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# Particle Theory

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## Particle Theory - Introduction

The kinetic theory of matter (particle theory) says that all matter consists of many, very small particles which are constantly moving or in a continual state of motion. The degree to which the particles move is determined by the amount of energy they have and their relationship to other particles. The particles might be atoms, molecules or ions. Use of the general term 'particle' means the precise nature of the particles does not have to be specified.

Particle theory helps to explain properties and behaviour of materials by providing a model which enables us to visualise what is happening on a very small scale inside those materials. As a model it is useful because it appears to explain many phenomena but as with all models it does have limitations.

Solids, liquids and gases

 n solids the particles n liquids the particles In gases the particles are held tightly and packed fairly close together - they are strongly attracted to each other o are in fixed positions but they do vibrate are fairly close together with some attraction between them are able to move around in all directions but movement is limited by attractions between particles have little attraction between them are free to move in all directions and collide with each other and with the walls of a container and are widely spaced out

Fig 1 Particles in solids, liquids and gases

The model can be used to help explain:

1. the properties of matter
2. what happens during physical changes such as melting, boiling and evaporating

The properties of matter

Solids
Liquids
Gases
• have a definite shape
• maintain that shape
• are difficult to compress as the particles are already packed closely together
• are often dense as there are many particles packed closely together
• do not have a definite shape
• flow and fill the bottom of a container. They maintain the same volume unless the temperature changes
• are difficult to compress because there are quite a lot of particles in a small volume
• are often dense because there are quite a lot of particles in a small volume
• do not have a definite shape
• expand to fill any container
• are easily compressed because there are only a few particles in a large volume
• are often low density as there are not many particles in a large space

Why do you think that the same volume of different materials have differing masses
E.g. 1cm3 of gold weighs 19.3g but 1cm3 of aluminium weighs 2.7g

Website maintainer: R. Jones Updated: November 13, 2000