Educerie · IB Diploma · Chemistry

Structure 1 — Models of the particulate nature of matter

Structure 1 is the foundation the rest of the course stands on: what an atom is, where its electrons sit, how chemists count particles they cannot see, and how gases behave. Nothing later in the course works if the mole is shaky.

S1.1 The particulate nature of matter

Matter is made of particles in constant motion. Temperature is a measure of their average kinetic energy — note average, since particles in a sample have a spread of energies. That spread reappears in Reactivity 2 as the Maxwell–Boltzmann distribution and explains why a small temperature rise can double a reaction rate.

Absolute temperature. T(K) = θ(°C) + 273.15. Every gas calculation uses kelvin, without exception. Substituting Celsius is the single most common error in this section, and it produces answers that are wrong by a large, obvious factor — which at least makes it easy to catch.

Concentration is amount of solute per volume of solution, mol dm⁻³. Distinguish it from amount (in mol) and mass (in g). Schemes are literal about this.

S1.2 The nuclear atom

An atom is a dense positive nucleus of protons and neutrons surrounded by electrons.

• Atomic number Z — the number of protons. It defines the element.
• Mass number A — protons plus neutrons.
• Isotopes — same Z, different numbers of neutrons. They therefore have identical chemical properties, because chemistry is determined by electron arrangement, but different physical properties such as density and rate of diffusion.

Relative atomic mass is the weighted mean of the isotope masses:

Ar = Σ (isotope mass × relative abundance) ÷ Σ (relative abundances)

If abundances are given as percentages summing to 100, the denominator is simply 100. If they are given as raw peak heights from a mass spectrum, you must divide by their total. Forgetting to do so is the standard error.

S1.3 Electron configurations

Electrons occupy discrete energy levels. The evidence is the emission spectrum: when excited electrons fall back to lower levels they emit photons of specific energies, giving discrete lines rather than a continuum. Those lines converge at higher frequency, because the energy levels themselves converge as they get further from the nucleus — a favourite explain question, and the convergence is the part students omit.

Sub-levels fill in the order 1s 2s 2p 3s 3p 4s 3d 4p. Note 4s fills before 3d, and note that 4s empties first on ionisation — so Fe²⁺ is [Ar]3d⁶, not [Ar]3d⁴4s².

Two exceptions are expected: chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹, because a half-full or full d sub-level is more stable than the alternative.

Ionisation energy is the energy to remove one mole of electrons from one mole of gaseous atoms. It provides the evidence for this whole model:

• Successive ionisation energies always increase, because each electron is pulled from an increasingly positive ion.
• A large jump between successive values marks the point where an electron is removed from a new, closer shell. The position of the jump reveals the number of outer electrons, and therefore the group.

Across a period, ionisation energy generally rises as nuclear charge increases with no extra shielding. The two dips — from group 2 to 13, and group 15 to 16 — are caused by a new sub-level starting and by electron pair repulsion respectively.

S1.4 Counting particles by mass: the mole

One mole is 6.02 × 10²³ particles, the Avogadro constant, L. The three relationships:

n = m / M          amount = mass ÷ molar mass
n = c × V          amount = concentration × volume   (V in dm³)
n = N / L          amount = number of particles ÷ Avogadro constant

Almost every stoichiometry question is the same three-step journey: convert to moles, use the equation ratio, convert back. Students who go straight from grams to grams get it wrong.

Two traps:

• V must be in dm³. A volume in cm³ must be divided by 1000. This is the most frequent single error in chemistry calculations.
• Specify the particle. "One mole of oxygen" is ambiguous — one mole of O atoms is 16 g, one mole of O₂ molecules is 32 g.

Percentage yield = (actual ÷ theoretical) × 100. Atom economy = (mass of desired product ÷ total mass of reactants) × 100. They are different measures: a reaction can have a high yield and a poor atom economy if it necessarily produces a large by-product.

Limiting reagent. Convert both reactants to moles, divide each by its coefficient, and the smaller value is limiting. Everything downstream is calculated from it.

S1.5 Ideal gases

An ideal gas is a model assuming particles have negligible volume and no intermolecular forces, and that collisions are perfectly elastic.

pV = nRT

with p in Pa, V in m³, T in K, and R = 8.31 J K⁻¹ mol⁻¹ from the data booklet. The unit conversions are where the marks go: 1 dm³ = 10⁻³ m³, 1 kPa = 10³ Pa.

The combined form p₁V₁/T₁ = p₂V₂/T₂ needs no conversion as long as units are consistent on both sides — a genuine time-saver when a question describes a change of conditions.

Real gases deviate from ideal behaviour at high pressure and low temperature, because under those conditions particle volume is no longer negligible relative to the container and intermolecular forces are no longer negligible relative to kinetic energy. Both halves of that sentence are needed for full marks.

What actually loses marks in this topic

1. Using °C instead of K in a gas calculation.
2. Leaving volume in cm³ when the formula needs dm³.
3. Not dividing by the total when abundances are given as peak heights, not percentages.
4. Writing Fe²⁺ as [Ar]3d⁴4s² — 4s electrons leave first.
5. Forgetting the chromium and copper exceptions.
6. Explaining a convergent emission spectrum without mentioning that the energy levels converge.
7. Omitting units, or giving significant figures inconsistent with the data.
8. Going from grams to grams without passing through moles.

Educerie · written from the published IB syllabus structure for Chemistry Structure 1, first assessment 2025. Original text; constants quoted are from the IB data booklet. Last reviewed 5 September 2026.