Overview

What you'll learn

Describe the three states of matter. Particle arrangement, energy and movement in solids, liquids and gases — plus how heating drives the changes of state.

Explain diffusion. Use kinetic particle theory to explain why gases mix — and why lighter particles diffuse faster.

Read atomic structure. Use proton number, nucleon number and electron configuration to work out any atom or ion.

Distinguish atoms, ions and isotopes. Tell them apart from their protons, neutrons and electrons, and predict ion charge from group number.

Tutor's Insight

"The chapter everything else builds on."
Close to 100% of the syllabus leans on this topic, and it shows up in both Paper 1 and Paper 2. If your atomic structure is shaky, bonding falls apart — then energetics, then rates, then the Periodic Table all stop making sense. It's only two subtopics, and you only need the first 20 elements cold. Get them clean now and the rest of the syllabus clicks into place.

2.1 Kinetic Particle Theory

2.1 Kinetic Particle Theory

Syllabus asks
  • Describe the three states of matter and how they interconvert — in terms of particles and the energy involved.
  • Explain diffusion in terms of moving particles, and how molecular mass and temperature change its rate.

2.1 Kinetic Particle Theory

Solid, liquid, gas — it's all about arrangement

Solid
Fixed and orderly
Particles very closely packed in a regular pattern. They vibrate about fixed positions. Strong forces. Fixed shape and volume.
Liquid
Close, but free to move
Particles still close but disordered. They slide over one another. Fixed volume. Takes the shape of its container.
Gas
Far apart and fast
Particles far apart and random, moving at high speed. Weak forces. Fills its container completely.
Heating adds energy — particles move more and forces break: solid → liquid → gas. Cooling reverses it.

Free Notes · O-Level Pure Chemistry

Read the full chapter

Changes of state, diffusion, subatomic particles, electron configuration, ions, isotopes and 4 worked exam questions.

2.1 Kinetic Particle Theory

Why the temperature pauses mid-change

The heating curve
Two flat steps
Heat a solid: the temperature climbs, then holds steady while it melts, climbs again, then holds steady while it boils.
Why it happens
Energy breaks forces — not raising heat
During melting and boiling, the energy you put in goes into breaking the forces between particles — not into raising kinetic energy. So the temperature won't budge until the change of state is finished.
Cooling is the mirror image — energy is released, and the temperature pauses again as gas condenses and liquid freezes.

2.1 Kinetic Particle Theory

Particles spread out on their own

The evidence · ammonia meets hydrogen chloride

Diffusion: particles spread from high concentration to low. In the classic glass-tube experiment, ammonia and hydrogen chloride diffuse toward each other. A white ring of ammonium chloride forms closer to the HCl end — because lighter NH₃ molecules diffuse faster.

Factor 01
Lighter particles, faster
The smaller the relative molecular mass, the faster a gas diffuses.
Factor 02
Hotter surroundings, faster
A higher temperature gives particles more kinetic energy, so they diffuse faster.

2.2 Atomic Structure

2.2 Atomic Structure

Syllabus asks
  • Describe the structure of the atom — protons, neutrons, electrons — and use proton and nucleon numbers and nuclide notation.
  • Deduce the particles in atoms and ions, and define isotopes.

2.2 Atomic Structure

Three particles make an atom

Charge +1
Proton
Relative mass 1. Sits in the nucleus. The number of protons is what defines the element.
Charge 0
Neutron
Relative mass 1. Sits in the nucleus next to the protons.
Charge −1
Electron
Almost massless (1/1840 of a proton). Sits in shells around the nucleus.
An atom is neutral — equal protons and electrons. Protons + neutrons = nucleons.

2.2 Atomic Structure

Every atom carries two numbers

Reading an atom
Proton number & nucleon number
Proton (atomic) number = number of protons, which equals the number of electrons. Nucleon (mass) number = protons + neutrons. The nuclide symbol writes the nucleon number above the proton number.
Deducing the particles
Subtract to find the rest
Neutrons = nucleon number − proton number. For an ion, adjust the electron count for the charge — a 2+ ion has 2 fewer electrons than protons.
e.g. magnesium-24, proton number 12 → 12 protons, 12 electrons, 12 neutrons.

2.2 Atomic Structure

Electron shells map the Periodic Table

The filling rule
2, then 8, then 8
For the first 20 elements: shell 1 holds up to 2 electrons, shells 2 and 3 hold up to 8 each. Fill from the inside out.
Down a group
Shells → Period
Number of electron shells = the Period number.
Across a period
Valence → Group
Number of outer-shell (valence) electrons = the Group number.
e.g. argon, 2.8.8 — three shells, eight valence electrons → Period 3, Group 18.

2.2 Atomic Structure

Gain or lose electrons — and you've made an ion

Cation — positive
Metals lose electrons
A metal atom loses its valence electrons to reach a noble-gas configuration. More protons than electrons → positive charge. e.g. Na → Na⁺ + e⁻
The cation is smaller than the atom — it has lost a whole shell.
Anion — negative
Non-metals gain electrons
A non-metal atom gains electrons to fill its valence shell. More electrons than protons → negative charge. e.g. O + 2e⁻ → O²⁻
The anion is a similar size to the atom — the electrons go into the same shell.
Either way — the ion ends up with the electron arrangement of the nearest noble gas.

2.2 Atomic Structure

Same element, different mass

What isotopes are
Same protons, different neutrons
Isotopes: same element, same number of protons and electrons. Different number of neutrons — and so a different nucleon number.
Why it matters
Same chemistry, different physics
Same valence electrons → same chemistry. Isotopes react the same way. Different mass → slightly different physical properties (density, m.p., b.p.).
Relative atomic mass is the weighted average across an element's isotopes — which is why it's rarely a whole number.

2.2 Atomic Structure

Ion or isotope — what actually changes?

Compared to the neutral atom An ion An isotope
Protons Same Same
Electrons Different Same
Neutrons Same Different
Nucleon number Same Different
Chemical properties Different Same
Physical properties Same Different
The pattern: ions change the electrons. Isotopes change the neutrons. Everything else follows.

Practice

Exam-style questions

Question 01
The Question

Counting the particles

A particle has a proton number of 17 and a nucleon number of 35, and carries a charge of 1−. How many protons, neutrons and electrons does it contain?


Hint: start from the neutral atom. Then adjust the electrons for the charge.

Worked Answer
17 protons · 18 neutrons · 18 electrons
  1. Protons = proton number = 17.
  2. Neutrons = nucleon number − proton number = 35 − 17 = 18.
  3. Electrons = 17 in the neutral atom; the 1− charge means one extra electron → 18.
Question 02
The Question
MCQ

Same element?

Two particles, X and Y, are described below. Which statement is correct?

  • X: 12 protons, 12 neutrons, 12 electrons
  • Y: 12 protons, 14 neutrons, 12 electrons

A They are ions of different elements    B They are isotopes of the same element    C Y is an ion of X    D They have different chemical properties

Worked Answer
B — isotopes of the same element
  1. Both have 12 protons, so both are the same element (magnesium) — rules out A.
  2. Both are neutral (protons = electrons), so neither is an ion — rules out C.
  3. They differ only in neutrons (12 vs 14), so they are isotopes. Same valence electrons means the same chemistry — rules out D.
  4. Answer: B.
Question 03
The Question
MCQ

Diffusion race

Cotton wool soaked in ammonia and cotton wool soaked in hydrogen chloride are placed at opposite ends of a glass tube at the same time. Where does the white ring of ammonium chloride first appear?

  • A  Exactly in the middle of the tube
  • B  Closer to the ammonia end
  • C  Closer to the hydrogen chloride end
  • D  At both ends simultaneously
Worked Answer
C — closer to the HCl end
  1. Rate of diffusion depends on relative molecular mass: the lighter the particle, the faster it diffuses.
  2. NH₃ (Mᵣ = 17) is lighter than HCl (Mᵣ = 36.5), so NH₃ diffuses faster and travels further before they meet.
  3. The gases meet nearer the slower HCl end, so the white ring forms closer to the hydrogen chloride end.
  4. Answer: C.
Question 04
The Question
Structured

Configuration and position

An atom has 17 electrons.

  1. Write its electron configuration.
  2. State its Period and Group in the Periodic Table.
  3. Predict the charge on the ion it forms, and explain why.
Worked Answer
2.8.7 · Period 3, Group 17 · forms 1−
  1. Fill from the inside out: 2, then 8, then 7 → 2.8.7.
  2. Three shells → Period 3. Seven valence electrons → Group 17.
  3. It is a non-metal with 7 valence electrons, so it gains 1 electron to reach a full outer shell (the argon configuration 2.8.8), forming a 1− ion.

Frequently Asked Questions

The Particulate Nature of Matter — FAQ

What is The Particulate Nature of Matter in the O-Level Chemistry syllabus (6092)?
The Particulate Nature of Matter (Topic 2 of Syllabus 6092) has two subtopics: 2.1 Kinetic Particle Theory — the arrangement, energy and movement of particles in solids, liquids and gases, changes of state and diffusion; and 2.2 Atomic Structure — protons, neutrons and electrons, proton and nucleon numbers, electron configuration, ions and isotopes. Because bonding, energetics, rates and the Periodic Table all build on it, this topic underpins the rest of the course.
How do particles differ in a solid, a liquid and a gas?
In a solid, particles are very closely packed in a regular pattern, vibrate about fixed positions and are held by strong forces — fixed shape and volume. In a liquid, particles are still close but disordered and slide over one another — fixed volume but takes the shape of its container. In a gas, particles are far apart and random, move at high speed with weak forces — a gas fills its container completely.
Why does temperature stay constant during melting and boiling?
During melting and boiling, the energy supplied goes into breaking the forces of attraction between particles rather than into increasing their kinetic energy. Because the average kinetic energy does not rise, the temperature stays constant until the change of state is complete. On a heating curve this appears as two flat steps.
What affects the rate of diffusion of a gas?
Two factors. The smaller the relative molecular mass, the faster the gas diffuses — lighter particles move faster. A higher temperature also speeds diffusion because particles gain more kinetic energy. In the classic ammonia and hydrogen chloride experiment, the white ring of ammonium chloride forms nearer the HCl end because the lighter NH₃ molecules diffuse faster.
How do you work out the number of protons, neutrons and electrons in an atom or ion?
Protons = proton (atomic) number. Neutrons = nucleon (mass) number − proton number. Electrons = protons in a neutral atom. For an ion, adjust the electrons for the charge: a 2+ ion has 2 fewer electrons than protons, and a 1− ion has 1 more electron than protons.
What is the difference between an ion and an isotope?
An ion is a charged particle formed when an atom gains or loses electrons — same protons and neutrons as the atom but a different number of electrons, so its chemical properties change. An isotope has the same number of protons and electrons but a different number of neutrons, so it has the same chemistry but slightly different physical properties. In short: ions change the electrons; isotopes change the neutrons.

O-Level Pure Chemistry  ·  Syllabus 6092  ·  Topic 2 of 12  ·  © 2026 Overmugged. For personal study use only.