Re: why did John Dalton suggest his atomic theory?

     In 1808 John Dalton published "A New System of Chemical Philosophy" 
where he outlined his atomic theory of matter.   Let's briefly write out 
what that theory states.

Dalton's Atomic Theory

1. Matter consists of indivisible atoms.
2.  All of the atoms of a given chemical element are identical in mass and 
in all other properties.
3. Different chemical elements have different kinds of atoms; in 
particular, their atoms have different masses.
4. Atoms are indestructible and retain their identities in chemical 
reactions.
5. A compound forms from its elements through the combination of atoms of 
unlike elements in small whole number ratios.

     The first part of Dalton's theory was nothing really new.  This idea 
dates back to Greek times and seems almost intuitive.  There has to be some 
limit at which particles cannot be "divided" anymore.  You can imagine a 
leaf, a coin, or anything else.  If you divide (or cut) that into smaller 
pieces, eventually you will get a piece that can't be cut anymore - an 
atom!  

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Actually, this wasn't all that obvious to the ancients; most saw no reason 
that matter could not be subdivided indefinitely. Greek atoms were purely
speculative!
                                                     -- Moderator
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But we can use scientific reasoning also to come upon this 
conclusion.  During Dalton's time, stoichiometry was just beginning to 
develop.  (Stoichiometry is the study of mass relationships in chemical 
reactions).  The fact that there is some regularity to chemical compounds 
and reactions leads us to the idea that there must be small particles 
(atoms!) combining in a regular way to produce compounds.

     The second idea stems to a certain extent for the first idea.  Let's 
look at water as an example.  Dalton knew that water was composed of 
hydrogen and oxygen.  Today we know the chemical formula to be H2O, but 
Dalton believed the formula to be HO.  Let's ignore the formula conflict 
right now and pretend that water is made up of 1 oxygen atom and 1 hydrogen 
atom.  We know all water to be the same (same weight, same physical 
properties).  That is, a given molecule or a given volume of pure water 
will weight the exact same amount.  The only way this can be true is if we 
assume that atoms of a given element all weigh the same and that hydrogen 
and oxygen consistently have the same properties.  If atoms of the same 
element all have the same properties, given elements will always react the 
same way given certain conditions.  We can see how the third idea relates 
to the second.  It merely assumes that there are different elements.

    The fourth idea tries to explain the law of conservation of mass and 
can be related to an experiment performed by French chemist Antoine 
Lavoisier.  Lavoisier performed some of his experiments with mercury.  
Essentially, he heated mercury with air in a sealed flask.  After a few 
days, mercury (II) oxide was produced.  Lavoisier took a carefully weighed 
amount of the mercury (II) oxide and heated it strongly.  This reaction 
produced elemental mercury and oxygen gas.  After weighing the amount of 
mercury and oxygen produced, Lavoisier noted that these products had the 
same combined mass as the mercury (II) oxide he started with!  Mass stayed 
the same, so no matter was created or destroyed (no atoms were created or 
destroyed).  We can also see that atoms retain their identities in a 
reaction.  That is, we don't form any new elements in a chemical 
reaction.  We started with mercury (II) oxide, which we know contains 
the elements mercury and oxygen, and we end up with elemental oxygen and 
mercury after heating.

     The fifth part of Dalton's theory pulls in something called the law of 
definite proportions.  This idea started as a debate between two prominent 
French chemists, Claude Berthollet and Joseph Proust.  Berthollet claimed 
that given two samples of the same compound, those samples do not have to 
have identical weights.  For example, we know today that water contains 
11.1% hydrogen by mass.  According to Berthollet's idea, water will not 
always be 11.1% hydrogen by mass, but rather the amount of hydrogen could 
vary.  Proust vehemently disagreed and claimed that Berthollet's 
conclusions were the result of experimental error (for example, using a 
scale that is not properly calibrated) and impurities in samples.  So the 
law of definite proportions essentially says that in a given chemical 
compound, the proportions by mass of the elements that compose it are 
fixed, independent of the origin of the compound or its mode or 
preparation.  For example, pure sodium chloride (better known as table 
salt) will always contain 60.66% chlorine and 39.34% sodium by mass, 
whether it is obtained from seawater, mines, or by synthesis in the 
laboratory.  From this we can draw another conclusion: elements will 
combine in certain whole number ratios.  (As a side note, today we know 
that a few compounds - called berthollides - do not combine as whole number 
ratios.  This happens because some elements can accept different numbers of 
electrons - that is, they have a wide variety of oxidation numbers.  Iron 
is one example.)

     Though Dalton did rely on the experiments of others as proof for his 
theory, he also did a lot of experimentation himself.  He experimented 
mostly with gases and used that data to support his ideas.  Specifically, 
he noted that water existed as a gas in air.  Since two particles cannot 
occupy the same space at the same time, he realized that the air and water 
must somehow mix, and that the agents of mixing must be tiny particles 
(atoms!).

     As I found, the actual events that brought Dalton's atomic theory to 
fruition are long and complicated.  To list and describe the experiments 
would require the length of a book, so I have tried to condense everything 
a bit.

     If you are curious, the information contained above is from a first 
year college chemistry text:

Oxtoby, David W. and Norman H. Nachtrieb.  "Principles of Modern Chemistry: 
     Third Edition."  Fort Worth: Saunders College Publishing, 1996.

     The following website also has information on Dalton's atomic theory. 
http://antoine.fsu.umd.edu/chem/senese/101/atoms/dalton.shtml

     If you are even more curious, I found another text that goes into 
extreme (and sometimes confusing!) detail concerning Dalton's experiments 
as well as reactions from some contemporary scientists:

Rocke, Alan J.  "Chemical Atomism in the Nineteenth Century: From Dalton to 
     Cannizzaro."  Columbus, OH: Ohio State University Press, 1984.