INTRODUCTION

It dates back to the origin of the scientific thought the hypotesis that every material body, althougth showing a continuos structure, is made up of tiny particles which determine the macroscopical properties of the body itself, of course originally, this hypothesis was developed on phylosophical grounds, rather than on sound experimental basis.

The modern concept of atom arised together with modern chemistry in the century XVIII and was clarified times by times while the experimental results were explained. But it was at the beginning of XIX century that it needs an atom hypothese coming from fondamental laws of chemistry:

In 1738, Bernoulli formulated the hypotesis that a gas is made of microscopic particles that are always in motion and that this particles determine the observed pressure because of the collision with the walls of the container.
In 1789 Lavoisier formulated the principle of matter conservation and affirmed that in a chemical reaction the total mass of the reagents is the same as the mass of the products; and the mass of every single element of the reagents is conserved as well.
In 1795 Proust formulated the law of the definite and costant proportions (called "the Proust law" after him) which affirms that, in a pure chemical compound, the costituting elements are always found to be in a definite and constant mass ratio.
In 1800 Dalton formulated the law of the multiple proportions (or simply "Dalton law") which affirms that the mass of a given element to form several different compounds appears to be in simple ratios between (usually small) integral numbers.
In 1802 the first Gay-Lussac's experimental work concerned the thermic expansion of some gasses as air, hydrogen, ammonia and gasses originating from sulphureus acid with a temperature varing between 32°F and 212°F. His research showed that these gasses expanded in away which is the same for an identical temperature variation independently from the actual mean temperature near which the variation is performed.

All these results concurred for the possibility of a real existence of the atoms, but were to predict the formulas for chemical compounds, nor they could predict the ratio between the atomic weights of the different atoms forming a compound. This was achieved only thanks to the great work of Avogadro who, in 1811, showed that identical volumes of different gasses in idetical conditions of temperatures and pressures contain the same number of particles (called Avogadro's number), this implying that the densities of the different gasses are the measures of the masses of their molecules while the volumes ratios between different gasses reppresent the ratios between the number of molecules which combines to form the molecules of the final compound, this extending and clarifyng the work sterted by Gay-Lussac.

It isn't possible for chemistry to foresee the weight of a single atom but it allows to estimate its order of magnitude; to this end it is necessary to know how many atoms are conteined in a given quantity of matter, keeping in mind that a gram-molecule of any substance contains the same number of molecules.

Initially, hidrogen was chosen as the standard to which compare atomic weights, and this because it is the simplest element. Then, when isotopes were discovered, ¹²C was chosen as the new standard

It was then possible to compose a table the atomic wieghts of all the known element were reported. Of course, these weren't absolute weights, in fact they were (and are) relative weights, all referred to the standard, the most abundant isotope of the carbon (the ¹²C ); its atomic weight imposed to be equal to 12,0000.