocalised electrons, regular lattice, layers that slide, and an explicit electrostatic attraction. Candidates who reach for the model rather than the label — who say what is moving, what is staying, and what is being disrupted — typically anchor their responses in the top two mark bands on both Paper 1 and Paper 2. The most efficient preparation is to drill the diagram, the four property mechanisms, and the alloy extension (HL) as a single connected argument, then test it against past Paper 2 extended-response mark schemes. IB Courses' one-to-one IB Chemistry programme drills Structure 2.3 metallic-model comparison responses against the published IB mark scheme, with a focus on the property → particle → mechanism template that moves a Paper 2 extended response from band 4 to band 6.