Compounds have a fixed composition involving small numbers of atoms in whole number ratios which remain constant from sample to sample of the compound.
Atoms link up in small whole number proportions to form molecules, although more than one combination may be possible, as in carbon monoxide and carbon dioxide.
When two compounds have the same atoms and different proportions, the properties of the compounds will be quite different, as in water and hydrogen peroxide.
A compound is often formed of an element and a group which remains linked during chemical reactions, even as it changes partners, behaving almost as an element.
The existence of chemical compounds with fixed proportions is further evidence for the reality of atoms as the base unit of matter as we experience it.
In 1865, Josef Loschmidt estimated the number of molecules in a fixed volume (1 cc, today, one millilitre) of gas, from kinetic theory, Loschmidt's number.
In 1873, James Clerk Maxwell estimated Loschmidt's number as 1.9 x 10^19, equivalent to an Avogadro's number of 4.3 x 10^23, about 2/3 of the accepted value.
In 1908, Jean Perrin studied Brownian motion in water, relating this to the size of the water molecules, getting a good estimate of the size of the molecules.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatscompounds.htm, first created on January 25, 2006. Last recorded revision (well I get lazy and forget sometimes!) was on January 25, 2006.