"If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words?  I believe it is the atomic hypothesis that all things are made of atoms - little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another." (Feynman)

Richard Feynman made this statement during one of his introductory lectures, at the California Institute of technology in 1961, to a class of freshman and sophomores.  In his next ten pages of lecture notes he explains and validates this statement.

Feynman starts off by describing the physical appearance of the atoms using water molecules as an example.
 

To illustrate the power of the atomic idea, suppose that we have a drop of water a quarter of an inch on the side.  If we look at it very closely we see nothing but water - smooth, continuous water.  Even if we magnify it with the best optical microscope available - roughly two thousand times - then the water drop will be roughly forty feet across, about as big as a large room, and if we looked rather closely, we would still see relatively smooth water - but here and there small football-shaped things swimming back and forth.  Very interesting.  These are paramecia.  You may stop at this point and get so curious about the paramecia with their wiggling cilia and twisting bodies that you go no further, except perhaps to magnify the paramecia still more and see inside.  This, of course, is a subject for biology, but for the present we pass on and look still more closely at the water material itself, magnifying it two thousand times again.  Now the drop of water extends about fifteen miles across, and if we look very closely at it we see a kind of teeming, something which no longer has a smooth appearance - it looks something like a crowd at a football game as seen from a very great distance.  In order to see what this teeming is we will see something similar to what is shown in Figure 27.  This is a picture of water magnified a billion times, but idealized in several ways.  In the first place, the particles are drawn in a simple manner with sharp edges, which is inaccurate.  Secondly, for simplicity, they are sketched almost schematically in two-dimensional arrangement, but of course they are moving around in three dimensions.

Feynman goes on to describe other properties of atoms:

1 Atom's vibrating motion.
2.    Atom density in a gaseous form
3.    Properties of gas
     a.    Pressure is proportional to density
     b.    Gas compression
4.    Properties of solids.
5.    Dissolving
6.    Atom arrangement in molecules