Overview
What you’ll learn
- Describe the solid, liquid and gaseous states and their interconversion using the kinetic particle theory and the energy changes involved.
- State the relative charges and approximate relative masses of a proton, a neutron and an electron.
- Describe the structure of an atom as a nucleus of protons and neutrons with electrons arranged in shells.
- Define proton (atomic) number and nucleon (mass) number, and interpret nuclide notations.
- Define isotopes and deduce the numbers of protons, neutrons and electrons in atoms and ions.
Key points
- 1Solid: particles touch in a regular lattice and only vibrate about fixed positions. Liquid: touching but irregular, particles slide past one another. Gas: far apart, fast and random.
- 2Melting, boiling and sublimation take in energy; solidifying and condensation give energy out.
- 3During a change of state the temperature stays constant: the energy changes the arrangement, not the average speed.
- 4A proton: charge +1, mass ≈ 1. A neutron: charge 0, mass ≈ 1. An electron: charge −1, mass ≈ 1/1840 (negligible).
- 5Proton number = protons; nucleon number = protons + neutrons; a neutral atom has electrons = protons.
- 6Nuclide notation puts the nucleon number top-left and the proton number bottom-left of the symbol.
- 7Isotopes have the same proton number but different nucleon numbers — they differ only in neutrons, so their chemical properties are identical.
- 8In an ion, protons never change: electrons are lost from cations and gained by anions.
Notes · 1 of 5
The three states of matter
Arrangement, separation and motion explain every property of a state.
The kinetic particle theory says all matter is made of tiny particles in constant motion. The three states differ in three things examiners always want named together: the arrangement of the particles, their separation, and their motion.
| State | Arrangement | Separation | Motion |
|---|---|---|---|
| Solid | regular lattice | touching | vibrate about fixed positions |
| Liquid | irregular | touching | move around, sliding past each other |
| Gas | random | far apart | fast, random, straight lines between collisions |
The properties follow straight from the table. Solids keep their shape (fixed positions) and cannot be compressed (no space). Liquids flow (particles slide) but keep a fixed volume (still touching). Gases fill their container and are easily compressed because the particles are mostly empty space.
Notes · 2 of 5
Changes of state and energy
Melting and boiling take in energy; solidifying and condensation give it out.
- Melting (solid → liquid) and boiling (liquid → gas): particles gain energy from the surroundings.
- Freezing (solidification) and condensation: particles lose energy to the surroundings.
- Sublimation skips the liquid: solid → gas directly (and the reverse, deposition).
On a heating curve, the temperature rises while one state is heated but holds a flat plateau while the substance changes state. Why? During melting, every joule added is used to pull particles out of their fixed lattice positions — it increases their potential energy, not their average speed. Temperature measures average kinetic energy, so it holds steady until melting is complete. The same logic gives a plateau at the boiling point.
Notes · 3 of 5
Protons, neutrons and electrons
Three particles, two of them heavy and one of them negligible.
| Particle | Relative charge | Approximate relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1/1840 (negligible) |
Almost all the mass sits in the nucleus — protons and neutrons together are called nucleons. The electrons zoom around the nucleus in shells (energy levels). For the first twenty elements the shells fill 2, then 8, then 8: sodium (11 electrons) is 2.8.1, calcium (20) is 2.8.8.2.
Because protons and neutrons have nearly equal masses and electrons are negligible, an atom's mass lives in the nucleus while its size and chemistry live in the electron shells. That split explains later chapters: nuclear numbers (this chapter) versus electron arrangements (bonding, Chapter 3).
Notes · 4 of 5
Proton number, nucleon number and nuclide notation
Two numbers pin an atom down completely.
- Proton number (Z): the number of protons — it fixes the element's identity and its place in the Periodic Table.
- Nucleon number (A): protons + neutrons — the mass number.
- In a neutral atom: electrons = protons.
Nuclide notation writes the nucleon number as a superscript before the symbol and the proton number as a subscript: carbon-14 is C with 14 top-left and 6 bottom-left. Reading any nuclide: protons = bottom number, neutrons = top − bottom, electrons = protons (for atoms).
Notes · 5 of 5
Isotopes, atoms and ions
Isotopes differ in neutrons; ions differ in electrons.
Isotopes are atoms of the same element (same proton number) with different nucleon numbers — different numbers of neutrons. Carbon-12 and carbon-14 both have 6 protons, but 6 versus 8 neutrons. Because isotopes have the same electron arrangement, they have identical chemical properties; their different masses show up in physical properties such as density and in nuclear behaviour.
An ion forms when an atom gains or loses electrons. Metal atoms lose electrons to form positive ions (cations): magnesium, 2.8.2, loses two electrons to become Mg²⁺ with 10 electrons. Non-metals gain electrons to form negative ions (anions): sulfur, 2.8.6, gains two to become S²⁻ with 18 electrons. The nucleus never changes in ordinary chemistry — protons and neutrons stay put.
Revise
Key terms
The words examiners expect you to use precisely.
- kinetic particle theory
- The model that all matter is made of tiny moving particles whose arrangement, separation and motion explain the states of matter.
- sublimation
- A change of state directly from solid to gas (or gas to solid) without passing through the liquid state.
- nucleon
- A particle in the nucleus — a proton or a neutron.
- shell
- An energy level holding electrons at a fixed set of distances from the nucleus; the first three hold 2, 8 and 8 electrons for the first twenty elements.
- proton number (Z)
- The number of protons in a nucleus; it identifies the element.
- nucleon number (A)
- The total number of protons and neutrons in a nucleus; also called the mass number.
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- nuclide notation
- Writing an atom as its symbol with the nucleon number top-left and the proton number bottom-left.
- isotope
- One of two or more atoms of the same element with the same proton number but different nucleon numbers.
- cation
- A positively charged ion, formed when an atom loses electrons.
- anion
- A negatively charged ion, formed when an atom gains electrons.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Saying gas particles "expand" when heated.
Individual particles do not grow; the space between them grows because they move faster.
Instead: Heating makes particles move faster and spread further apart; the sample expands because of the extra separation.
Mistake: Saying the temperature rises while a substance melts.
During melting the energy breaks the fixed-position arrangement, not the average speed.
Instead: Temperature stays constant on the plateau while the state changes.
Show 9 more mistakesShow fewer mistakes
Mistake: Confusing boiling with evaporation in energy terms.
Students mix up "fast particles escape" with the bulk process at a fixed temperature.
Instead: Both are liquid → gas and take in energy; boiling happens throughout the liquid at one temperature while evaporation happens only at the surface at any temperature.
Mistake: Giving the electron a relative mass of 1.
Copying the proton row; the electron is about 1/1840 — effectively zero on this scale.
Instead: Proton ≈ 1, neutron ≈ 1, electron ≈ 1/1840 (negligible).
Mistake: Reading the nucleon number as the number of neutrons.
The top-left number is protons plus neutrons.
Instead: Neutrons = nucleon number − proton number.
Mistake: Thinking isotopes have different chemical properties.
The different mass suggests different behaviour, but chemistry is electronic.
Instead: Same proton number → same electron arrangement → same chemical properties.
Mistake: Changing the proton number when an ion forms.
Ions are electronic changes; the nucleus is untouched in ordinary chemistry.
Instead: Ions form by losing or gaining electrons only.
Mistake: Writing the nuclide numbers on the wrong sides.
A guessed layout loses the mark even when both values are right.
Instead: Nucleon number top-left, proton number bottom-left.
Mistake: Saying a solid's particles stop moving when frozen.
Fixed positions are not still positions.
Instead: Solid particles vibrate about fixed points — they never stop entirely.
Mistake: Describing a liquid's particles as far apart.
Liquids are barely expandable — their particles touch.
Instead: Liquid particles are touching but irregular, free to slide past one another.
Mistake: Stating that all atoms of an element have identical masses.
Elements with isotopes have a spread of masses.
Instead: Atoms of an element share the proton number, not necessarily the mass.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Describe the solid, liquid and gaseous states and their interconversion using the kinetic particle theory and the energy changes involved.
Which row correctly describes the particles in a liquid?
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Practise
Topical questions
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- 4Foundation
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12 questions worth 69 marks, each with a model answer, mark scheme and examiner tips.
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Question paper
Chapter 2 Practice Paper
30 marks · 40 min · 5 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 2 (states of matter and changes of state, sub-atomic particles, atomic structure, nuclide notation, isotopes, atoms and ions).
Mark scheme
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Model answers with a mark for each creditworthy point — for marking at home or in class.
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Questions
Frequently asked questions
What are the three things to mention when comparing states of matter?
Arrangement (regular, irregular, random), separation (touching, touching, far apart) and motion (vibrate fixed, slide past, fast and random). Name all three every time — answers with only one rarely earn full marks.
Why does the temperature stay flat during melting or boiling?
Because the energy being added goes into changing the arrangement of the particles — overcoming the forces holding them in place — rather than into their average speed, and temperature measures average speed (kinetic energy).
How do I read a nuclide notation quickly?
Bottom-left number = protons. Top-left number = protons + neutrons, so neutrons = top − bottom. Electrons = protons for a neutral atom; adjust by the charge for ions.
Do isotopes behave differently in chemical reactions?
No. Isotopes share the same proton number, so they have identical electron arrangements and identical chemical properties. Only physical properties linked to mass (like density) and nuclear stability differ.
What changes when an ion forms?
Only the electrons. Cations lose electrons (charge = number lost); anions gain them. Proton and neutron numbers never change in ordinary chemistry.