S2 Models of bonding and structure

Guiding question

Official titleModels of bonding and structure

Editorial framingHow do the ionic, covalent and metallic models explain the properties of materials, and why is bonding a continuum rather than three separate boxes?

The one big idea

Three bonding models — ionic, covalent and metallic — are idealised corners of a continuum, not separate boxes. Together they explain the properties of every material, from salt to steel to DNA.

What this rests on

Our editorial judgement.The dependencies in this section are this site’s own assessment of what a sub-topic rests on. They have notbeen verified by a chemistry teacher and are not part of the official IB guide. By contrast, section 8 (Linking questions) reproduces the IB’s own wording verbatim.

Prerequisites are recorded at the sub-topic level only. The list below aggregates the prerequisites of this topic’s sub-topics.

EditorialS1.1 Introduction to the particulate nature of matterIntermolecular forces presuppose the particle model and states of matter. (via S2.2)
EditorialS1.3 Electron configurationsPredicting ionic charge is done from electron configuration (explicit skill in 2.1.1). (via S2.1)
EditorialS1.3 Electron configurationsLewis formulas and the octet rule require valence-shell counting. (via S2.2)
EditorialS2.1 The ionic modelThe guide explicitly contrasts the ionic and covalent models (linking question S2.2.1). (via S2.2)
EditorialS2.1 The ionic modelThe bonding triangle synthesises the ionic, covalent and metallic models. (via S2.4)
EditorialS2.2 The covalent modelThe bonding triangle synthesises the ionic, covalent and metallic models. (via S2.4)
EditorialS2.3 The metallic modelThe bonding triangle synthesises the ionic, covalent and metallic models. (via S2.4)
EditorialS3.1 The periodic table: Classification of elementsMetallic character and cation charge/radius trends come from periodic position. (via S2.3)
EditorialS3.2 Functional groups: Classification of organic compoundsHL onlyFunctional groups (HL S3.2.2) are needed to form condensation polymers (HL S2.4.6); you cannot form an amide/ester link without knowing the groups. (via S2.4)
EditorialR2.1 How much? The amount of chemical changeAtom economy of addition polymerization is linked by the guide (R2.1.5 to S2.4.5). (via S2.4)

What rests on this

EditorialS2.2 The covalent model
EditorialS2.4 From models to materials
EditorialR1.2 Energy cycles in reactions
EditorialR3.1 Proton transfer reactions
EditorialR3.2 Electron transfer reactions
EditorialS2.4 From models to materials
EditorialS3.2 Functional groups: Classification of organic compounds
EditorialR1.2 Energy cycles in reactions
EditorialR3.3 Electron sharing reactions
EditorialR3.4 Electron-pair sharing reactions
EditorialS2.4 From models to materials
EditorialR3.2 Electron transfer reactions

Sub-topics in this topic

Core concepts that must be mastered

Structure 2 takes the particulate model from Structure 1 and asks: how do particles stick together? Four sub-topics present three bonding models — ionic, covalent and metallic — and then dissolve the boundaries between them. The insight you should carry away is that these are three idealised corners of a continuum, not three separate boxes.

The ionic model(2.1.1–2.1.3)Metals lose electrons to form cations; non-metals gain electrons to form anions (2.1.1). You can predict ionic charge from the atom's electron configuration — the drive toward a noble-gas configuration is the pattern, though not the cause, of ionic bond formation. The ionic bond is the electrostatic attraction between oppositely charged ions (2.1.2). You must know the common polyatomic ions by name and formula: NH₄⁺, OH⁻, NO₃⁻, HCO₃⁻, CO₃²⁻, SO₄²⁻, PO₄³⁻. Binary ionic compounds are named with the cation first and the anion taking the "-ide" suffix. Ionic compounds form three-dimensional lattices represented by empirical formulas, and their physical properties — high melting points, conductivity only when molten or in solution, solubility in polar solvents — follow directly from the lattice structure (2.1.3). Lattice enthalpy measures the bond strength and depends on ion radius and charge: smaller ions and higher charges give stronger lattices.
The covalent model(2.2.1–2.2.16)A covalent bond is the electrostatic attraction between a shared electron pair and the two nuclei (2.2.1). Lewis formulas represent these shared pairs, and the octet rule guides their construction for molecules and ions with up to four electron pairs on each atom. Single, double and triple bonds correspond to one, two and three shared pairs, with increasing bond strength and decreasing bond length (2.2.2). A coordination bond is a covalent bond where both electrons come from one atom — important for HL transition-element complexes (2.2.3). VSEPR predicts molecular shape from electron-domain repulsion: two domains give linear, three give trigonal planar, four give tetrahedral, with non-bonding pairs compressing bond angles (2.2.4). Bond polarity arises from electronegativity difference between bonded atoms (2.2.5), and molecular polarity depends on both bond polarity and geometry — dipoles may cancel in symmetric molecules like CO₂ but not in H₂O (2.2.6). Carbon and silicon form covalent network structures — diamond, graphite, fullerenes, graphene, silicon, SiO₂ — whose properties follow from extended bonding rather than discrete molecules (2.2.7). Intermolecular forces (IMFs) explain physical properties of molecular substances: London (dispersion) forces, dipole–dipole, dipole–induced-dipole and hydrogen bonding (2.2.8). "Van der Waals forces" is the inclusive term for the first three. For comparable molar mass, the strength ordering is London < dipole–dipole < hydrogen bonding (2.2.9). Chromatography separates mixtures by relative IMF attractions to mobile and stationary phases, quantified by R_F values (2.2.10). At HL, the model deepens considerably: resonance and delocalisation explain molecules that no single Lewis formula can represent, with benzene as the key example (2.2.11, 2.2.12); expanded octets allow five and six electron domains (e.g. SF₆, PCl₅) with their own VSEPR geometries (2.2.13); formal charge distinguishes the preferred Lewis formula when more than one is possible (2.2.14); sigma (σ) bonds form by head-on orbital overlap (electron density along the bond axis) and pi (π) bonds by lateral p-orbital overlap (density above and below the axis) (2.2.15); and hybridisation — sp, sp², sp³ — connects electron-domain count to molecular geometry to bonding type (2.2.16).
The metallic model(2.3.1–2.3.3)A metallic bond is the electrostatic attraction between a lattice of cations and delocalised electrons (2.3.1). This "sea of electrons" explains electrical conductivity (electrons are free to move), thermal conductivity, malleability and ductility (the lattice can slide without breaking bonds because the bonding is non-directional). Bond strength depends on cation charge and metal ion radius, explaining melting-point trends in s- and p-block metals (2.3.2). At HL, transition elements have delocalised d-electrons that contribute to their characteristically high melting points and electrical conductivity (2.3.3) — their chemical properties (variable oxidation states, coloured complexes, catalysis) are covered in Structure 3.1 and Reactivity 3.4.
From models to materials(2.4.1–2.4.6)The decisive insight comes last: bonding is a continuum, not three separate boxes (2.4.1). The bonding triangle (van Arkel–Ketelaar) places every binary compound on a surface whose corners are pure ionic, covalent and metallic. Position is set by two numbers from data-booklet electronegativities — the difference Δχ (ionic character) and the average χ̄ (metallic vs covalent) — and predicts properties from position (2.4.2). Pure covalent and fully ionic are the limits, not the norm. Alloys are mixtures of a metal with other elements, with enhanced properties explained by non-directional bonding disrupting the lattice (2.4.3). Polymers are macromolecules built from repeating monomers (2.4.4): addition polymers form by breaking a C=C double bond in each monomer (2.4.5), and at HL condensation polymers (polyamides, polyesters) form by reaction between functional groups with release of a small molecule like water (2.4.6). All biological macromolecules — proteins, nucleic acids, polysaccharides — form by condensation and break down by hydrolysis.
Why this mattersStructure 2 is where the GCSE "ionic or covalent" binary gets retired. The bonding continuum (2.4) is the bridge from Structure to Reactivity: bond polarity and IMFs determine how molecules interact, which is the starting point for every reaction mechanism in Reactivity 3. The linking questions in §8 connect bonding to the periodic table (Structure 3.1), to energy changes (Reactivity 1.2), and to the design of materials (Structure 2.4). Many of the 83 catalogued misconceptions live here — the octet-as-explanation, "polar bonds mean polar molecule", and "boiling breaks covalent bonds" are all addressed in §6.

Quantitative non-negotiables

Predict ionic charge from electron configuration; deduce formula and name of ionic compounds including polyatomic ions (NH₄⁺, OH⁻, NO₃⁻, SO₄²⁻, CO₃²⁻, PO₄³⁻).
Draw Lewis formulas for molecules and ions with up to four electron pairs per atom.
Apply VSEPR to predict electron-domain and molecular geometry for up to four domains.
Deduce bond polarity and molecular polarity from electronegativity values and geometry.
Calculate and interpret retardation factor R_F for chromatography.

Common failure modes

M-08 — Atoms bond because they want a full outer shell.
Confidence: verified
Why it’s wrong: Circular and causally backwards. This is Taber's 'full outer shells explanatory principle'.
Correction: Bonding is electrostatic attraction; full-shell configurations correlate with low energy, they don't cause bonding.
M-09 — Sodium gives its electron to chlorine, so Na⁺ and Cl⁻ are bonded to each other.
Confidence: verified
Why it’s wrong: The molecular framework for ionic bonding — treating NaCl as discrete ion pairs.
Correction: An ionic solid is a 3D lattice in which each Na⁺ is attracted to all surrounding Cl⁻ (six nearest neighbours in rock salt); there is no privileged partner and no memory of which electron came from where. NaCl is an empirical formula, not a molecular one.
M-10 — Electrons remember which atom they came from.
Confidence: verified
Why it’s wrong: The 'ownership of electrons' conception.
Correction: Electrons are indistinguishable; once transferred or shared, the electronic history is chemically irrelevant.
M-11 — A compound is either ionic or covalent.
Confidence: verified
Why it’s wrong: Bonding is not a binary classification.
Correction: Bonding is a continuum; use Δχ and χ̄ and the bonding triangle. Most bonds are polar covalent to some degree.
M-12 — If a molecule has polar bonds, the molecule is polar.
Confidence: verified
Why it’s wrong: Molecular polarity depends on bond polarity and geometry.
Correction: Symmetric arrangements cancel the dipoles. CO₂, CCl₄, BF₃ and SF₆ have polar bonds and zero net dipole.
M-13 — Boiling water breaks the covalent bonds in H₂O.
Confidence: verified
Why it’s wrong: Boiling breaks intermolecular hydrogen bonds, not covalent bonds.
Correction: The O–H covalent bonds survive — the product is still water molecules, just gaseous.
M-14 — Non-polar molecules have no intermolecular forces.
Confidence: verified
Why it’s wrong: London dispersion forces act between all particles.
Correction: London forces arise from instantaneous fluctuations in electron density; they strengthen with more electrons / greater polarisability. That's why Br₂ is liquid and I₂ solid.
M-15 — Hydrogen bonding happens whenever there's a hydrogen atom.
Confidence: verified
Why it’s wrong: Hydrogen bonding has specific requirements.
Correction: Requires H covalently bonded to N, O or F, and an accessible lone pair on an N, O or F of a neighbouring molecule. HCl, CH₄ and CH₃F do not hydrogen bond.
M-16 — A hydrogen bond is a type of covalent bond.
Confidence: verified
Why it’s wrong: A hydrogen bond is an intermolecular force, not a covalent bond.
Correction: It is an intermolecular force, roughly 5–10% of the strength of a typical covalent bond.
M-17 — Bigger molecule = stronger bonds = higher boiling point.
Confidence: verified
Why it’s wrong: Boiling point depends on intermolecular forces, not bond strength.
Correction: Say 'stronger London dispersion forces because more electrons', never 'stronger covalent bonds'.
M-18 — Resonance means the molecule flips back and forth between two structures.
Confidence: verified
Why it’s wrong: The real molecule is a single species with delocalised electrons.
Correction: The resonance forms are limitations of Lewis notation, not states the molecule visits. Benzene has six identical C–C bonds of intermediate length.
M-19 — All molecules with four things around the central atom are 109.5°.
Confidence: verified
Why it’s wrong: Lone pairs repel more strongly than bonding pairs.
Correction: Count electron domains including lone pairs; lone pairs repel more, so NH₃ is 107° and H₂O 104.5°.
M-20 — Every atom must obey the octet rule.
Confidence: verified
Why it’s wrong: Period 3 onwards can expand the octet; some species are electron-deficient or odd-electron.
Correction: SF₆, PCl₅ expand the octet; BF₃ is electron-deficient; radicals have an odd electron.
M-21 — Giant covalent substances have high melting points because covalent bonds are strong, and simple molecular ones have low melting points because covalent bonds are weak.
Confidence: verified
Why it’s wrong: The covalent bonds are strong in both.
Correction: The difference is that melting a molecular solid only overcomes intermolecular forces, whereas melting diamond or SiO₂ requires breaking covalent bonds throughout the lattice.
M-22 — Graphite conducts because the layers slide.
Confidence: verified
Why it’s wrong: Two separate properties conflated.
Correction: It conducts because each carbon uses only three of four valence electrons in σ bonds, leaving one delocalised electron per atom; it is slippery because the layers are held only by London forces.

What "HL standard" actually looks like

HL extends the covalent model to resonance and delocalisation (benzene), expanded octets (5–6 electron domains), formal charge, σ and π bonds, and sp/sp²/sp³ hybridisation. The metallic model gains delocalised d-electrons in transition elements. Materials adds condensation polymers (polyamides, polyesters).

See the command terms reference →

Linking questions

Official IB.The questions in this section are the IB’s own linking questions, reproduced verbatim from the guide. They are not this site’s editorial judgement — see section 3 for that distinction.

Linking questions are recorded at the sub-topic level only. The list below aggregates the linking questions of this topic’s sub-topics.

Official IB
How do intermolecular forces influence the type of mixture that forms between two substances?
Structure 1.1.1 → S2.2 The covalent model
(via S2.2)
Official IB
Why are alloys generally considered to be mixtures, even though they often contain metallic bonding?
Structure 1.1.1 → S2.3 The metallic model
(via S2.3)
Official IB
Why are some substances solid while others are fluid under standard conditions?
Structure 1.1.2 → S2.4 From models to materials
(via S2.4)
Official IB
Why are some changes of state endothermic and some exothermic?
Structure 1.1.2 → S2.1 The ionic model
(via S2.1)
Official IB
Under comparable conditions, why do some gases deviate more from ideal behaviour than others?
Structure 1.5.2 → S2.2 The covalent model
(via S2.2)
Official IB
How does the position of an element in the periodic table relate to the charge of its ion(s)?
Structure 2.1.1 → S3.1 The periodic table: Classification of elements
(via S2.1)
Official IB
How does the trend in successive ionization energies of transition elements explain their variable oxidation states?
Structure 2.1.1 → S1.3 Electron configurations
(via S2.1)
Official IB
Why is the formation of an ionic compound from its elements a redox reaction?
Structure 2.1.2 → R3.2 Electron transfer reactions
(via S2.1)
Official IB
How is formal charge used to predict the preferred structure of sulfate?
Structure 2.1.2 → S2.2 The covalent model
(via S2.2)
Official IB
Polyatomic anions are conjugate bases of common acids. What is the relationship between their stability and the conjugate acid's dissociation constant, Ka?
Structure 2.1.2 → R3.1 Proton transfer reactions
(via S2.1)
Official IB
How can lattice enthalpies and the bonding continuum explain the trend in melting points of metal chlorides across period 3?
Structure 2.1.3 → S3.1 The periodic table: Classification of elements
(via S2.1)
Official IB
Why do noble gases form covalent bonds less readily than other elements?
Structure 2.2.1 → S1.3 Electron configurations
(via S2.2)
Official IB
Why do ionic bonds only form between different elements while covalent bonds can form between atoms of the same element?
Structure 2.2.1 → S2.1 The ionic model
(via S2.1)
Official IB
How does the presence of double and triple bonds in molecules influence their reactivity?
Structure 2.2.2 → R2.2 How fast? The rate of chemical change
(via S2.2)
Official IB
Why do Lewis acid–base reactions lead to the formation of coordination bonds?
Structure 2.2.3 → R3.4 Electron-pair sharing reactions
(via S2.2)
Official IB
What properties of ionic compounds might be expected in compounds with polar covalent bonding?
Structure 2.2.5 → S2.1 The ionic model
(via S2.1)
Official IB
What features of a molecule make it "infrared (IR) active"?
Structure 2.2.6 → S3.2 Functional groups: Classification of organic compounds
(via S2.2)
Official IB
Why are silicon–silicon bonds generally weaker than carbon–carbon bonds?
Structure 2.2.7 → S3.1 The periodic table: Classification of elements
(via S2.2)
Official IB
To what extent can intermolecular forces explain the deviation of real gases from ideal behaviour?
Structure 2.2.8 → S1.5 Ideal gases
(via S2.2)
Official IB
How do the terms "bonds" and "forces" compare?
Structure 2.2.8 → S1.1 Introduction to the particulate nature of matter
(via S2.2)
Official IB
To what extent does a functional group determine the nature of the intermolecular forces?
Structure 2.2.9 → S3.2 Functional groups: Classification of organic compounds
(via S2.2)
Official IB
Why are oxygen and ozone dissociated by different wavelengths of light?
Structure 2.2.11 → S1.3 Electron configurations
(via S2.2)
Official IB
How does the resonance energy in benzene explain its relative unreactivity?
Structure 2.2.12 → R2.1 How much? The amount of chemical change
(via S2.2)
Official IB
What are the structural features of benzene that favour it undergoing electrophilic substitution reactions?
Structure 2.2.12 → R3.4 Electron-pair sharing reactions
(via S2.2)
Official IB
How does the ability of some atoms to expand their octet relate to their position in the periodic table?
Structure 2.2.13 → S3.1 The periodic table: Classification of elements
(via S2.2)
Official IB
What are the different assumptions made in the calculation of formal charge and of oxidation states for atoms in a species?
Structure 2.2.14 → S3.1 The periodic table: Classification of elements
(via S2.2)
Official IB
What experimental data demonstrate the physical properties of metals, and trends in these properties, in the periodic table?
Structure 2.3.1 → S3.1 The periodic table: Classification of elements
(via S2.3)
Official IB
What trends in reactivity of metals can be predicted from the periodic table?
Structure 2.3.1 → R3.2 Electron transfer reactions
(via S2.3)
Official IB
What are the features of metallic bonding that make it possible for metals to form alloys?
Structure 2.3.2 → S2.4 From models to materials
(via S2.4)
Official IBwording unverified against the guide
Why is the trend in melting points of metals across a period less evident across the d-block?
Structure 2.3.3 → S3.1 The periodic table: Classification of elements
(via S2.3)
Official IB
How do the trends in properties of period 3 oxides reflect the trend in their bonding?
Structure 2.4.1 → S3.1 The periodic table: Classification of elements
(via S2.4)
Official IB
What are the limitations of discrete bonding categories?
Structure 2.4.1 → S2.1 The ionic model
(via S2.1)
Official IB
Why do composites like reinforced concretes, which are made from ionic and covalently bonded components and steel bars, have unique properties?
Structure 2.4.2 → S2.1 The ionic model
(via S2.1)
Official IB
Why are alloys more correctly described as mixtures rather than as compounds?
Structure 2.4.3 → S1.1 Introduction to the particulate nature of matter
(via S2.4)
Official IB
What are the structural features of some plastics that make them biodegradable?
Structure 2.4.4 → S3.2 Functional groups: Classification of organic compounds
(via S2.4)
Official IB
What functional groups in molecules can enable them to act as monomers for addition reactions?
Structure 2.4.5 → S3.2 Functional groups: Classification of organic compounds
(via S2.4)
Official IB
Why is the atom economy 100% for an addition polymerization reaction?
Structure 2.4.5 → R2.1 How much? The amount of chemical change
(via S2.4)
Official IB
What functional groups in molecules can enable them to act as monomers for condensation reactions?
Structure 2.4.6 → S3.2 Functional groups: Classification of organic compounds
(via S2.4)
Official IB
How do differences in bonding explain the differences in the properties of metal and non-metal oxides?
Structure 3.1.5 → S2.1 The ionic model
(via S2.1)
Official IB
What are the arguments for and against including scandium as a transition element?
Structure 3.1.8 → S2.3 The metallic model
(via S2.3)
Official IB
What is unique about carbon that enables it to form more compounds than the sum of all the other elements' compounds?
Structure 3.2.1 → S2.2 The covalent model
(via S2.2)
Official IB
What are the advantages and disadvantages of different depictions of an organic compound (e.g. structural formula, stereochemical formula, skeletal formula, 3D models)?
Structure 3.2.1 → S2.2 The covalent model
(via S2.2)
Official IB
What is the nature of the reaction that occurs when two amino acids form a dipeptide?
Structure 3.2.2 → S2.4 From models to materials
(via S2.4)
Official IB
What is the influence of the carbon chain length, branching and the nature of the functional groups on intermolecular forces?
Structure 3.2.4 → S2.2 The covalent model
(via S2.2)
Official IB
How does the fact that there are only 3 isomers of dibromobenzene support the current model of benzene's structure?
Structure 3.2.6 → S2.2 The covalent model
(via S2.2)
Official IB
What features of a molecule determine whether it is IR active or not?
Structure 3.2.9 → S2.2 The covalent model
(via S2.2)
Official IB
Most combustion reactions are exothermic; how does the bonding in N₂ explain the fact that its combustion is endothermic?
Reactivity 1.1.3 → S2.2 The covalent model
(via S2.2)
Official IB
How would you expect bond enthalpy data to relate to bond length and polarity?
Reactivity 1.2.1 → S2.2 The covalent model
(via S2.2)
Official IB
Would you expect allotropes of an element, such as diamond and graphite, to have different ΔH⦵f values?
Reactivity 1.2.3 → S2.2 The covalent model
(via S2.2)
Official IB
What are the factors that influence the strength of lattice enthalpy in an ionic compound?
Reactivity 1.2.5 → S2.1 The ionic model
(via S2.1)
Official IB
The atom economy and the percentage yield both give important information about the "efficiency" of a chemical process. What other factors should be considered in this assessment?
Reactivity 2.1.5 → S2.4 From models to materials
(via S2.4)
Official IB
What are the conjugate acids of the polyatomic anions listed in Structure 2.1?
Reactivity 3.1.2 → S2.1 The ionic model
(via S2.1)
Official IB
The surface oxidation of metals is often known as corrosion. What are some of the consequences of this process?
Reactivity 3.2.1 → S2.3 The metallic model
(via S2.3)
Official IB
Under what conditions can ionic compounds act as electrolytes?
Reactivity 3.2.8 → S2.1 The ionic model
(via S2.1)
Official IB
How is it possible for a radical to be an atom, a molecule, a cation or an anion? Consider examples of each type.
Reactivity 3.3.1 → S2.1 The ionic model
(via S2.1)
Official IB
What is the reverse process of homolytic fission?
Reactivity 3.3.2 → S2.2 The covalent model
(via S2.2)
Official IB
Chlorine radicals released from CFCs are able to break down ozone, O₃, but not oxygen, O₂, in the stratosphere. What does this suggest about the relative strengths of bonds in the two allotropes?
Reactivity 3.3.2 → S2.2 The covalent model
(via S2.2)
Official IB
Why are alkenes sometimes known as "starting molecules" in industry?
Reactivity 3.4.5 → S2.4 From models to materials
(via S2.4)
Official IB
Do coordination bonds have any different properties from other covalent bonds?
Reactivity 3.4.7 → S2.2 The covalent model
(via S2.2)
Official IB
What are the features of benzene, C₆H₆, that make it not prone to undergo addition reactions, despite being highly unsaturated?
Reactivity 3.4.13 → S2.2 The covalent model
(via S2.2)

6 further official linking questions target Tool, Inquiry or Nature of Science strands and are not drawn as edges on the map:

Editorial The decision to surface these off-graph questions here is our editorial judgement — the IB does not prescribe where they should appear.

Official IB
What experimental data demonstrate the physical properties of ionic compounds?
Structure 2.1.3 → Tools 1
(via S2.1)
Official IB
What are some of the limitations of the octet rule?
Structure 2.2.1 → Nature of Science
(via S2.2)
Official IB
How useful is the VSEPR model at predicting molecular geometry?
Structure 2.2.4 → Nature of Science
(via S2.2)
Official IB
How can advances in technology lead to changes in scientific definitions, e.g. the updated IUPAC definition of the hydrogen bond?
Structure 2.2.8 → Nature of Science
(via S2.2)
Official IB
What experimental data demonstrate the physical properties of covalent substances?
Structure 2.2.9 → Tools 1
(via S2.2)
Official IB
How can a mixture be separated using paper chromatography or thin layer chromatography (TLC)?
Structure 2.2.10 → Tools 1
(via S2.2)

Bridge: GCSE → IB HL

Derived. These are the bridge items tagged to this topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.

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Open questions

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