The Particle Theory - states of matter

The Particle Model is a model of matter. That means it is a way to make our thinking about the complex reality of matter much simpler. It explains most types of matter very well and is easy to picture in our heads.

Rules for being a 'Particle'

Particles are very small - their size is negligible.

Particles are identical - to simplify our model we will ignore Chemistry - same mass - no awkward shape - no polarity (charge differences within the molecule)

Particles attract one another: the closer they are the stronger the attraction (therefore in solids and liquids attraction is very strong but in gases it is negligible).

Particles are in constant random motion (hence the word 'kinetic' in the name of the theory - random means no preferred direction - as many move in one direction as the opposite - all of the movements are 'balanced' or cancel each other out - no nett (resultant) movement.

The motion is a combination of these three types:

vibrational motion - back and forth about a point

rotational motion - spinning round - changing direction

translational motion - moving from place to place in a straight line.

The arrangement of particles within each of the three main states of matter can be used to explain their general properties.

There are some substances that are difficult to categorize - for example sponge, rubber etc. Their structures make them not quite fit the model's examples. In an examination the examiner will be thinking of a rigid solid (like a metal block) when speaking of a solid... but the ones that are difficult to categorize do have interesting structures and you will learn more about them at higher levels.

This excellent graphic was taken from - click to go there - worth a visit!In a solid there is no translation - they are stuck in a fixed 3-dimensional arrangement (often a 'regular' arrangement - one with a pattern).

They do not have enough energy to overcome the attraction from their neighbours - that is why solids are a fixed shape and size.

They are restricted to rotational and vibrational movement. The hotter they are the faster they vibrate and rotate and the further apart they are able to get (that is why objects expand on heating!).

Solids are often referred to as a condensed phase because the particles are very close together.

Solids generally have a fixed shape. They cannot flow to take on new shapes. This is because the particles cannot move from place to place - they have no translational energy.

Solids cannot be easily compressed, squashed or stretched. This is because the particles are as close together as they can be. and therefore have no spaces to move into.

This excellent graphic was taken from - click to go there - worth a visit!In a liquid they are still greatly under the influence of the particles around them but they are able to move around - they have translational energy but it is very limited.

They are not in a regular patern at all.

They constantly move and 'mingle' with each other - like a person in a crowd they move in contorted paths as they weave their way around.

Liquids therefore have a fixed size - no 'liquid particle' has the energy to escape completely from its neighbours - but the shape is determined by the shape of the base of the container that the liquid is poured into (gravity ensures it settles as low as possible in the gravitational field - therefore the liquid has a flat surface. (But - see meniscus!).

It is difficult to compress liquids because the particles are so closely packed - but as they slide past each other it is easy to change the shape!

A liquid flows easily because the particles can move/slide past one another.

Liquids are often referred to as a condensed phase because the particles are very close together.

This excellent graphic was taken from - click to go there - worth a visit!In a gas the particles are so wide apart that they are totally free from each other's influence!

A gas flows easily because the particles can move past one another so easily.

They whizz around in straight lines and their translational energy is so great that we can ignore the other two forms most of the time!

Their speeds are very high.

They bump into each other and the atoms in the container they are in.

When they bump into the container sides they create a 'pressure' on the container - the faster they move (i.e. the higher the temperature) the harder they hit the container and the greater the pressure on it - heating a gas sealed in a container is very dangerous as it can explode if the pressure gets too great!

No matter how low or high the temperature is a gas always fills the container it is in because the particles whizz round and fill the space - it is the pressure on the container that changes.

If the container is flexible (like a balloon) then the increase in pressure as the gas gets hotter (and particles move faster) causes the container to expand until the pressure pushing from the inside of the container (from the gas) is equal to the pressure on the outside (Atmospheric pressure).

Animated gifs: Copyright 2003, Purdue University - Reproduced with kind permission - click on an image to go through to the originator's site!

This is a link to a page of videoclips on the states of matter and this is a link to a page explaining about internal energy of particles (Advanced level).

to go to a page on changing state

to go to a page on evaporation and condensation