We think of matter as being made up of tiny particles. A particle is a 'tiny part' - it is the smallest part of the whole you can have with it still being the same substance. If you took a lump of something and cut it in half again and agin until you were left with something that you couldn't divide any more - that would be indivisible - and you would have a particle of that substance.

The smallest 'particle' of a compound is a molecule - you'll study those in Chemistry!

The smallest 'particle' of an element is an atom - the details of the interactions and properties of those is again the realm of Chemistry!

All atoms break down into protons, neutrons and electrons.

In physics we them break atoms down even further.... but that is the realm of particle physics at A level!

Up to KS4 (Y11) we only look at the atoms and molecules as particles. We totally ignore Chemistry - which looks into the fact that all atoms are not the same but that there is a whole periodic table of them! Instead we look at the 'big picture' of particle behaviour and consider them to all be and behave the same!

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 movement.

The motion is a combination of these three types:

In a solid there is no translation - they are stuck in a fixed 3-dimensional arrangement (often a 'regular' one - 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!). It is difficult to compress them because the particles are so closely packed. Solids are often referred to as a condensed phase because the particles are very close together

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 them 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.

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!

Internal Energy (A level work)

In the realm of thermodynamics we are interested in the internal workings of mattter. That is the sum total of all of the energies of the particles in it.

The energy that a particle has is down to two factors:

The restraining influence of the other particles works against the liberating kinetic energy. So, if a particle has net 'negative' energy it means that the 'potential' enegy is greater than the kinetic energy and the particle is not 'free' - it will be constrained in a solid or liquid form. If it has no potential energy - only kinetic energy - then it is free! (an ideal gas particle).