Full chapter
Particles and Atoms
From changes of state to counting the particles in atoms and ions.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
States and changes of state
Explain what particles do and where energy goes.
Matter consists of particles. In a solid, particles remain close together in an ordered arrangement and vibrate about fixed positions. In a liquid, they remain close but move past one another. In a gas, they are widely spaced and move rapidly in random directions. A gas fills its container because its particles spread throughout the available space; compression mainly reduces the empty spaces between particles.
| State | Arrangement and movement | Observable behaviour |
|---|---|---|
| Solid | Close, regular; vibrate in fixed positions | Fixed shape and volume |
| Liquid | Close, irregular; move past one another | Fixed volume; takes container shape |
| Gas | Far apart; random movement | No fixed shape or volume; readily compressed |
- Solid to liquid: melting
Energy is absorbed to overcome enough attraction for particles to move past each other.
- Liquid to gas: boiling or evaporation
Energy is absorbed to separate particles much further. Evaporation occurs at the surface and can occur below the boiling point.
- Gas to liquid: condensation
Particles come closer and energy is transferred to the surroundings.
- Liquid to solid: freezing
Particles settle into fixed positions and energy is released. Sublimation changes solid directly to gas; deposition is the reverse.
During heating within one state, average particle kinetic energy increases and temperature rises. During the melting or boiling of a pure substance at constant pressure, supplied energy changes particle separation rather than raising temperature, so a plateau can occur. Cooling reverses these energy transfers. Particles themselves do not become larger or disappear.
Heating a pure substance
A rising line alternates with two horizontal plateaus: melting then boiling. Heat continues to enter during each plateau.
Check your understandingIce is melting while its temperature stays constant. Is it absorbing energy?Think it through, then reveal the answer
Inside an atom
Use charges, masses and shells to account for an atom.
An atom has a tiny central nucleus containing protons and neutrons, collectively called nucleons. Electrons occupy shells, or energy levels, around it. Almost all the mass lies in the nucleus. A neutral atom has equal numbers of positive protons and negative electrons.
| Particle | Relative charge | Approximate relative mass | Position |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | -1 | 1/1840; negligible compared with a proton | Shells around nucleus |
A simple shell model
A lithium-7 nucleus contains three protons and four neutrons, with two electrons in the first shell and one in the second.
For the first twenty elements, use the familiar shell arrangements up to 2,8,8,2. For example, sodium has 11 electrons arranged 2,8,1 and calcium has 20 arranged 2,8,8,2. The Periodic Table supplies proton numbers. This course does not require orbital labels such as s, p, d or f.
Check your understandingNeutral sodium has proton number 11. Give its proton and electron counts and electron arrangement.Think it through, then reveal the answer
Atomic numbers, isotopes and ions
Keep the identity of the element separate from its mass and charge.
Proton number Z is the number of protons; it identifies the element. Nucleon number A is protons plus neutrons. In nuclide notation 126C, carbon has Z = 6 and A = 12, hence 6 protons and 6 neutrons. A neutral carbon atom also has 6 electrons.
- Protons = Z
The element identity fixes the proton count.
- Neutrons = A - Z
Only protons and neutrons contribute to nucleon number.
- Electrons: start with Z
Subtract electrons for a positive ion; add electrons for a negative ion.
Isotopes are atoms of the same element with the same proton number but different neutron numbers. Carbon-12 and carbon-14 both have six protons; they have six and eight neutrons respectively. Their neutral atoms have the same electron arrangement, so their chemical behaviour is similar. An ion forms when electrons are lost or gained; its nucleus does not change.
Worked example
Count particles in ions
Find protons, neutrons and electrons in 2412Mg2+ and 3517Cl-.
- Magnesium: protons = 12; neutrons = 24 - 12 = 12. A 2+ charge means two electrons lost: 12 - 2 = 10.
- Chlorine: protons = 17; neutrons = 35 - 17 = 18. A 1- charge means one electron gained: 17 + 1 = 18.
Mg2+: 12 p, 12 n, 10 e. Cl-: 17 p, 18 n, 18 e.
Check your understandingTwo particles both have 8 protons, but have 8 and 10 neutrons. One has 10 electrons. What can you conclude?Think it through, then reveal the answer
Quick revision
Revisit the essentials, then return to an explanation when you need it.
State changes rearrange particles; heating does not enlarge them. Z counts protons, A counts protons plus neutrons. Isotopes change neutron number; ions change electron number.
| Quantity | Rule |
|---|---|
| Protons | Z |
| Neutrons | A - Z |
| Electrons | Z minus positive charge, or Z plus magnitude of negative charge |
Scope and references
Learning outcomes and sources
2. The Particulate Nature of Matter (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
2.1(a) Explain states and their interconversion
- Solid, liquid and gas particle arrangements and movements
- Melting, freezing, boiling/evaporation, condensation, sublimation
- Energy changes during state changes
2.2(a) Compare subatomic particles
- Relative charges and approximate relative masses of proton, neutron, electron
2.2(b) Represent atomic structure
- Nucleons in nucleus
- Electrons in shells shown in a diagram
- No s/p/d/f classification
2.2(c) Define proton and nucleon numbers
- Proton number Z
- Nucleon number A
2.2(d) Read and write nuclide notation
- Carbon-12 notation
- Atomic and mass-number positions
2.2(e) Recognize isotopes
- Same proton number; different neutron number
2.2(f) Count particles in atoms and ions
- Use proton/nucleon numbers
- Account for positive and negative ion charges
- 2026 Pure Chemistry 6092
Official topic 2, pages 11. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Pure Chemistry K324
Official topic 2, pages 11. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2026 Combined Chemistry 5086 / 5088
Official topic 2, pages 28. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.
- 2027 Combined Chemistry K326 / K328
Official topic 2, pages 28. Original explanations mapped to the stated outcomes; 2026 and 2027 topic content agrees.