Dalton and atoms

Development of the Periodic Chart

Dalton and atoms

With few exceptions, the new chemistry of Lavoisier was adopted by chemists everywhere. With the nature of the airs, metals, and earths understood, 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 (1754 to 1826), 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 the Law of Constant Proportions.

Claude Louis Berthollet (1748 to 1822), 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 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 was accepted by nearly all chemists.

John Dalton (1766 to 1844) 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.

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

Substances are composed of indivible particles called atoms.
Each element consists of a characteristic kind of identical atom. Thus there are as many kinds of atoms as elements. Atoms of a particular element are perfectly alike.
Atoms are unchangeable.
When elements combine to form a compound, the smallest portion of the compound is a group containing a definite number of atoms of each element.
In chemical reactions, atoms are never created nor destroyed, but only rearranged.

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 atomic weights** (1808)
Hydrogen 1 Azote 7 Carbon 6 Oxygen 8 Phosphorus 9 Sulphur 13 Magnesia 20	Lime 24 Soda 28 Potash 42 Strontian 46 Barytes 68 Iron 50 Zinc 56	Copper 56 Lead 90 Silver 190 Gold 190 Platina 190 Mercury 167

In 1808 Scottish Thomas Thomson (1773 to 1852) and his English friend, William Hyde Wollaston (1766 to 1828) 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.

***For fast time transport, click on the timeline above or use this alternate Menu.***
introduction	Greeks	alchemy	Lavoisier	↑	Berzelius	molecules	spectra	electron	radiation	Bohr	isotopes	synthesis
to site menu				Discovery and Naming of Chemical Elements					chemistry		physics
page created 23 March 2002 latest revision 4 November 2006				by D Trapp