Draft:Dalton's atomic model

Various atoms and molecules, based on their atomic model, from Dalton's book A New System of Chemical Philosophy (1808).

Dalton's atomic model was the first scientifically based atomic model and it was proposed in many steps between 1803 and 1808 by John Dalton, although the author categorized it as an "atomic theory". The model allowed to clarify for the first time why chemical compounds reacted in static stoichiometric proportions (Law of definite proportions), and why when two substances react to form two or more different compounds, then the proportions of these relations are integer numbres (Law of multiple proportions). The model also clarified that even with the existance of a great variety of different substances, these could be explained in terms of a rather small quantity of elemental or elements constituents.

Dalton's main postulates

Dalton formed his atomic theory using a series of simple postulates:

1. Matter is formed by very small particles named atoms, these are indivisible and cannot be destroyed.
2. Atoms of the same element are all equal, they have the same mass (historically, Dalton called this weight) and the same properties (now we know this is not completely true because one element can have lots of different isotopes, whose distinctive characteristic is the number of neutrons in the nucleus, which changes the atomic mass of each of them and physical properties such as their density). Atoms from different elements have different mass (comparing the mass of the elements to the mass of the hydrogen, as a unit, he proposed the concept of atomic weight, although for his lack of knowledge on the existence of isotopes, he really meant what we now know as relative atomic mass, which averages the masses of different isotopes of one same element).
3. Atoms are indivisible even when they are combined in chemical reactions.
4. Chemical compounds are formed in a chemical reaction after the union of two or more different elements (for example, hydrogen and oxygen atoms can combine and form water molecules H₂O).
5. Atoms, when they combine to form compounds, mantain simple numeric relations (for example, in the formation of water, the relation is 2:1, two hydrogen atoms with one oxygen atom).
6. Atoms of different elements can combine in different proportions and form more than one compound (for example, the compund made from two hydrogen atoms and one oxygen atom is water H₂O; as the compund made from two hydrogen atoms and two oxygen atoms gives a different result known as hydrogen peroxide H₂O₂, also known as liquid un-bleach).

Model's insufficiencies

John Dalton's hypothesis, that stated elements in gaseous state were monoatomic and that the atoms of elements combined in the smallest possible proportion to form atoms from compounds that we now call molecules, caused some difficulties. For example, Dalton thought water was HO. In consequence, many mistakes such as incorrect calculations regarding mass and weight of some basic compounds were made.

In 1805, Gay-Lussac y Alexander von Humboldt proved water was formed by two hydrogen atoms and one of oxygen. In 1811, Amedeo Avogadro confirmed the exact composition for water, basing his work in what is today known as Avogadro's Law and the evidence of the existence of diatomic homonuclear molecules. Nonetheless, these results were mostly ignored until 1860. This was, in part, because of the belief that atoms of the same element did not have any chemical affinity between them. Also, some concepts of molecular dissociation were not explained by Avogadro's Law.

In 1860, in Karlsruhe Congress about stable masses and atomic weights, Cannizzaro revived Avogadro's ideas and used them to elaborate a periodic table regarding relative atomic mass that was very similar to the actual values. These weights were an important prerequisite for the discovery of Dmitri Mendeléyev and Lothar Meyer's Periodic Table.

Other limitation of Dalton's original model is that it ignores the existence of different isotopes of a determinated element, when in reality slightly different versions of atoms of each element can exist that differ in number of neutrons, but always mantain the same quantity of electrons and protons and their chemical properties are very similar, almost indistinguishable.

Dalton also stated that atoms differentiated according to their size, being hydrogen the smallest one.

Finally, until the last half of the XIX century, no evidence of atoms being divisible or being constituted by more elemental parts appeared. For that reason, Dalton's model was not questioned for decades. It properly explained facts, even when it was generated to explain chemical compounds and stoichiometric regularities, it could not explain periodic regularities in chemical element's properties as they appeared in the periodic table of elements of Mendeleiev (this would only be explained by models that presumed the atom was formed by electrons displayed in different layers). Dalton's model also could not count with research made on cathode rays that suggested atoms were not divisible but contained smaller electrically charged particles.

References

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