Development of the Periodic Chart

Dalton and atoms


Proust With few exceptions, the new chemistry of Lavoisier was adopted by chemists everywhere.  With the much improved understanding of the nature of the airs, metals and earths, chemists turned their attention to determining the composition of compounds, and the nature of the affinity that holds them together.

The French chemist Joseph Louis Proust (born 1754, died 1826, shown at right→), teaching at Madrid in 1799 showed that the composition of copper carbonate is fixed, no matter where it is obtained or how it is synthesized.  Over the next 9 years he purified and analyzed numerous compounds to support what became known as the Law of Constant Proportions which states that each compound has a fixed composition no matter where or how it was formed.

Claude Louis Berthollet (b1748, d1822), a collaborator with Lavoisier on the new nomenclature, approached chemical composition in his books Recherches sur les lois de l'affinite (1801) and Essai de statique chimique chimique (1803) as a matter of chemical affinities similar to gravity.  Believing that there is no difference between what we now distinguish as solutions and compounds, he argued that Proust's Law of Constant Proportions was an accidental effect of saturated solutions.

However by 1808 Proust's Law of Constant Composition had been accepted by nearly all chemists.  (Some of the substances Berthollet used as his evidence turned out to be confusing mixtures of substances with multiple proportions.)

John Dalton (b1766, d1844, portrait at right→) of Manchester, as part of a life-long series of weather observations and research on gases, speculated upon the nature and constitution of the atmosphere.  Issac Newton had describe an elastic fluid composed of small particles or atoms repelled by an inverse square force.

Dalton It occured to Dalton to contemplate the effect of the difference of size of the particles in such a fluid under different pressures and temperatures.  He tried to determine the relative sizes and weights of atoms from the numbers of atoms in a given volume.  This led to consideration of combinations of gases and the numbers of atoms in such combinations.

In 1801 Dalton applied the atomic concept to account for a mixture of gases exerting a pressure equal to the sum of the partial pressures.  (I.e., each gas exerts its own pressure independent of other gases.) In 1803 he announced that the amount of gas dissolved in water from a mixture of gases is proportional to the partial pressure of that gas.

In studying two gases made of only Carbon and Hydrogen, olefiant gas and carburetted Hydrogen, Dalton found the latter had exactly twice as much Hydrogen in relation to Carbon.  After applying the procedure to carbonic oxide, ammonia, and water, he published in 1808 New System of Chemical Philosophy describing how the Atomic Theory could account for chemical compositions.

Dalton's Atomic Theory

Dalton listed the atomic weights of each element relative to the lightest element, Hydrogen, being set equal to 1.  In determining relative weights, Dalton relied on the results of quantitative analysis of compounds, and the assumed formulas for the compounds.  Dalton presumed the simplest of formulas:  thus water was composed of one atom Hydrogen to one atom of Oxygen.

Dalton's symbols & atomic weights (1808)
dalton's elements

In 1808 Scottish Thomas Thomson (b1773, d1852) and his English friend, William Hyde Wollaston (b1766, d1828) presented confirmation of the Law of Multiple Proportions:  they showed that when oxalic acid combines with potash and strontia to form two sets of salts, one of the salts has twice as much base as the other.

The Atomic Theory not only explained Proust's Law of Constant Composition, but also predicted the Law of Multiple Proportions:  when two compounds are formed from the same two elements, the weights of one element from each compound are in a simple ratio to each other when combining with a fixed weight of the other element.

The second and third volumes of Dalton's New System of Chemical Philosophy were published in 1810 and 1827.

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page created 23 March 2002
latest revision 26 April 2010
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