IB DP Chemistry Structure 1: Models of the Particulate Nature of Matter

This guide covers Structure 1 — Models of the Particulate Nature of Matter, the first content component of IB Diploma Programme Chemistry (17 guided hours SL, 21 HL), first assessment 2025. For the full six-component syllabus map, see the IB DP Chemistry syllabus guide.

Where this fits in the syllabus

Structure 1 builds the particle-level model that every later Structure and Reactivity component assumes: what matter is made of, how atoms are arranged and counted, and how gases behave. The mole (1.4) is the sub-topic most later content leans on directly — stoichiometric calculations throughout Reactivity 1 and 2 assume fluent mole arithmetic, so treat it as a skill to over-learn rather than a topic revised once and set aside. Structure 1 is mostly SL-depth content shared between SL and HL students, but it carries 4 hours of HL-only extension: one additional hour in 1.2 on interpreting mass spectra, and three additional hours in 1.3 on first ionisation energy (its trends and discontinuities, and its value from the convergence limit of spectral lines) and successive ionisation energies — do not assume the whole component is SL-only just because most of it is shared.

Syllabus coverage

IB DP CHEMISTRY — STRUCTURE 1: MODELS OF THE PARTICULATE NATURE OF MATTER

• 1.1 Introduction to the particulate nature of matter — the three classical states and particle-kinetic state changes; physical versus chemical change; pure substances versus mixtures
• 1.2 The nuclear atom — protons, neutrons, electrons; atomic number and mass number; isotopes and relative atomic mass; ion formation
• 1.3 Electron configurations — emission spectra as evidence of quantised energy levels; energy levels, sub-levels (s, p, d and f) and orbitals; the aufbau principle, Pauli exclusion principle and Hund’s rule together; the chromium and copper exceptions; at HL only (1.3.6-1.3.7), the trends and discontinuities in first ionisation energy across a period and down a group, first ionisation energy from the convergence limit of the emission spectrum, and successive ionisation energies (the periodicity of ionisation energy is also SL content, in Structure 3.1.3)
• 1.4 Counting particles by mass: the mole — the mole and the Avogadro constant; molar mass; empirical and molecular formulae; concentration
• 1.5 Ideal gases — the ideal gas model’s assumptions and where real gases deviate from it; the molar volume of an ideal gas, a constant at a specific temperature and pressure; the combined and ideal gas equations (pV = nRT)

How to approach it

For 1.2, keep neutron count precise: neutrons equal mass number minus atomic number, not mass number alone — a common source of confusion. For 1.3, practise writing full electron configurations for unfamiliar elements under timed conditions rather than memorising a fixed list, since Paper 1 multiple-choice questions frequently test the underlying aufbau logic with elements not drilled in class; remember that transition-metal configurations must be re-sorted into energy-level order after filling, which is a genuinely separate skill from the filling order itself, and that chromium and copper are named exceptions where one electron moves from 4s into 3d for extra stability. For 1.4, treat the mass-to-moles-to-particles chain as a single reusable method rather than three separate calculation types. For 1.5, always convert Celsius to kelvin before substituting into pV = nRT — forgetting this conversion is a very common, easily avoidable arithmetic slip that produces a plausible-looking but incorrect answer.

Worked example: mass to moles to particles

Given 4.40 g of CO₂ (M = 44.01 g mol⁻¹), find the number of molecules present.

n = m / M = 4.40 / 44.01 = 0.0999... mol

number of molecules = n x Avogadro constant
                     = 0.0999... x 6.02 x 10^23
                     ~= 6.02 x 10^22 molecules

The same three-step chain (mass → moles → particles, or the reverse) underlies most 1.4 exam questions; the only thing that changes is which quantity is being solved for.

Worked example: emission spectra as evidence for quantised energy levels

A question asks why a line emission spectrum, rather than a continuous band of colour, is evidence that electron energy levels are quantised.

Excitation:      heating (or another energy input) lets an atom's
                 electrons absorb energy and jump to a higher energy
                 level

Emission:        when an electron falls back down to a lower level,
                 it emits a photon whose energy -- and therefore
                 frequency -- equals the energy gap between the two
                 levels

Observation:     because only specific, discrete energy gaps exist
                 between levels, only specific, discrete frequencies
                 of light are emitted -- producing separate coloured
                 lines rather than a continuous spread of colour

Conclusion:      if energy levels were continuous rather than
                 quantised, electrons could fall through any energy
                 gap, producing a continuous spectrum instead of
                 discrete lines -- the existence of separate lines is
                 therefore direct evidence for quantisation

For the hydrogen spectrum specifically, the lines converge (get closer together) at higher frequency, and this convergence limit corresponds to the electron being removed from the atom entirely — the HL route to calculating ionisation energy directly from spectral data.

HL only: mass spectra and ionisation energies

These are the HL-only extensions named above: the extensions to 1.2 (one additional hour) and 1.3 (three additional hours).

• Interpreting a mass spectrum of an element (1.2, HL). Each peak is one isotope. The m/z value identifies the isotope (for an ion with a 1+ charge, m/z equals the isotope’s relative mass) and the peak height, or percentage abundance, gives its relative abundance. Relative atomic mass is the abundance-weighted mean:

  Peaks: m/z 24 (79%), m/z 25 (10%), m/z 26 (11%)   -> magnesium
  Ar = (24 x 79 + 25 x 10 + 26 x 11) / 100 = 2432 / 100 = 24.32

• Trends and discontinuities in first ionisation energy (1.3, HL). First ionisation energy generally increases across a period (greater nuclear charge, similar shielding) and decreases down a group (outer electron further from the nucleus, more shielding). The discontinuities come from sub-levels: it falls from group 2 to group 13 (the electron is removed from a higher-energy p sub-level) and from group 15 to group 16 (the electron is removed from a doubly occupied p orbital, where electron-electron repulsion makes it easier to remove).

• Ionisation energy from the convergence limit (1.3, HL). At the convergence limit of the hydrogen emission spectrum the electron is removed completely, so the photon energy there is the ionisation energy of one atom: E = hν (or E = hc/λ if a wavelength is given). Multiply by the Avogadro constant for a molar value:

  convergence frequency = 3.29 x 10^15 Hz
  E(one atom) = 6.63 x 10^-34 J s x 3.29 x 10^15 Hz = 2.18 x 10^-18 J
  IE = 2.18 x 10^-18 J x 6.02 x 10^23 mol^-1 = 1.31 x 10^6 J mol^-1 (1310 kJ mol^-1 to 3 s.f.)

• Successive ionisation energies (1.3, HL). Each successive ionisation energy is larger than the one before, because each electron is removed from an increasingly positive ion. A large jump appears when the next electron must come from a new shell closer to the nucleus, with less shielding. The number of electrons removed before the first large jump is the number of outer-shell electrons, which gives the group:

  aluminium, kJ mol^-1:  IE1 578   IE2 1817   IE3 2745   IE4 11577
  largest jump between IE3 and IE4 -> 3 outer electrons -> group 13

Common mistakes

Confusing mass number with atomic number when working out neutron count. Writing an electron configuration in filling-order sequence but forgetting to re-sort into energy-level order for transition metals. Treating relative atomic mass as the mass of the most common isotope, rather than the isotopic-abundance-weighted average the syllabus defines it as. Confusing the molar volume at STP (the value this course and its data booklet supply) with the RTP value used in some other courses. Forgetting to convert Celsius to kelvin before substituting into the ideal gas equation.

Quick revision checklist

• Practise neutron-count calculations (A − Z) until automatic.
• Write electron configurations for unfamiliar elements, including correctly re-sorting transition-metal configurations into energy-level order.
• Know the chromium and copper exceptions and why they occur.
• Treat the mole as an over-learned skill, not a one-off revision topic, given how heavily later components depend on it.
• Always convert to kelvin before using pV = nRT.
• (HL) Read isotope identity and abundance from a mass spectrum and calculate Ar from it.
• (HL) Calculate an ionisation energy from the convergence-limit frequency or wavelength (E = hν, then × Avogadro constant), and deduce a group from successive ionisation energies.

Official syllabus

International Baccalaureate Organization, Chemistry guide, Diploma Programme, first assessment 2025 (published February 2023, updated May 2023) — copy consulted via a school-hosted mirror, not ibo.org. The same source is already cited by the full syllabus guide. Verified 2026-09-06.