from identical atoms, such as hydrogen with hydrogen (H 2 ), or, oxygen with
oxygen (O 2 ). Berzelius dualism, the hypothesis that only non-identical atoms can
combine in molecules, was with this discovery finally rejected. That was one of the
most important steps in the development of structural theory.
Avogadro’s and Canizzaro’s discovery of two-atomic molecules has been the
direct consequence of the fundamental law of volumetry formulated by Joseph
Louis Gay-Lussac (1778–1850) in 1808: in chemical reaction gases combine in
constant ratios of volumes. This is the same year when Dalton has published the
law about multiple ratios of masses in his A New System of Chemical Philosophy,
i.e. the basic law of gravimetry!
However, Dalton’s law of gravimetry, and Gay-Lussac’s law of volumetry were
apparently in contradiction. While Dalton’s law has confirmed the Lavoisier’s
discovery about the conservation of masses before and after chemical reaction, what
means that the sum of masses of substances before reaction is equal to the sum of
masses of substances after the reaction, from the Gay-Lussac’s law does not follow
that the volumes are the same before and after the chemical process. For example,
Gay-Lussac and Friedrich Wilhelm Heinrich Alexander von Humboldt (1769–
1859), who is better known by his ideas about the basic principles of modern
universities, have already in the year 1805 discovered that two volumes of hydrogen
and one volume of oxygen yield two volumes of water. Thus, since before the
reaction there are three volumes (one of oxygen, and two of hydrogen), and after
the reaction we have only two volumes of water vapour, the total volume before and
after the reaction is not the same.
The nonexistence of the law of conservation of volume has stimulated Cannizzaro and Avogadro to propose that hydrogen, oxygen and other gasses exist as
two-atomic molecules H 2 , O 2 , etc. Berzelius representation of the reaction must to
be reshaped in chemical equations, such as we use today:
2H 2 þ O 2 ! 2H 2 O
Law of conservation of masses is conserved by the discovery of the two-atom
molecules, because from the upper equation it is clear that the number of atoms is
the same before and after the reaction (in summary, 4 hydrogen atoms and 2
oxygens on both sides of the equation). Let me mention that Gay-Lussac has also
formulated an additional physical law about the correlation of pressure or volume
with temperature. This law plays the important role in classical thermodynamics.
Accumulated discoveries of new phenomena such as atoms, molecules, amount
of substance, atomic masses molecular masses, etc., were required a new kind of the
general systematization, a sort of convention between chemists. Such convention,
which was especially supported by August Kekulé, and which comprises definitions
of atoms, molecules, and the values of relative atomic masses of all known elements, was the result of the first world congress of chemists organized in Karlshuhe
(Germany) from 3 till 5 September 1860. The Karlsruhe congress has been the
beginning of standardization in chemistry, necessary for the further development of
science and its applications. The agreed values of atomic masses, nearly 1 for
112
11 Models and Reality
oxygen (O 2 ). Berzelius dualism, the hypothesis that only non-identical atoms can
combine in molecules, was with this discovery finally rejected. That was one of the
most important steps in the development of structural theory.
Avogadro’s and Canizzaro’s discovery of two-atomic molecules has been the
direct consequence of the fundamental law of volumetry formulated by Joseph
Louis Gay-Lussac (1778–1850) in 1808: in chemical reaction gases combine in
constant ratios of volumes. This is the same year when Dalton has published the
law about multiple ratios of masses in his A New System of Chemical Philosophy,
i.e. the basic law of gravimetry!
However, Dalton’s law of gravimetry, and Gay-Lussac’s law of volumetry were
apparently in contradiction. While Dalton’s law has confirmed the Lavoisier’s
discovery about the conservation of masses before and after chemical reaction, what
means that the sum of masses of substances before reaction is equal to the sum of
masses of substances after the reaction, from the Gay-Lussac’s law does not follow
that the volumes are the same before and after the chemical process. For example,
Gay-Lussac and Friedrich Wilhelm Heinrich Alexander von Humboldt (1769–
1859), who is better known by his ideas about the basic principles of modern
universities, have already in the year 1805 discovered that two volumes of hydrogen
and one volume of oxygen yield two volumes of water. Thus, since before the
reaction there are three volumes (one of oxygen, and two of hydrogen), and after
the reaction we have only two volumes of water vapour, the total volume before and
after the reaction is not the same.
The nonexistence of the law of conservation of volume has stimulated Cannizzaro and Avogadro to propose that hydrogen, oxygen and other gasses exist as
two-atomic molecules H 2 , O 2 , etc. Berzelius representation of the reaction must to
be reshaped in chemical equations, such as we use today:
2H 2 þ O 2 ! 2H 2 O
Law of conservation of masses is conserved by the discovery of the two-atom
molecules, because from the upper equation it is clear that the number of atoms is
the same before and after the reaction (in summary, 4 hydrogen atoms and 2
oxygens on both sides of the equation). Let me mention that Gay-Lussac has also
formulated an additional physical law about the correlation of pressure or volume
with temperature. This law plays the important role in classical thermodynamics.
Accumulated discoveries of new phenomena such as atoms, molecules, amount
of substance, atomic masses molecular masses, etc., were required a new kind of the
general systematization, a sort of convention between chemists. Such convention,
which was especially supported by August Kekulé, and which comprises definitions
of atoms, molecules, and the values of relative atomic masses of all known elements, was the result of the first world congress of chemists organized in Karlshuhe
(Germany) from 3 till 5 September 1860. The Karlsruhe congress has been the
beginning of standardization in chemistry, necessary for the further development of
science and its applications. The agreed values of atomic masses, nearly 1 for
112
11 Models and Reality