Evidence of Atom

Preamble

The idea that matter is made of tiny indivisible particles called atoms is very old. It goes back at least to the Greeks. It wasn’t until the end of the nineteenth century that something more was known about atomic structure. In this period Physicists began to debate whether atoms existed or were only a convenient fiction used by chemists.
In a few years, from 1895 to 1930, we pass from knowing almost nothing about the atoms, to a firm theory letting us explain many physical and chemical phenomena.
The presence of electrons in the atom was discovered in 1897, while the existence of the nucleus was cleared up in 1911. During those years, Niels Bohr developed new ideas, providing building blocks for a more complete and successful theory of atoms called Quantum Mechanics, a branch of modern Physics concerning the study of physical micro-systems.

Elements and compounds: a clear distinction.

Since certain elementary substances always seemed to be retrievable in the experiments of early chemists, they assumed that every elementary substance has irreducible particles that retained their form. The idea of elementary substances (elements), made up of tiny particles (atoms), represents a successful model to explain experimental evidence: atoms of one element might combine with atoms of other elements to form different non-elementary substances (compounds).
Helium, hydrogen, oxygen, copper, watertight, fluorine, nitrogen, zinc… are examples of elements.
Water, salt, bronze, brass… are examples of compounds. Water is made up of hydrogen and oxygen; salt is made up of Chlorine and Sodium, bronze is made up of copper and watertight, brass is made up of copper and zinc…
Until 1803 ten thousands compounds had been discovered, reducible to thirty-five elements. Nowadays we know approximately one million of compounds reducible to about one hundred elements.

The mass conservation principle and the law of definite proportions.

Systematic study of how elements combined with each other revealed the first laws and regularities that form the basis of chemistry.
The first important result was due to Antoine Lavoisier (1743-1794) who discovered the mass conservation principle:
TOTAL MASS OF CHEMICAL REACTANTS AND PRODUCTS IS CONSERVED IN CHEMICAL REACTIONS.
This principle is an important achievement for that time. Think for instance of the combustion of a piece of wood. Combustion is a chemical reaction during which oxygen in the air combines with carbon in the wood to form carbon dioxide and water vapour. If we don’t know the role of oxygen, we can be misled into thinking that some of the mass of the wood disappears during the combustion (in effect the final mass of ash is lighter than the initial mass of wood). Only if we recognize and measure the quantities of oxygen in the air involved in the reaction do we reach the right conclusion that mass is conserved.
As a consequence of the mass conservation principle, John Dalton (1766-1844) deduced many details about the masses of elements and compounds involved in a chemical reaction. He discovered that WHEN A CHEMICAL REACTION TAKES PLACE, THE REACTANTS COMBINE ALWAYS IN THE SAME DEFINITE PROPORTION.
For instance when oxygen combines with hydrogen to form water(), the ratio of quantities involved is eight to one. This means that:

Each reaction has its own specific mass proportion. In the formation of carbon dioxide() the ratio is eight oxygen parts to 3 carbon parts, that means:

(table B)

The evidence of these ratios during chemical reactions, suggested to Dalton the idea that different elements had atoms with different atomic masses. ANY ATOM COULD BE CHARACTERIZED WITH A NEW PROPERTY CALLED ATOMIC MASS. The characteristic masses of atoms were responsible for the regular ratios observed in the reactions to form molecules of different compounds (molecule: two or more atoms bound together). The idea of atomic mass makes the physical distinction between atoms possible. This distinction can be made in relative terms by referring to the hydrogen atom. For instance, it’s possible to establish the mass of oxygen as a function of the mass of hydrogen: we know that each molecule of water is made up of one O atom and two H atoms. This proportion must be respected during the chemical reaction of O and H. Hence, by referring to Table A, it’s possible to state that: the mass of one atom of O is to the mass of two atom of H, as 1000 grams of O are to 125 grams of H.
It’s easier to write it in the mathematical language:

this proportion leads to

                                                                                                               (C)

and from the last one we deduce that the Oxygen atomic mass (a.m.) is sixteen times the hydrogen atomic mass.

With a similar argument, we can express the mass of one atom of carbon as a function of the hydrogen atomic mass too. It’s known that each molecule of carbon dioxide is made up of two O atoms and one C atom. Hence, by referring to Table B, it’s possible to state that: the mass of one atom of C is to the mass of two atoms of O as, for instance, 375 grams of C are to 1000 grams of O. In other words:

this proportion leads to

by inserting the relation C into the last one we have

hence the Carbon atomic mass is twelve times the hydrogen atomic mass.

Going on this way, elements can be ordered by the criterion of their increasing masses:

On the other hand, Chemists grouped elements in families that present chemical regularities and similarities. For example chlorine Cl, fluorine F, bromine Br (the group now known as the family of halogens) formed similar compounds when reacting with metals as sodium Na, potassium K, Lithium Li (the group now known as the family of alkali metals).

By crossing both the criterions, Dmitri Mendeleev (1834-1907) published in 1869 his periodic table of the elements. In Mendeleev’s table, elements are ordered by the increasing atomic mass criterion along each row, by chemical similarity criterion along each column.