S3.1
The periodic table: Classification of elements
7 SL · +4 HL (SL 3.1.1-3.1.6) (HL 3.1.7-3.1.10)
Unverified
← View on the mapParent topic: Classification of matter
Guiding question
Guiding questionHow does the periodic table help us to predict patterns and trends in the properties of elements?
The one big idea
The periodic table organises elements by electron configuration. Period number equals the outer energy level; group number equals the valence electron count. Trends in radius, ionisation energy, electronegativity and oxidation state follow from this structure — and at HL, transition element chemistry reveals the richness of the d-block.
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.
- EditorialS1.2 The nuclear atomAtomic number determines an element's position in the periodic table (linking question S1.2.1).
- EditorialS1.3 Electron configurationsElectron configuration is the explanation; the periodic table is the pattern being explained (linking questions S1.3.3, S1.3.4).
- EditorialS1.4 Counting particles by mass: The moleAtoms increase in mass as their position descends in the periodic table; Ar and the mole relate to periodic trends (linking question S1.4.2).
What rests on this
Core concepts that must be mastered
- Periodic table structure(3.1.1, 3.1.2)The periodic table consists of periods, groups and blocks. A period is a horizontal row; a group is a vertical column. The four blocks — s, p, d, f — correspond to the sublevel being filled. You must identify the positions of metals, metalloids and non-metals: metals occupy the left and centre, non-metals the upper right, and metalloids (B, Si, Ge, As, Sb) sit along the staircase boundary. The period number equals the outer occupied energy level, and elements in a group share the same valence electron count. From an element's position you can deduce its electron configuration up to Z = 36, and vice versa. Know the group names: alkali metals (group 1), halogens (group 17), transition elements (groups 3–12), noble gases (group 18).
- Periodic trends(3.1.3)Periodicity means trends across a period and down a group. You must explain the periodicity of atomic radius (decreases across a period as nuclear charge rises and pulls the cloud in; increases down a group as a new shell is added), ionic radius (cations are smaller than their atoms, anions larger; trends follow atomic radius), ionisation energy (increases across a period, decreases down a group — with discontinuities, see HL below), electron affinity (generally increases across a period), and electronegativity (increases across a period, decreases down a group). The driving force for all of these is the balance between nuclear charge and electron shielding: across a period, protons are added while the outer electrons enter the same main level, so effective nuclear charge rises.
- Group trends and the metallic continuum(3.1.4, 3.1.5)Down a group, metallic character increases (group 1: Li → Na → K → Rb → Cs, increasingly reactive metals) and non-metallic character decreases (group 17: F → Cl → Br → I, decreasingly reactive non-metals). Group 1 metals react with water to form hydrogen and the hydroxide: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Group 17 elements react with halide ions in displacement reactions: Cl₂ oxidises Br⁻ and I⁻ but not F⁻. Metallic and non-metallic properties are a continuum: across period 3, the oxides move from basic (Na₂O, MgO) through amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₃). You must deduce equations for reactions of group 1 and 2 metal oxides with water (Na₂O + H₂O → 2NaOH), and of carbon and sulfur oxides with water (CO₂ + H₂O → H₂CO₃, SO₃ + H₂O → H₂SO₄). Gaseous non-metal oxides cause acid rain and ocean acidification.
- Oxidation state(3.1.6)Oxidation state is the charge an atom would have if the compound were ionic. It is assigned by electronegativity: the more electronegative element takes the negative value. Rules: the oxidation state of an element is zero; H is +1 (except −1 in metal hydrides like NaH); O is −2 (except −1 in peroxides like H₂O₂). Use sign-then-Arabic-numeral notation (+2, −1). "Oxidation number" and "oxidation state" are interchangeable. Oxyanions may be named with Roman numerals (iron(III) sulfate) but generic names (nitrate, sulfate) are acceptable. Oxidation state is not the same as ionic charge: in SO₄²⁻, sulfur is +6 by bookkeeping, but there is no actual S⁶⁺ ion.
- HL: IE discontinuities, transition elements and complex colour(3.1.7–3.1.10)Discontinuities in the trend of increasing first IE across a period provide evidence for the existence of energy sublevels. The group 2 → 13 dip (e.g. Be → B) occurs because the group 13 outer electron enters a higher-energy p orbital, easier to remove. The group 15 → 16 dip (e.g. N → O) occurs because the group 16 electron is the first to pair up in a p orbital, and repulsion between the paired electrons makes it easier to remove. The explanation must be based on the energy of the electron removed, not on "special stability" of filled or half-filled sublevels — that framing is explicitly rejected by the guide. Transition elements have incomplete d-sublevels giving characteristic properties: variable oxidation state, high melting points, magnetic properties, catalytic properties, coloured compounds, and complex ion formation with ligands. Types of magnetism are not assessed. Variable oxidation states arise because successive ionisation energies are close in value — the 3d and 4s electrons are similar enough in energy that removing different numbers is feasible. You must deduce electron configurations of ions of first-row transition elements: remember to remove 4s electrons before 3d (Fe is [Ar]3d⁶4s², Fe²⁺ is [Ar]3d⁶, Fe³⁺ is [Ar]3d⁵). Complexes are coloured because light is absorbed when an electron is promoted between orbitals in the split d-sublevels; the colour absorbed is complementary to the colour observed. Apply the colour wheel (data booklet) and c = λf to deduce the absorbed and observed wavelengths.
- How this sub-topic connectsThe periodic table is the organising map of chemistry. Electron configuration (S1.3) explains position, and position predicts bonding (S2), reactivity (R2.2, R3.1, R3.2) and properties (S2.4). Oxidation state is the language of redox (R3.2). At HL, transition element chemistry connects to complex ion formation (R3.4: ligands and coordination bonds) and to catalysis (R2.2: transition elements catalyse by changing oxidation state). The colour of complexes links to spectroscopic techniques (S3.2 HL: colorimetry, NMR). The periodic trends in metallic character connect to the bonding continuum (S2.4).
Quantitative non-negotiables
- Deduce electron configuration up to Z = 36 from an element's position and vice versa.
- Deduce oxidation states in an ion or compound, including hydrogen in metal hydrides (−1) and oxygen in peroxides (−1).
- Write equations for reactions of group 1 metals with water and group 17 elements with halide ions.
- Write equations for reactions of group 1 and 2 metal oxides, carbon and sulfur oxides with water.
- At HL: deduce electron configurations of ions of first-row transition elements.HL
- At HL: apply the colour wheel to deduce wavelengths/frequencies absorbed and observed for transition element complexes.HL
Common failure modes
M-04 — Ionisation energy increases smoothly across a period.
Confidence: verified
Why it’s wrong: There are dips at group 2→13 and group 15→16.
Correction: The group 13 dip is because the outer electron is in a higher-energy p orbital; the group 16 dip is because of repulsion between the first pair of paired p electrons.
M-07 — The atom gets bigger across a period because you're adding electrons.
Confidence: verified
Why it’s wrong: Atomic radius decreases across a period.
Correction: Electrons are added to the same main energy level while nuclear charge increases, so effective nuclear charge rises and the electron cloud is pulled in.
M-62 — Oxidation state is the same as ionic charge.
Confidence: verified
Why it’s wrong: They coincide for monatomic ions but not otherwise.
Correction: Oxidation state is a formal bookkeeping value assigned by electronegativity even in purely covalent species (S in SO₄²⁻ is +6, though there's no S⁶⁺ ion).
What "HL standard" actually looks like
HLHL adds ionisation-energy discontinuities as evidence for sublevels, transition element characteristic properties (variable oxidation state, coloured complexes, catalysis, magnetism), electron configurations of first-row transition element ions, and the colour of complexes from d-orbital splitting. +4 HL hours.
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.
- Official IB
How does the atomic number relate to the position of an element in the periodic table?
- Official IB
How does an element's highest main energy level relate to its period number in the periodic table?
- Official IB
What is the relationship between energy sublevels and the block nature of the periodic table?
- Official IB
How does the trend in IE values across a period and down a group explain the trends in properties of metals and non-metals?
- Official IB
How do patterns of successive IEs of transition elements help to explain the variable oxidation states of these elements?
- Official IB
Atoms increase in mass as their position descends in the periodic table. What properties might be related to this trend?
- Official IB
How does the position of an element in the periodic table relate to the charge of its ion(s)?
- Official IB
How can lattice enthalpies and the bonding continuum explain the trend in melting points of metal chlorides across period 3?
- Official IB
Why are silicon–silicon bonds generally weaker than carbon–carbon bonds?
- Official IB
How does the ability of some atoms to expand their octet relate to their position in the periodic table?
- Official IB
What are the different assumptions made in the calculation of formal charge and of oxidation states for atoms in a species?
- Official IB
What experimental data demonstrate the physical properties of metals, and trends in these properties, in the periodic table?
- Official IBwording unverified against the guide
Why is the trend in melting points of metals across a period less evident across the d-block?
- Official IB
How do the trends in properties of period 3 oxides reflect the trend in their bonding?
- Official IB
How has the organization of elements in the periodic table facilitated the discovery of new elements?
- Official IB
How do differences in bonding explain the differences in the properties of metal and non-metal oxides?
- Official IB
How can oxidation states be used to analyse redox reactions?
- Official IB
What are the arguments for and against including scandium as a transition element?
- Official IB
What is the nature of the reaction between transition element ions and ligands in forming complex ions?
- Official IB
What are the features of transition elements that make them useful as catalysts?
- Official IB
What is the periodic trend in the acid–base properties of metal and non-metal oxides?
- Official IB
Why does the release of oxides of nitrogen and sulfur into the atmosphere cause acid rain?
- Official IB
What are the advantages and limitations of using oxidation states to track redox changes?
- Official IB
Why does metal reactivity increase, and non-metal reactivity decrease, down the main groups of the periodic table?
- Official IB
Why is there a colour change when an alcohol is oxidized by a transition element compound?
- Official IBwording unverified against the guide
How can oxidation states be used to show that the following molecules are given in increasing order of oxidation: CH₄, CH₃OH, HCHO, HCOOH, CO₂?
- Official IB
Why is the iodide ion a better leaving group than the chloride ion?
2 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
Why are simulations often used in exploring the trends in chemical reactivity of group 1 and group 17 elements?
- Official IB
How can colorimetry or spectrophotometry be used to calculate the concentration of a solution of coloured ions?
Bridge: GCSE → IB HL
Derived. These are the bridge items tagged to this sub-topic— places where the GCSE model gets redefined, genuinely new territory, or carry-over strengths. The tagging is this site’s editorial judgement.