The particle model shows particles as solid spheres
Definition
Particle model
A model that represents all matter as tiny particles whose arrangement, spacing and movement explain the properties of solids, liquids and gases.
- Particle diagrams draw every particle of a substance as an identical solid sphere.
- The model treats the particles as inelastic spheres, like bowling balls that are hard and never change shape.
- The spacing of the spheres shows the state: packed in rows for a solid, touching but jumbled for a liquid and far apart for a gas.
- The model explains why solids keep their shape, why liquids flow and why gases can be compressed.
- Because it is a simplified picture, the model leaves out several features of real particles that matter during a change of state.
Key Idea
A model is judged by what it explains, so a limitation is anything real particles do that the spheres in a diagram cannot show.
Real particles are not solid spheres drawn to scale
- Real atoms are not solid all the way through, because each one is mostly empty space.
- Many particles are not spheres at all, since a water molecule is bent and a carbon dioxide molecule is a straight line of three atoms.
- Particles of different substances have different sizes, so one set of identical circles cannot represent every substance.
- Diagrams cannot show the size of particles compared with the spaces between them.
- In steam, the water molecules themselves take up less than a thousandth of the volume, so a diagram drawn to scale would look almost empty.
Example
- About 1 cm31\,\text{cm}^31cm3 of liquid water boils to make roughly 1700 cm31700\,\text{cm}^31700cm3 of steam, yet the number of water molecules stays the same.
- Almost all of that extra volume is empty space between the molecules, which an ordinary particle diagram has no room to show.
The model leaves out forces of attraction
- Real particles attract their neighbours, but the spheres in a diagram have no forces drawn between them.
- These forces of attraction are what hold a solid in shape and keep a liquid together.
- The strength of the attractions varies from one substance to another, which a set of identical circles cannot show.
- Oxygen boils at −183 ∘C-183\ ^{\circ}\text{C}−183 ∘C while water boils at 100 ∘C100\ ^{\circ}\text{C}100 ∘C, a difference caused by how strongly their particles attract.
- The attractions act between particles, so melting and boiling overcome them without breaking the particles themselves apart.
Common Mistake
Do not describe melting ice as breaking the bonds inside water molecules, because the molecules stay whole and only the attractions between them are overcome.
The model cannot show energy in changes of state
Definition
Kinetic energy
The energy an object or particle has because it is moving.
- A particle diagram is a still picture, so it cannot show how fast the particles are moving.
- Heating a substance gives its particles more kinetic energy, which a diagram can only hint at with arrows.
- During melting and boiling, the energy supplied goes into overcoming attractions between particles, and no sphere diagram shows where that energy goes.
- During condensing and freezing, energy is released as the attractions pull the particles closer together, which the diagram cannot show either.
- Two diagrams side by side show the arrangement before and after a change of state, but not the energy transfer that caused it.
Exam technique
- When a question asks for limitations of the particle model, name the feature the diagram leaves out and then say what real particles do instead.
- Missing forces, non-spherical shapes, particle size compared with the spaces and energy transfer are separate limitations, so choose different ones when more than one is asked for.
Self review
- What shape does the particle model give to every particle?
- Why is a water molecule a poor fit for the solid sphere picture?
- Which forces between particles are missing from a particle diagram?
- Why can a particle diagram of steam never be drawn to scale?
- What happens to the energy supplied to a solid while it melts?
