Many of the ancient Greek philosophers believed that all
substances were composed of tiny particles or atoms; but it was not until the nineteenth century that this idea developed into a useful theory for explaining some of the chemical and physical properties of matter. 

   In 1808, the English chemist, John Dalton ( 6 September 1766 – 27 July 1844), produced experimental evidence to show that chemical compounds consist of molecules which are groups of atoms of various elements united in the same simple numerical proportion.

  An element is a substance which cannot be split into simpler substances, while an atom is the smallest portion of an element which can take part in a chemical change.

  Some scientists thought of atoms as being like tiny billiard-balls, but since then we have learned a great deal about the nature of the atoms themselves.

 The structure of atoms will be discussed later. In this post we shall show how the molecular theory is used in physics to explain some of the elementary properties of gases, liquids and solids.


So let's begin...

   Brownian movement


      I do not want to repeat myself. So please read : Einstein’s theory for Brownian motion  . It is discussed in details, with gif picture:) . 

  Nature of the force between atoms and molecules
 
   Newton's law of universal gravitation which works so  well in calculations of the force between two  pieces of matter whose distance apart is large compared with their sizes, fails to give the right answer when applied to two molecules which are very close together. This does not mean that the gravitational attraction no longer acts but that incomparably greater forces of a different kind come into action.

 Now we shall deal with this subject very briefly. As most students know ( I hope ) that electricity can produce electric and magnetic forces of attraction and repulsion.

Atoms themselves contain particles of electricity in motion and so we  get  electric and magnetic forces between them.

We shall, therefore, sum up the situation by saying that, when atoms are very close together, the forces between them are electromagnetic in nature.   The net result is that, when their centers are a certain distance apart, the resultant force between two atoms of molecules is zero. When closer than this they repel one another and when further apart they attract one another.

Furthermore, these electromagnetic forces differ from one kind of atom to another and even between atoms of the same kind depending on whether a substance is in the liquid state or some particular kind of solid state. A good example of a substance which can exist in more than one kind of solid state is carbon ( graphite or, of course, diamond in the picture above).

Three states of matter, or four?

   Matter commonly exists in either the solid, liquid or gas state.( of course there is a plasma state of matter which could not be discussed now ).
  In a solid substance the molecules vibrate about their zero resultant force position, alternately attraction and repelling one another.

All true solids have a crystalline structure in which the atoms are arranged in a regular pattern called the lattice.
There is, however, a borderline class of materials which appear to be solids but actually are very viscous liquids. Pitch is a good example. When struck with a hammer, it readily splinters, but if placed in a funnel and left for several years it slowly flows out.

    In a liquid, the molecules are also vibrating to and fro alternately attracting and repelling one another with forces which can be just as strong as those in a solid.

 At the same time, however, the liquid molecules can move freely among one another, exchanging partners as they go. It is this freedom of movement which enables a liquid to take up the shape of any vessel in which it is placed.

It is worth mentioning that experimental evidence indicates that small groups of liquid molecules can arrange themselves for very short period of time into the same kind of regular pattern found in solids.
   In a gas the molecules are much further apart than those in solids and liquids. They move at high velocities colliding with one another and with the walls of their containing vessel.

Except at the moment of collision, the short-range  inter-molecular forces we have been describing do not come into action.

   Unless the gas is highly compressed, the molecules are, for the greater part of the time, so far apart that the attractive force is effectively negligible.

 Consequently, a gas is perfectly free to expand and completely fill the vessel containing it.
 
   The average distance moved by a molecule between collisions is called its mean free path.
 Rudolf Clausius ( 2 January 1822 – 24 August 1888) applied the laws of mechanics to these collisions and showed how they explained the relation between the pressure and volume of a gas.

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Coming up: Surface tension, diffusion, and more. :)